richard g. carson b.sc, (wons) psychology, university of...
TRANSCRIPT
THE ASYMMETRICAL REGULATION OF POINTING MOVEMENTS
RICHARD G. CARSON
B.Sc, (Wons) Psychology, University of Bristol U.K., 1985
THESIS S U B T D IN PARTIAL W m N T OF THE REQUIREmWS FOR TEEi DEGREE OF
MASTER OF SCIENCE
in the School of
KINESIOLOGY
0 RICHAm. G. CARSON 1988 SIMON FRASER UNIVERSm
MAY, 1988
All rights reserved. This work may not be
reproduced in whole or in part, by photocopy or other means, without permission of the author*
APPROVAL
NAME : Richard Carson
DEGREE : Master of Science (Kinesiology)
TITLE OF THESIS: The Asymmetrical Regulation of Pointing
Movements
Examining Committee:
Chairman: Dr. E.W. Banister
---CC--4---LY&x;----
Dr. D. Goodman Senior Supervisor
Dr. D. El iott McMaster University External Examiner
Date Approved F7 y,kd
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wi thou t my w r i t t e n pe rmiss ion ,
Author : , L C - - C - , , ,
( s i g n a t u r e )
(name )
ABSTRACT
Consideration was given to the way in which factors relating to the presumed
complexity of spatial processing required to locate a target in extrapersonal space, might
exert a consistent and asymmetrical effect upon the overt characteristics of pointing
movements. The role of ambient illumination in the regulation of such movements was also
examined. Target positions were never explicitly revealed but were indicated by brief,
linear, quadratic, cubic and quartic function, display sequences from which subjects were
required to extrapolate to the target location. Contrary to initial expectations, the complexity
of the pattern, indicating target position, had a symmetrical effect upon movement
parameters. Movements made by the preferred and non-preferred hands were, however,
observed to differ in terms of the Peak Velocity achieved during movement, and in terms of
movement duration. The pattern manipulation did however have consistent effects upon
Movement Time, F(3,21) = 11.18, p < 0.01, the Mean Peak Velocity, F(3,21) = 9.29, p
< 0.0005, and upon measures of Constant Error and Variable Error. Collectively these
indices may reflect the incidence of time consuming, modifications of the movement
trajectory.
The availability of ambient illumination during the execution of pointing was
associated with an increase in the absolute accuracy of responses relative to the non-
illuminated conditions, as expressed by Radial Error, F(l, 7) = 7.99, p < 0.05, even
though subjects were never afforded the opportunity for comparison of concurrent visual
information pertaining to limb and target positions. The manipulation of visual conditions
was not however associated with variations in Movement Time. Responses made by the
preferred and non-preferred hands did not differ with respect to the terminal accuracy of the
movement.
The results are discussed in terms of a vkety of "mechanisms" through which
visual information may be utilized in the regulation of goal directed action.
ACKNOWLEDGEMENTS
The author wishes to express his gratitude to the Faculty and students in the School
of Kinesiology for their friendship and advice. My particular thanks are extended to the
support staff in the department, in particular Rob Taylor, and to George Mah, Dale Parkyn
and Paul Nagelkerke for the benefit of their expertise.
I am grateful to Dr. David Goodman for granting me the freedom and
encouragement to pursue lines of investigation which reflected personal philosophies and
interests, and to Dr. Arthur Chapman for his practical advice throughout.
Thanks also to Joe, Rekha, Tsily and Dario for their help, and numerous 'personal
communications'.
DEDICATION
To my parents
HARRY AND MARY for their love,
and constant encouragement.
Heraclitus.. .says... that it is by something in motion that what is in motion is known; for he, like most philosophers, conceived all that exists to be in motion
ARISTOTLE
TABLE OF CONTENTS
PREFACE .......................................................................................... 1
INTRODUCTION ................................................................................ -2
1.1 HISTORICAL ANTECEDENTS ................................................ 2
1.2 THE POSSIBLE ANATOMICAL BASES OF HEMISPHERIC DIFFERENTIATION .................................................................... 3
FUNCTIONAL ASYMMETRIES OF THE CEREBRAL HEMISPHERES .............. 6
2.1 CLINICAL STUDIES ............................................................ 6
2.2 EVIDENCE FROM ADDITIONAL SOURCES . SPLIT BRAIN STUDIES .................................................................................. 9
.............................. 2.3 ASYMMETRIES IN THE "NORMAL" BRAIN 11
THE NATURE OF HEMISFXERIC SPECIALIZATION ................................... 15
3.1 LOCAL AND GLOBAL MODELS .............................................. 15
3.2 THE ANALYTIC . HOLISTIC DICHOTOMY ................................ 17
3.3 SPATIAL VERSUS TEMPORAL? ............................................ -18
3.4 A RESTRAINED RESOLUTION ............................................... 20
HANDEDNESS AND CEREBRAL SPECIALIZATION .................................... 23
4.1 PREFERENCE AND PROFICIENCY ......................................... 23
................. 4.2 THE CONTRIBUTION OF DESCENDING PATHWAYS 25
................. 4.3 DEFICIENCIES IN THE "STRUCTURAL" APPROACH 27
AN "INDIRECT" APPROACH ................................................................. 29
5.1 THE SIGNIFICANCE OF THE STIMULUS INPUT ....................... 29
5.2 THE DUAL TASK PARADIGM ................................................ 31
5.3 APRAXIA AND RELATED DISORDERS .................................... 38
5.4 THE LEFT HEMISPHERE AS A "FEEDBACK PROCESSOR" .......... 39
CHARACTERISTICS OF THE MOTOR OUTPUT .......................................... 42
6.1 REASONS FOR CAUTION ..................................................... 42
6.2 TAPPING TASKS ................................................................ 42
vi i
6.3 THE NON-ROLE OF ATTENTION ............................................ 47
.......... THE ROLE OF VISION IN THE REGULATION OF AIMED MOVEMENTS 53
7.1 A MINIMUM PROCESSING TIME FOR VISION? ......................... 53
7.2 THE MULTIDIMENSIONAL CONTRIBUTION OF VISION ............. 60
7.3 THE CONTRIBUTIONS OF CENTRAL AND PERIPHERAL VISION ................................................................................... -63
ASYMMETRIES OF RAPID AIMED MOVEMENTS ....................................... 66
8.1 THE ROLE OF FEEDBACK PROCESSING ................................. 66
8.2 THREE RECENT STUDIES ................................................... -68
THE SIGNIFICANCE OF SPATIALITY IN MOVEMENT ................................ 76
9.1 ARGUMENTS FOR ECOLOGICAL VALIDITY ............................. 76
9.2 CONVERGING EVIDENCE .................................................... 77
9.3 AN EVOLUTIONARY PERSPECTIVE ....................................... 79
EXPERIMENT 1 .................................................................................. 81
10.1 INTRODUCTION ................................................................. 81
10.2 METHODS ......................................................................... 85
SUBJECTS ....................................................................... 85
APPARATUS FOR DATA COLLECTION .................................. 85
MATERIALS .................................................................... -86
................................................................... PROCEDURE -88
10.3 RESULTS ......................................................................... -90
MEDIAN REACTION TIMES ................................................. 90
MEDIAN MOVEMENT TIMES ............................................... 94
10.4 DISCUSSION ..................................................................... 100
EXPERIMENT 2 .................................................................................. 103
INTRODUCTION ................................................................. 103
......................................................................... 11.2 METHODS 108
SUBJECTS ...................................................................... -108 viii
APPARATUS FOR DATA COLLECTION .................................. 108
MATERIALS ..................................................................... 110
PROCEDURE .................................................................... 110
DATA REDUCTION ............................................................ 110
.......................................................................... 11.3 RESULTS 113
MEDIAN REACTION TIMES ................................................. 113
MEDIAN MOVEMENT TIMES ............................................... 116
RADIAL ERROR ................................................................ 122
X CONSTANT ERROR ........................................................ 126
Z CONSTANT ERROR ................................................. 133
X VARIABLE ERROR ......................................................... 138
Z VARIABLE ERROR .......................................................... 141
VARIABILITY IN THE TIME TO PEAK VELOCITY .................... 145
MEAN PEAK VELOCITIES ................................................... 147
11.4 DISCUSSION .................................................................... -157
SUMMARY AND CONCLUSIONS .......................................................... -170
12.1 SUMMARY OF RESULTS ...................................................... 170
12.2 CONCLUDING REMARKS ................................................... -171
APPENDIX A ..................................................................................... -175
APPENDIX B ...................................................................................... 176
APPENDIX C ...................................................................................... 177
APPENDIX D ..................................................................................... -182
REFERENCE NOTES ............................................................................ 219
REFERENCES ................................................................................... -220
LIST OF TABLES
Table Page
10.1 Median Reaction Time (ms) as a Function of Hand and Visual Field ................ 90 10.2 Median Reaction Time (ms) as a Function of Hand and Pattern ...................... 92 10.3 Median Movement Time (ms) as a Function of Hand and Visual Field .............. 94 10.4 Median Movement Time (ms) as a Function of Hand and Pattern .................... 96
.......................... 10.4b Median Movement Times (ms) Differences among Means 96
10.5 Median Movement Time (ms) as a Function of Hand and Relation to ....................................................................................... Midpoint -97
10.6 Median Movement Time (ms) as a Function of Hand. Visual Field and Target . . .................................................................................... Eccentricity -98
11.1 Median Reaction Time (ms) as a Function of Hand and Visual Field ................ 113 1 1.2 Median Reaction Time (ms) as a Function of Hand and Pattern ...................... 114
1 1.3 Median Movement Time (ms) as a Function of Hand and Visual Field .............. 116 .................... 11.4 Median Movement Time (ms) as aFunction of Hand and Pattern 118
11.4b Median Movement Time (ms) Differences among Pattern Means ................... 118 11.5 Median Movement Time (ms) as a Function of Hand and Relation to
....................................................................................... Midpoint -120
11.6 Radial Error (mm) as a Function of Hand and Visual Condition ...................... 122 11.7 Radial Error (mm) as a Function of Hand. Visual Field and Relation to
....................................................................................... Midpoint -124
1 1.8 X Constant Error (mm) as a Function of Hand, Visual Condition and Visual ............................................................................................ Field -126
11.9 X Constant Error (rnm) as a Function of Visual Field. Relation to Midpoint . . and Target Eccentricity ........................................................................ 127 11.10 X Constant Error (mm) as a Function of Pattern. Relation to Midpoint and
............................................................................ Target Eccentricity -130
1 1 . lob X Constant Error (mm) Differences among Pattern Means ......................... 131 1 1.1 1 Z Constant Error (mm) as a Function of Hand and Visual Condition ............... 133 11.12 Z Constant Error (mm) as a Function of Hand. Visual Field and Pattern .......... 134 11.13 Z Constant Error (mm) as a Function of Pattern. and Relation to Midpoint ........ 136
X
1 1.14 X Variable Error (mm) as a Function of Hand, Visual Condition and Visual Field ............................................................................................ -138
1 1.15 X Variable Error (mm) as a Function of Visual Field and Pattern ................... 139
11.15b X Variable Error (rnm) Differences among Pattern Means .......................... 140
1 1.16 Z Variable Error (mm) as a Function of Hand. Visual Condition and Visual Field ............................................................................................ -141
11.17 Z Variable Error (mm) as a Function of Visual Condition and Pattern .............. 143
.......................... 1 1.17b Z Variable Error (mm) Differences among Pattern Means 143
11.18 Variability in the Time to Peak Velocity as a Function of Hand and Visual Field ............................................................................................ -146
1 1.19 Mean Peak Velocity ( d s ) as a Function of Hand. Visual Condition and Visual Field ................................................................................... -147
1 1.20 Mean Peak Velocity (m/s) as a Function of Hand and Visual Field ................. 148
11.21 Mean Peak Velocity ( d s ) as a Function of Visual Condition and Visual Field ............................................................................................ -149
........ 11.22 Mean Peak Velocity ( d s ) as a Function of Pattern and Target Eccentricity 150
1 1.22b Mean Peak Velocity (ds). Differences among Pattern Means ..................... 150
..... 1 1.23 Mean Peak Velocity ( d s ) as a Function of Pattern and Relation to Midpoint 152
1 1.24 Mean Peak Velocity ( d s ) as a Function of Hand, Visual Field and Target . . Eccentricity. ................................................................................... -153
11.25 Mean Peak Velocity ( d s ) as a Function of Hand, Visual Condition and Pattern ......................................................................................... -155
1A Mean Number of Zero Crossings as a Function of Hand and Pattern .................. 178
............... 1.1A Mean Number of Zero Crossings. Differences among Pattern Means 178
......... 2A Number of Zero Crossings as a Function of Hand and Relation to Midpoint 180
3A Number of Zero Crossings as a Function of Visual Condition. Visual Field and Relation to Midpoint. ........................................................................ -181
LIST OF FIGURES
Figure Page
10A Schematic representation of the position of the subject relative to the display ........................................................................................... panel -86
...................................... 10.1 Median Reaction Time (ms) Hand by Visual Field 91
............................................ 10.2 Median Reaction Time (ms) Hand by Pattern 92
.................................... 10.3 Median Movement Time (ms) Hand by Visual Field 95
.......................................... 10.4 Median Movement Time (ms) Hand by Pattern 97
.......................... 10.5 Median Movement Time (ms) Hand by Relation to Midpoint 198
10.6 Median Movement Time (ms) Hand by Visual Field and Target Eccentricity ....... 99
................................ 1 1.1 Median Reaction Times (ms) by Hand and Visual Field 114
...................................... 1 1.2 Median Reaction Times (ms) by Hand and Pattern 115
1 1.3 Median Movement Times (ms) by Hand and Visual Field .............................. 117
1 1.4 Median Movement Times (ms) by Hand and Pattern ................................. 119
.................... 11.5 Median Movement Times (ms) by Hand and Relation to Midpoint 120
....................................... 1 1.6 Radial Error(mm) by Hand and Visual Condition 123
................... 1 1.7 Radial Enor(rnm) by Hand. Visual Field and Relation to Midpoint 124
.................... 11.8 X Constant Error (mm) by Visual Field and Relation to Midpoint 128
....................... 11.9 X Constant Error (mm) by Visual Field and Target Eccentricity 128
11.10 X Constant Error (mm) by Visual Field. Relation to Midpoint and Target . . .................................................................................... Eccentricity -129
1 1.11 X Constant Error (mm) by Pattern and Relation to Midpoint ........................ 131
11.12 X Constant Error(mrn) by Pattern. Relation to Midpoint and Target . . .................................................................................... Eccentricity -132
................................... 1 1.13 Z Constant Error(mm) by Visual Field and Pattern 135
........................... 1 1.14 Z Constant Error (mm) Hand by Visual Field and Pattern 135
......................... 1 1.15 Z Constant Error(rnrn) Pattern . by Relationship to Midpoint 137
11.16 X Variable Error(mm) Hand by Visual Condition ..................................... 139
....................................... 1 1.17 X Variable Error (mm) Visual Field By Pattern 140 xii
1 1.18 Z Variable Error(mm) Bind by Visual Field ........................................... 142
11.19 Z Variable Error (mm) Visual Condition by Pattem ................................... 144
11.20 Variability in the Time to Peak Velocity as a Function of Hand and Visual Condition ...................................................................................... -145
11.21 Variability in the Time to Peak Velocity as a Function of Hand and Visual Field ............................................................................................ -146
1 1.22 Mean Peak Velocity (m/s) as a Function of Hand and Visual Field ................. 148
1 1.23 Mean Peak Velocity ( d s ) as a Function of Visual Condition and Visual ............................................................................................. Field 149
1 1.24 Mean Peak Velocity (m/s) as a Function of Pattern and Target Eccentricity ........ 151
..... 1 1.25 Mean Peak Velocity (m/s) as a Function of Pattern and Relation to Midpoint 152
1 1.26 Mean Peak Velocity (m/s) as a Function of Hand. Visual Field and Target . . Eccentricity. ................................................................................... -154
1 1.27 Mean Peak Velocity (m/s) as a Function of Hand, Visual Condition and Pattern .......................................................................................... 156
.................................. 1A Mean Number of Zero Crossings by Hand and Pattern 179
2A Mean Number of Zero Crossings by Hand and Relation to Midpoint ................... 180
3A Mean Number of Zero Crossings by Visual Condition. Visual Field and Relation to Midpoint .......................................................................... 182
... X l l l
PREFACE
In recent years a large number of studies have been conducted with the apparent aim
of clarifying the mechanisms which give rise to the almost universal superiority of the
'preferred hand' on a variety of manual tasks and in particular aimed movements. It has
been customary to link exhibited manual asymmetries, at least in part, to the presumed
processing characteristics of the cerebral hemispheres. This paper examines in some detail
the historical, theoretical and experimental background to this approach and highlights
ways in which apparently conflicting accounts may be reconciled. Consideration is also
given to the manner in which visual infoxmation is used in the regulation of such
movements, and the implications this may have for the examination of manual
asymmetries.
CHAPTER 1
INTRODUCTION
1.1 HISTORICAL ANTECEDENTS
Throughout the modern history of the "Mind's Sciences" there have been recurring
attempts to establish specificity of neural functions. These can be clearly discerned in the
writing of Descartes who was among the f ~ s t to recognize that different bodily functions
were subject to control exerted by various regions of the brain.
Perhaps the most infamous proponent of localization was the German anatomist
Franz Gall, whose writings, in the hands of his most ardent followers were expressed in
the doctrine of phrenology. It is of little surprise that Gall's own extreme view, that an
individual's intellectual and emotional profile could be revealed by examination of the
unique configuration of his or her skull, was dismissed in serious scientific circles.
Nonetheless, there was at least some merit in his claim that the faculty for speech was
located in the most anterior parts of the cerebral cortex, the frontal lobes. It was to be some
time however before there would appear evidence that would provide some limited support
for this notion.
The most obvious attempts to establish localization of neural function have been
along the lines of the physical demarcation of the left and right cerebral hemispheres. It has
variously been suggested (e.g., Boring, 1950; Gardner, 1985) that, the presumed,
specialization of the cerebral hemispheres for particular functions was articulated first by
the French surgeon Paul Broca in the early 1860's (Broca 1861), although the conclusion
that speech is controlled by the left hemisphere had been drawn almost thirty years prior to
Broca's presentation. A French country doctor, Marc Dax had in 1836 presented data
demonstrating that aphasia was accompanied by left hemisphere damage, yet his conclusion
that speech is controlled by the left half of the brain was seemingly overlooked. It was
however Broca who compiled evidence of a good deal more substantial nature, including
comprehensive details of case histories and circumscribed anatomical damage.
John Hughlings Jackson's proposal in 1868 of a "leading hemisphere" was itself a
reflection of merely five years of coordinated clinical research during which the concept of
cerebral dominance had already crystallized. There rapidly developed a widely held view of
hemispheric disparity, as Jackson (1958) concludes "that in most people the left side of the
brain is the leading side - the side of the so called will, and that the right is the automatic
side".
The conviction that the right hemisphere was essentially the minor hemisphere was
compounded by evidence revealing difficulties in understanding speech, created by damage
to the rear of the temporal lobe in the left hemisphere (Wernicke, 1874), and by the
catalogued loss of reading comprehension and writing which followed unilateral left
hemisphere damage. In contrast, even large scale damage to the right hemisphere appeared
to have negligible effect upon language functions. In spite of occasional proposals that the
right hemisphere could to some extent compensate for left hemisphere damage (e.g.,
Dejerine & Thomas, 1912; Goldstein, 1917), and that the right hemisphere might even
possess capabilities of its own (Jackson, 1865), there was general adherence to the notion
of strict dominance. It has only been in the second half of this century that this
conceptualization of hemispheric asymmetry has been reassessed
1.2 1 DIFFERENTIATION
The view that cerebral dominance may arise as a consequence of anatomical
asymmetries is a venerable one, nevertheless, in spite of a body of data indicating some
differences between the hemispheres in specific regions (e.g., Connolly, 1950; Pfeifer,
1936; Von Economo & Horn, 1930), the standard view of cerebral dominance for
language, throughout the greater part of this century, was that it possessed no anatomical
correlate, had no analogue in other species and that its evolution in man was not amenable
to study (Damasio & Geschwind, 1984). The accepted view was most lucidly expressed by
von Bonin in his 1962 review, in which, although justifiably dismissing as unimportant
asymmetries such as mass, specific gravity, surface area or length von Bonin displayed a
corresponding disregard for those studies which had already revealed specific structural
differences between the hemispheres. It was in this climate that Geschwind and Levitsky
(1968) published what was to become a seminal report demonstrating, unequivocally, the
existence of asymmetries in the brain regions thought to be important for speech and
language, specifically, in the temporal plane. A number of subsequent studies, using a
variety of procedures, have confirmed the presence of distinct differences between the
temporal planes (e.g., Wada, Clark & Hamin, 1975; Witelson & Paillie, 1973).
Although the magnitude of the asymmetries was impressive, of potentially greater
importance was the observation that virtually all involved the Sylvian fissure or structures
within or along its banks (Geschwind, 1974). This structure is not found in non primate
mammals and indeed has changed considerably in the course of evolution (Connolly,
1950). It is in the region of the Sylvian fissure that are found the major speech areas such
as those of Broca and Wernicke, these are the areas in which lesions are found in most 3
cases of severe aphasia. Geschwind and Levitsky (1968) demonstrated the presence of
structural between hemisphere differences in the Sylvian fissure, leading Geschwind
(1974) to the conclusion that particular cortical areas, all intimately related to speech
function, were larger in the left hemisphere. This appeared to provide at least some initial
confirmation that some manifestations of dominance may arise from physiological
mechanisms. It has also been noted that, in right handers, the right occipital cortex appears
to be of larger size (McRae, Branch & Milner, 1968), these and other posterior regions,
which may be slightly larger in the right hemisphere, are the areas which have been
independently assessed to be most critically involved in visuospatial operations (Harris,
1978).
Indeed, in the past decade, the application of sophisticated techniques has permitted
examination of asymmetries at the microscopic level. Post mortem examinations have
revealed that in the auditory association cortex, a region involved in high level processing
of auditory information and in particular speech, there exist anatomically distinct cell layers,
referred to as Tpt, which are larger in the left hemisphere (Galaburda, Sanides and
Geschwind, 1978). Similarly, cell layers types lying between the temporal and parietal
lobes, PG, appear to be larger on the left side of the brain (Galaburda & Sanides, 1980).
There does then appear to be at least some linkage between asymmetries at the macroscopic
and microscopic levels, enlarged areas of Tpt and PG cells have been correlated with the
size of the temporal plane (Springer & Deutsch, 1985).
Recent research has also provided some substantiation for claims regarding the
presence of neurochemical asymmetries in the brain. In a fashion consistent with
indications from gross anatomical studies, the primary differences have been found in those
areas associated with language processing, for example, the neurotransmitter
norepinephrine appears to be distributed unequally on each side of the thalamus (Ode,
Keller, Mefford and Adams, 1978). Additionally there appear to exist neurochemical
asymmetries at the cortical level, enzymes associated with the activity of the
neurotransmitter acetylcholine, in particular choline-acetyl-transferase (CAT), demonstrate
greater activity in the left than the right temporal lobe, again an area thought to be intimately
associated with the facility for language (Amaducci, Sorbi, Albanese & Gainotti, 198 1).
There is additional evidence, obtained from the "living brain" using techniques such as
cerebral angiography (LeMay & Culebras, 1972) and computerized tomography (CT scan)
(LeMay & Geschwind, 1978), which is consistent with the asymmetries found in post
mortem examinations.
There are, however, a number of problems associated with the evaluation of
anatomical asymmetries, not least in interpreting these differences in a manner which is 4
meaningfully related to functional asymmetries. The clinical value of these insights is
undisputed, in addition the illustration that cerebral dominance has potentionally some
anatomical correlate gives some scope for considering the biological basis of hemispheric
differentiation. Nevertheless, as Springer and Deutsch (1985) point out, "ultimately it is
likely that dividing the brain in terms of 'where' will not completely answer the question of
'how' " (p. 115). It is to studies examining functional asymmetries of the cerebral
hemispheres to which one must turn in order to elucidate the role of the so called minor
hemisphere.
CHAPTER 2
FUNCTIONAL ASYMMETRIES OF THE CEREBRAL HEMISPHERES.
2.1 CLINICAL STUDIES
As outlined, the concept of cerebral dominance arose initially from observations of
the frequent relationship of aphasia and damage to the left cerebral hemisphere in right
handers. However, with the disclosure of multiple functional asymmetries which favour
the right rather than the left hemisphere, the notion of a major and minor hemisphere has
come to be reassessed (Weinstein, 1978). The rationale for clinical examination of
functional asymmetries is loosely described as follows, if individuals suffering damage to a
given hemisphere perform more poorly on a task than individuals with disruption limited to
the other side of the brain, the "mental abilities" for the task can be considered to dwell
primarily in the former hemisphere (Nebes, 1978). The problems of this approach will
soon become evident, although the examination of clinical cases has, however, proved
provocative if inconclusive.
The term aphasia has come to encompass a wide range of speech and language
disorders arising from a broad spectrum of, predominantly left hemisphere, neurological
damage. With the introduction of analytic techniques of increasing subtlety, it has become
clear that the right hemisphere plays a complimentary role in linguistic communication. The
right cerebral hemisphere appears not only to comprehend language in a manner which was
not previously contemplated, but may subserve the processing of nuances of speech such
as intonation and aspects of metaphor (Springer and Deutsch, 1985). A number of aphasics
with intact right hemispheres can derive the meaning of received speech (Danly & Shapiro,
1982), whilst individuals with right hemisphere damage often exhibit speech absent of
intonation (Heilman, Scholes & Watson, 1975).
It is however, with respect to "non-verbal" abilities that the contribution of the right
hemisphere appears most profound. There are aspects of musical processing for which the
right hemisphere seems particularly well equipped, for example; tonal memory, timbre,
melody recognition and intensity (Springer and Deutsch, 1985), and the capacity for
singing appears to remain in patients having undergone a complete left hemispherectomy
(Smith, 1966; Zaidel, 1973). Heilman, Scholes and Watson (1975) have suggested a
leading role for the right hemisphere in the processing of emotional information, whilst a number of studies have indicated that astereognostic conditions occur with greatest
frequency in patients who have suffered lesions in the area of the right parietal lobe (De
Renzi, Faglioni & Scotti, 1970; Fontenot & Benton, 1971). Astereognosis is characterized 6
by a loss of awareness of the spatial relations of ones's body parts or "body scheme" (Ban & Kiernan, 1983). Furthermore, such patients often experience interference with tactile-
kinesthetic memories and are unable to recognize familiar objects when using only tactile
cues (Springer & Deutsch, 1985).
Indeed it is patterns of.performance on what might liberally be classified as "spatial"
tasks which appear to indicate the wider significance of right hemisphere functioning. As
Benton (1969) points out, one of the most dramatic effects upon overt behaviour, in a
patient more usually suffering damage to the right side of the brain, is disruption of the
facility to percieve and manipulate the spatial relationships of objects, both in relation to
himself and to one another. Individuals may be unable to reproduce structures organized in
space and may become lost, unsure of their orientation in an otherwise familiar locale
(Nebes, 1978). Similarly striking is the phenomenon of unilateral neglect. Generally
observed in patients having suffered large scale disruption of the right posterior occipital
regions, the victim will act as if the entire left side of space, and on occasions the left side
of their own body, does not exist (Springer & Deutsch, 1985).
Although the precise distribution of spatial "abilities" between hemispheres remains
unclear, the right hemisphere percept appears to predominate at the primary level of line
orientation and direction of movement (Nebes, 1978; Warrington & Rabin, 1970). The
transition from consideration of the right hemisphere as minor to a view which considers
each side of the cortex as equivalent in importance evolved simultaneously with the
appreciation that the:
ability to scan extrapersonal space and locate stimuli with respect to one another and in relation to the observer mav be considered a ~rereauisite of every operation implying spatial and cognition (be ~ i n z i , 1978, p. 53).
This shift has been accompanied by a large and still expanding body of clinical
evidence demonstrating the importance of the right hemisphere in the mediation of a
number of spatial tasks. The ability to infer the organization and structure of the external
environment without recourse to detailed analysis of the sensory array, generating a percept
of the whole from fragmentary information, is viewed as an essentially spatial function
(Nebes, 1978), and following Thurstone (1944), "closure speed" is the rapidity with which
this operation may be accomplished. Right hemisphere damage appears to detrimentally
affect closure speed to a greater extent than left hemisphere damage @e Renzi & Spinnler,
1966; Lansdell, 1968). Individuals having sustained right posterior or parietal damage
demonstrate problems with spatial orientation and construction (Critchley, 1953; De Renzi,
1967; Patterson & Zangwill, 1944, McFie & Piercy, 1952), whilst deficiency pattern
recognition has resulted from lesions of the right temporal lobe (I(lmura, 1963; Lansdell,
1961, 1968; Meier & French, 1965; Milner, 1958). De Renzi (1978) has suggested that
there may exist a "supramodal mechanism guiding the sensorimotor neuronal networks
involved in scanning extrapersonal space through the various modalities" (p. 58).
Certainly, it appears that disorders of space perception are not restricted to the visual
modality, but occur in both auditorily and tactually guided tasks. Milner and Teuber (1968)
also recount that individuals with right temporal lobectomies did significantly poorer than
controls, and those with equivalent left side operations, on maze learning tasks. The same
pattern of results has been observed on a number of intelligence test subscales. Right
hemispherectomized patients do less well on the Block Design subtest of the Wechsler
Intelligence Test (Smith, 1969; Gott, 1973a) and when dealing with the geometrical figures
comprising the Raven's Coloured Progressive Matrices (Smith, 1969). Additionally, Kohn
and Dennis (1974) have independently confirmed that hemispherectomized patients with
intact right hemispheres perform more efficiently on spatial tasks than those possessing
only a left hemisphere. Patients with right hemisphere disease have also been observed to
perform more poorly than those with equivalent left hemisphere damage tasks involving;
recognition of abstract figures (Rubino, 1970), recognition of overlapping figures (De
Renzi et al, 1969; Gainotti & Tiacci, 1971), the matching of fragmented circles (Meier &
French, 1965) and the detection of a depth effect arising from the presentation of random
dot stereograms (Cannon & Bechtoldt, 1969; Benton & Hecaen, 1970).
The balance of clinical data appears initially to give lie to the notion of cerebral
dominance. As Nebes (1978, p. 120) concludes
when required to perform a spatial transformation on sensory input ..., the minor hemisphere is far superior to the major. This suggests that in man the right cerebral cortex is responsible for forming from the incomplete information provided by our senses the spatial and cognitive map of our surroundings in which the planning and organization of behaviour take place.
There are obviously likely to exist a number of potentially confounding variables of which
one must take account in any comparison between individuals suffering left or right
hemisphere disruption. Groups of patients should be matched for size and locus of
damage, and additionally upon dimensions such as age, sex, education and premorbid
intelligence (Nebes, 1978). The damage sustained is rarely well circumscribed and often
does not respect what are regarded as anatomical boundaries. This is also characteristic of
surgically inscribed lesions performed for medical reasons, and poses obstacles in the path
of attempts to establish whether there exists regional or focal representation of function
within a hemisphere. As Springer and Deutsch (1985) highlight, damage may be sustained 8
to areas directly subserving a given function, or to areas anatomically distinct but
functionally related, resulting in equivalent decrements in either instance. The behaviour
exhibited by an individual subsequent to trauma reflects the functioning of the remaining
brain tissue. With direct reference to the two sides of the brain, overt behaviour may result
from depressed performance of the damaged hemisphere, or from the undisrupted, but
initially inferior, activity of the intact hemisphere (Nebes, 1978). Indeed, diaschisis may
occur, the undamaged hemisphere operating more poorly as a consequence of damage to
the other side of the brain. Clearly, considerable caution should be exercised in the
unfettered interpretation of clinical data.
2.2 EVIDENCE FROM ADDITIONAL SOURCES - SPLIT BRAIN STUDIES.
Clinical evidence of the asymmetrical functioning of the cerebral hemispheres did
not remain uncorroborated, a major impetus towards rationalization of these strands of
belief was provided by what have become popularly known as the "Split Brain Studies".
Until even the late 1930's there was considerable debate as to the role sustained by
the commissural fibre tract, the corpus callosum, it might intuitively have been supposed
that the corpus callosum and other connecting commissures and subcortical connections
would serve some integrative function, yet evidence that this was the case was notably
absent. The only certain role appeared to be that the fibres aided the spread of epileptic
discharge from one side of the brain to the other (Erickson, 1940; Van Wagenen & Herren,
1940). Indeed, these observations merely provided fuel for the facetiae of commentators
such as McCullough (1940) who perhaps doubted the evolutionary utility of a structure
which accentuated the symptoms of localized seizures. In a similar vein, it was with some
irony that Lashley (1950) proposed that the major role of the corpus callosum was to "keep
the hemispheres from sagging" and was thus largely mechanical.
There did however prove to be some utility in the observations of this single
functional attribute. After an initial series of largely unsuccessful operations, complete
cornmissurotomies (the severing of all the fibres of the corpus callosum) appeared to be
successful in limiting the consequences of seizure activity in what were regarded as intractable epileptics. The medical benefits appeared initially to be bereft of accompanying
post surgical deficits (Akelaitis, 1941, 1944), however more extensive and ingeneous
testing orchestrated by Roger Sperry (c.f., Sperry, 1982) demonstrated that, under specific
conditions, the commissurotomized patients were severely limited in their abilities to
perform tasks which necessitated the performance of one hemisphere in isolation from the
other.
These patients appeared to be able to conduct their everyday lives in a conventional
fashion, as both hemispheres receive common information across all modalities even in the
absence of interhemispheric communication. For example the optic chiasm remains intact,
and thus, each hemisphere generally receives portions of the same visual image, if however
visual information is presented solely to one visual field by way of a brief, spatially
localized, exposure, in the absence of eye movement, the stimulus will be projected to the
opposite hemisphere. The perfoxmance of the individual hemisphere may subsequently be
examined in relative isolation.
Research on cornmissurotomized patients has provided confirmation of clinical
evaluations indicating that speech, for the majority of individuals, is the perogative of the
left hemisphere, yet again,the right hemisphere has not been seen to be without language
capabilities, a considerable degree of comprehension has been observed (e.g., Zaidel,
1978). In general however, the right hemisphere has again been observed to demonstrate a
specialization for what are complex, often non linguistic, visual and spatial processes.
Indeed, the right hemisphere has been found to be a good deal more proficient on a
varied selection of spatial and drawing tasks. This right hemisphere advantage appears to
be present at what, from an information processing perspective, might be considered as
relatively fundamental stages of processing. Durnford and Kimura (197 1) observed a right
hemisphere superiority for binocular depth perception, whilst Trevarthen and Leby (1973)
found an accompanying advantage for the detection of motion and the orientation of lines,
arrows and two dimensional objects located in space. A number of studies have concerned
themselves with potential differences in the transmodal transformations required to match
figures to the objects they represent. The left hand performs better when matching wooden
blocks to 2-dimensional drawings of the block in "opened up" form (Levy-Agresti &
Sperry, 1968), and when assembling coloured blocks to match a design (Gazzaniga,
1970). This superiority of the right hemisphere appears to increase as shapes become less
geometric and more free form (Franco & Sperry, 1977). Likewise, Nebes (1971) has
shown that the "left hand system" is superior in establishing the relationship between an arc
and the circle from which it came. This differential ability to establish the whole from its
parts was further emphasized using dot stimulus arrays (Nebes 1973) and line drawings
(Nebes 1972). Constructive reproduction in solving visuospatial problems has also been
examined by Bogen and Gazzaniga (1965) and by Levy (see Sperry, 1974), in the latter
case, a qualitative difference was observed, right hand performance was accompanied by a
running verbal commentary and appeared to be more hesitant. Furthermore, the left hand,
in split brain individuals, has been found to be more proficient when producing geometric
and perspective drawings (Bogen, 1969). 10
Although the conclusions drawn from research conducted with commissurotomized
patients do appear to be in substantial agreement with those obtained from clinical studies,
specifically that the right hemisphere may better assimilate space relations and
configurations of object form, and perform part-whole transformations (Trevarthen, 1974),
it is also prudent to articulate some widely held reservations.It is certainly the case that
information may be transferred from one half of the brain to the other by commissures
situated in lower regions, commissurotomy severs only those nerve fibres lying in the
cortex leaving most subcortical structures intact. Structures such as the superior colliculus,
lying subcortically, are important in their own right. This structure has been implicated in
visual processing, in particular with regard to the spatial locii of stimuli. The commissures
connecting the left and right superior colliculi are not intruded upon by commissurotomy,
thus, irrespective of the position in the visual field, information regarding the location of
that object is available to both hemispheres. This is potentially one reason why asymmetries
for the processing of visuospatial information tend to be less distinct than those for the
manipulation of linguistic material. There is obviously also a need for caution in extending
conclusions based on commissurotomized patients to "normals". The operations were
exclusively undertaken to relieve the symptoms of severe epilepsy it is almost impossible to
assess the damage sustained by the brain or indeed subsequent reorganization following
seizures. A sensitivity to individual neurological histories provides some measure of
accountability, however, one cannot realistically expect the severing of 200 million nerve
fibres to be without consequence. Certainly split brain research may be regarded as a single
element of a complex puzzle, though, the greatest contribution of these endeavors is likely
to have been the powerful impulse provided to the study of hemispheric asymmetries in
normal individuals.
2.3 ASYMMETRIES IN THE "NORMAL" BRAIN
In the period of almost four decades which has followed the instigation of split
brain studies, there has been a vast, one might conclude exponential, increase in the volume
of research which has aimed at elucidating the nature of hemispheric asymmetries in
normals. The flood of publications in recent years has approached a rate of almost four per
week (Allen, 1983). Obviously in face of this burgeoning accumulation of literature it is
possible to focus on barely a fragment of the field and to provide merely a flavour of the
research so delineated. Inevitably there will be omissions, however, the studies cited will
generally serve as illustrations of phenomena which have been widely and consistently
observed. Specific attention will again be given to the role of the right hemisphere in
mediating the manipulation of spatial relationships. 1 1
The investigation of visual asymmetries in normal individuals has resembled very
closely that of cornmissurotomized patients. There has been a marked reliance upon brief
tachistoscopic presentations which permit the projection of visual information initially to
one hemisphere or the other. Of course in nonnal subjects, for whom the corpus callosum
is intact, information directed to one side of the brain is very rapidly made available to the
other. Nevertheless, lateralized presentation of stimuli does appear to result in lateralized
performance.
A number of researchers have obtained faster reaction times to brief stimulus
"flashes" presented in the left visual field (e.g., Anzola, Bertoloni, Buchtel & Rizzolatti,
1977; Berlucchi, Heron, Hyman, Rizzolatti & Umilta, 197 1; Jeeves, 1969; Jeeves &
Dixon, 1970), and these are differences which De Renzi (1978) attributes to tapping of a
right hemisphere superiority for elementary cognitive visuospatial operations. Similarly,
Kimura and Durnford (1974) comment on the attraction of the claim which posits that the
right hemisphere propensity for complex visuospatial functions is based upon asymmetries
of rather fundamental processes.
Certainly the asymmetrical processing of visuospatial information does appear to
exist at a number of levels. Left visual field superiority has been observed for the spatial
localization of dot stimuli (Kimura, 1969; Bryden, 1976; Pohl, Butters & Goodglass,
1972), whilst a reaction time benefit for the left visual field was obtained for the recognition
of "matched" or "unmatched" figures (Gibson, Filbey & Gazzaniga, 1970; Rizzolatti,
Umilta & Berlucchi, 1971). When symmetrical half figures are presented to different
hemispheres matching is accomplished more quickly if both figures are flashed to the right
rather than the left hemisphere @imond & Beaumont, 1972b; Egeth, 1971). Perception of
line orientation is accomplished with greater rapidity when stimuli are presented in the left
visual field (Kimura & Durnford, 1974; Atkinson & Egeth, 1973), as is evaluation of
curvature (Longden, Ellis & Iverson, 1976). The enumeration of dots (La Grone, 1942)
and other non-alphabetic stimuli (Kimura, 1966) is also apparently accomplished more
successfully by the right hemisphere. In addition, there exists some degree of right
hemisphere superiority for visual depth perception @urnford & Kimura, 197 1) and for
stereopsis (Cannon & Bechtoldt, 1969).
These asymmetries are sustained at what, for want of a more generally recognizable
conceptual framework, might be classified as higher order levels of processing. Whilst
there appears to exist a left visual field advantage for pattern displays (Moscovitch, 1979),
there is considerable debate as to whether the often cited right hemisphere superiority for
faces (Geffen, Bradshaw & Wallace, 1971; Rizzolatti, Umilta & Berlucchi, 1971) and face
like stimuli (Patterson & Bradshaw, 1975) is reflective of a facilitation in dealing with
spatial or with emotional characteristics. However, it has been reliably demonstrated that
when employing rehearsal sets for which subjects are encouraged to employ either verbal
or imaginal mneumonics, a lateralized recognition probe will produce left visual field
benefit for the imagery condition (Seamon & Gazzaniga, 1973; Metzger & Antes, 1976;
Seamon, 1974), thus providing a least partial support for Bower's (1972) claim that the
right hemisphere would be dominant for information coded in terms of imagery. Similarly,
in the context of a study in which examination was made of effects due to memory set size,
Klatzky (1970) observed that responses to items presented to the left visual field became
more rapid for picture test stimuli.
Bogen (1969a) has suggested not only that on verbal tasks the left hemisphere
actually inhibits its partner, but also that the converse situation should occur when
visuospatial information is presented. Smith, Chu and Edmondston (1977) have reported
some supportive evidence employing a haptic discrimination task, and indeed, Hanis and
Can (1981) have suggested that a right hemisphere superiority for spatial manipulation may
potentially be elicited in a greater range of designs using tactual discrimination. The
consistently observed left hand superiority in the discrimination of braille letters (c.f.,
Harris, 1980) may arise as a consequence of this right hemisphere advantage (Hermelin &
O'Connor, 1971). Lateralized differences have been obtained for the localization of touch
upon the fingers (Nachson & Cannon, 1975; Bakker & van der Kleij, 1978). Data patterns
for tactual recognition also suggest a right hemisphere emphasis (Dodds, 1978). Clinical
evidence appears to indicate almost unilateral preeminence for the right hemisphere for the
manipulation of tactual information (Boll, 1974; Cannon & Benton, 1967, 1971). This
imbalance has been posited by Le Doux, Wilson and Gazzaniga (1977) to be that whcih
underlies the right hemisphere superiority for the mapping of all exteroceptive body space.
With the advent of sophisticated modern techniques it has been possible to utilize
"non-behavioural" indices of asymmetrical functioning in normals. Analyses of EEG
records during cognitive tasks have indicated that reduced levels of right hemisphere alpha
rhythm activity (alpha activity reflecting resting brain states) are associated with the solution
of block design problems (Galin & Omstein, 1972).
More recently, examination of evoked potentials (EP) has confirmed the greater
involvement of the right hemisphere in visuospatial tasks (Galin & Ellis, 1975;
Papanicolaou, Schmidt, Moore & Eisenberg, 1983). Associatively, Davis and Wada
(1977) concluded that there exists a differential hemisphere involvement in the cortical
processing of temporally and spatially ordered information. Techniques permitting the
study of regional blood flow in the two hemispheres have revealed greater right hemisphere
blood flow during a test of perceptual closure (Risberg, Halsey, Wills & Wilson, 1975). 1 3
Similarly, fledgling investigations employing positron emission tomography scans (PET) have confirmed anticipated asymmetrical changes in brain metabolism during verbal and
visuospatial tasks (Mazziotta & Phelps, 1983). Although the investigation of spatial ability has a hoary lineage, and there have been
a corresponding multitude of tests of "spatial sense" (Harris, 1978), there has been no
satisfactory definition of what spatial ability should comprisemd there has been a
corresponding failure to find an external correlate of spatial complexity per se. That which
constitutes an "operational definition", in the rare instances in which one has been
expressed, varies considerably between paradigms and even between studies ostensibly
manipulating the same parameter. There has been an associated problem in manipulating the
possible levels of spatial complexity. It is more often the case that a single task labelled
"visuospatial" is simply contrasted with one labelled "verbal". Efforts however, to move
from a descriptive to an analytic account of the potential mechanisms subserving, what are
clearly tangible asymmetries, have been altogether more problematic.
CHAPTER 3
THE NATURE OF HEMISPHERIC SPECIALIZATION.
3.1 LOCAL AND GLOBAL MODELS
There is now consideiable evidence to support the view that the cerebral
hemispheres are functionally asymmetric. In predictable moves, characteristic of those
disciplines which presume to provide some rational distillation of the workings of "the
mind", there have followed a multitude of attempts to provide some unifying theoretical
framework, some metatheory for the whole of laterality (Allen, 1983). These attempts have
taken the form of global models, initiated with the express intent of accounting for
hemispheric asymmetries in general, and those local models which, although initially
descriptive and typically confining attention to a particular experimental paradigm, have
become elevated to the level of explanation, most notably Cohen's (1972) seriUparalle1
distinction. Many have the status of having become "deeply rooted traditions" which can
seemingly fashion some accommodation of almost all apparently contrary evidence
(Sergent, 1983).
In addition to Cohen's (1973) separation of serial processing for the left
hemisphere, parallel for the right, it has been proposed that the left cerebral cortex is
specialized for, analytic as opposed to holistic, global, synthetic or gestaltic apprehension
(Nebes, 1978), verbal versus visuospatial activities (Kimura, 1961), focal rather than
diffuse processing (Semmes, 1968), name matching in contrast to physical matching
(Geffen, Bradshaw & Nettleton, 1972), and in terms of the traditional verbdnonverbal
distinction (c.f., Bradshaw & Nettleton, 1981). This list is by no means exhaustive. Allen
(1983) further classifies local models into basic mechanisms of; unilateral specialization,
bilateralization, hemispheric interaction (positive and negative), hemispheric parallelism and
allocation, though there could be similar application to global models. The first category is
self explanatory, whilst bilateralization entails that both hemispheres have equal capacity for
a given process. Positive interaction models provide for the hemispheres to interact
positively for a given function, whilst the reverse is true of negative interaction
formulations. Hemispheric parallelism suggests that the hemispheres operate
simultaneously and independently which contrasts with allocation, wherein normally only
one hemisphere processes information relevant to a given task at any one time.
Few now maintain, at least publicly, a strong version of unilateralization which
would correspond most closely to an acceptance of the strictest dichotomies inherent in the
local and global models, similarly, the form of bilateralization, which in its strongest 1 5
expression is equivalent to equipotentiality, is apparently held by no-one. Allen (1983)
concedes that the other four mechanisms are embraced by bilateralization, further, they are
themselves not mutually exclusive. Although conceptually separable from unilateral
specialization, in view of present methodology, all mechanisms become practically
inseparable when the only, presently, tenable forms of unilateralization or bilateralization
are employed. In spite of Allen's (1983) claim that a general model can be formulated, his
postulation of distributed subprocessors performing functions analogous to the indefensible
"basic mechanisms" acts quite contrary to the principle of a unifying mechanism. It is clear
that basic mechanisms or subprocessors cannot characterize a local or global model per se,
rather the selection of a subprocessor or indeed basic mechanism is likely to depend on the
paradigm or manipulation it is one's wont to consider. Allen merely substitutes one
inappropriate level of analysis for another, however, in doing so he perhaps inadvertently
stumbles upon recognition of one of the fundamental limitations of laterality theorizing to
date, that "there has often been a rather strong implication-by-usage that tasks or functions
are unitary" (Allen, 1983 p.94). Although clearly prudent in emphasizing the need for "fine
grained analyses" of the subprocesses of what are, in reality, complex tasks and functions,
Allen is misguided in the belief that correlation of what appear to be stages of processing
with mythical subpmcessors, makes the latter any more worthy as a level of analysis than
that associated with the local and global models. Indeed the local and global themselves can
be shown to be of relatively little value on the basis of empirical evidence and upon a priori
grounds.
The most obvious limitation of the local and global models proposed thus far is that
they are represented as dichotomies. Whilst the proviso is conventionally attached, that a
strict dichotomy does not prevail (e.g., Bradshaw & Nettleton, 198 I), that the capacities of
each hemisphere lie at points on a continuum between two poles, and that differences are in
degree rather than of kind (e.g., Corballis, 1981; Milner, 1971; Zangwdl, 1960), the
boundaries are imposed by the limits of our conceptualization rather than by the nature of
the stratum they presume to describe. It is by no means evident that the brain "divides up its
functions into categories that correspond to our concepts or vocabulary" (Bullock, 1965).
Allen (1983) claims that there is some basis for reconciliation, that the various labels are merely reflections of a unitary underlying mechanism, however it seems that "cerebral
specialization is not likely to be less complex or more 'captured' by labelling one of its
attributes" (McKeever, 198 1 p. 74). Although it does seem reasonable that there exists a
common underlying mechanism, the conflict between schemes indicates that this has been
an inappropriate level of analysis.
Various reviewers, apparently oblivious of the limitations of dichotomies per se, in
what might be construed as a metanalysis of metatheories, have attempted to distill from the
global models some glorious unifying principle. As Cohen (1981 p. 67) comments "when
old dichotomies collapse under the weight of exceptions, psychologists seem to have an
irresistible urge to tidy up the field by reclassifying the data, and shoe homing it into a new
dichotomy". Clearly, it is these exceptions which are likely to prove more informative,
there would appear to be a good deal more worth in descriptive local models than some
contrived and inevitably inadequate grand theory. The point is not lost on McKeever (198 1
p.74), "the drive to neatly subsume all the essential aspects of hemispheric functioning
under some perfect dichotomy is an exercise in futility".
3.2 THE ANALYTIC - HOLISTIC DICHOTOMY
The approach favoured by Bradshaw and Nettleton (1981) is perhaps the epitome
of this "reclassifying urge". Selecting as a "straw-man" the verbaVnonverba1 dichotomy, in
a form so narrow that its acceptance by anyone would be in considerable doubt, these
authors run through a series of potential alternatives before settling on an analyticlholistic
dichotomy. Distinctions such as focddiffuse and seriaVparallel are apparently to be special
cases of the analyticholistic dichotomy, which in itself may be reduced to a left hemisphere
characterized by temporal order, and a right hemisphere by some form of spatial ordering
(Bradshaw & Nettleton, 1981). These alternatives however fare little better than the original
verbaVnonverba1 distinction.
Cohen's (1973) seriaVparalle1 dichotomy, which has been favoured by a number of
theorists of motor control (e.g., Todor & Dome, 1978; Nachson & Cannon, 1973), has
become established in the literature in a form which bears only a tenuous link to Cohen's
original formulation. In its received guise, it is held that the left hemisphere processes all
information in a serial fashion, whilst the right operates upon stimuli in a parallel manner.
Cohen (1973) was more circumspect, limiting the claim to alphanumeric stimuli, however
this "weak" version is in itself difficult to sustain, discrimination between serial and parallel
systems requires more complete and precise information than is conventionally obtained
from "psychological experimentation" (Townsend, 1972). Indeed even within the context
of the living body considered as an information processing system, attempts to draw
distinctions between serial and parallel processing in this manner must be seen as quite
misguided. This approach to cognition has recently received considerable criticism, Allport
(1980) highlights that a model encapsulating multiple entry of information at multiple points
is more appropriate than a serial foxmulation of successive bit entries at a single point. By
this perspective there are essentially few limitations on the amount of information which
can be received.
The extent to which even language production and comprehension may be
considered as serial processes has recently been reassessed (Poeck & Huber, 1977).
Certain aspects of speech and memory for verbal items cannot be accounted for by a
sequential mode of operation in the left hemisphere and the right hemisphere can itself
engage in sequential processing (Moscovitch, 1979). Tactual patterns and melodies appear
to be apprehended as some unitary form and demonstrate a right hemisphere superiority,
although initially they may be scanned sequentially (Cannon, 1981; Witelson, 1974). As
Bradshaw and Nettleton (198 1) themselves admit "parallel processing may still be a
consequence of an analysis of a configuration into its component elements, each element
still being processed concurrently with its partners, however, though not in a holistic global
manner" (p. 58). The basis for distinction has thus been removed, assignation of serial or
parallel characteristics to a given process can seemingly be made post hoc in an arbitrary
fashion.
It seems the analytic/holistic resolution must itself suffer the same fate.
Insurmountable problems both theoretical and practical appear to preclude any useful
application of the distinction. Not least of the objections is that "there is no theory about
what 'holistic' or 'global' processing might be" (Churchland, 1986 p. 199). Operational
definitions have been no more successful, the consequence is a constantly shifting sense of
what holism or analysis may be, not altogether unwelcome to theorists who are incapable
or unwilling to relate what may constitute a practical test of a favoured dichotomy. The arcs
and circles test favoured by Nebes (e.g., 1978) illustrates the point. Right hemisphere
superiority was observed in the matching, test leading Nebes to conclude that holistic
processing was involved, subjects were ostensibly displaying the ability to form a whole
circle gestalt from the component arcs. Had alleft hemisphere advantage been observed, it
is likely the explanation would have emphasized the analytic procedure of dividing circles
into constituent arcs (Marshall, 1981). It is evident that the analytic/holistic distinction is
not presently, and cannot be, sufficiently well defined to serve as the basis on which to
predict experimental outcomes.
3.3 SPATIAL VERSUS TEMPORAL?
One is left with that distinction to which all dichotomies purportedly reduce,
temporal processing in the left hemisphere, spatial processing in the right (Bradshaw &
Nettleton, 1981). This has at least found favour with some commentators (e.g., Corballis,
198 1; Nottebohm, 1979,198 1; Tallal, 198 1; Wyke, 198 I), though once again the 1 8
distinction is beset with problems which quite dissolve its utility. Attempts to dissociate
space and time have existed more in the minds of philosophers than in the world with
which we deal, for instance, it is the very essence of movement that it comprises series
which are both spatial and temporal. The fundamental irrationality of a dissociation of the
spatial Trom the temporal has not been lost upon modern theorists. Marshall (1981) points
out that having postulated that there exist distinct mechanisms for the manipulation of
spatial and temporal information, one immediately needs to speclfy some unifying device
which would integrate this information within the overspreading tempomspatial continuum,
whilst Sergent (1983) considers that "temporal and spatial dimensions are such basic
properties of any experience that it is hardly conceivable that each hemisphere would be
bereft of one of them" (p. 482).
Even if one acquiesces that it is not space and time as such which are handled
differently by each hemisphere, but rather perception or execution of the spatial or temporal
aspects of tasks, it is not clear that the increasing ambiguity is accompanied by any
pragmatic advance. The problem exists that it is impossible to dissociate the bulk of
"perception" in this manner. Perception would appear to have as its goal the resolution of a
stable, spatially ordered world from, for example, a temporally ordered series of glances
(Morgan, 1981).
Quite apart Trom the theoretical objections one must raise toward a spatiaVtempora1
dichotomy, the data is in itself sufficient to dispel apirations for this illusory distinction.
Music has been considered the epitome of temporality (e.g., Jankelevitch, 1977), yet the
right hemisphere superiority for the processing of such has been extensively catalogued
(e.g., Critchey & Henson, 1977). Singing also appears to proceed normally in patients
with left hemisphere lesions suffering fiom Broca's aphasia (Yamadori, Osumi, Masuhara
& Okubo, 1977), yet few would dispute that singing requires the mediation of duration,
sequencing and temporal order (Marshall, 1981). Additional material specific laterality has
been demonstrated in brain damaged individuals, verbal and nonverbal sequencing abilities
were differentially affected by the side which sustained damage (Kim, Royer, Bonstelle, &
Boller, 1980). The same double dissociation appears to be present in visuospatial and
verbal sequencing tasks (e.g., De Renzi & Nichelli, 1975; De Renzi, Faglioni & Previdi,
1977). There is clearly a spatial component to writing, yet this is a skill which, it is
assumed, is mediated by the left hemisphere (Hecaen & Albert, 1978; Peters, note 1).
Visually presented words and letters are comprised of spatial features, as are tactual
presentations, nevertheless there is overwhelming evidence demonstrating that recognition
of alphabetic material is accomplished primarily by the left hemisphere. Indeed stimuli
which are physically identical may be processed differentially by each hemisphere in a 1 9
context specific fashion . Significantly, examples of spatiotempd interactions have also
been observed (c.f., Barlow, 1979; Morgan, 1980).
The spatial/temporal distinction is thus as deficient as any other previously
proposed. It should come as little surprise that this is the case, rather what is more puzzling
is why we should anticipate that hemispheric asymmetries are reducible to a single
principle. There is some case to be made for retention of the distinction as some heuristic,
to be used with clearly defined terms of reference, though Churchland (1986) is less
magnanimous, "the hypothesis is really a metaphor in search of a reality to give it
substance, and it may be more misleading than helpful" (p. 200).
3.4 A RESTRAINED RESOLUTION
One must necessarily conclude that the present confused portrayal of hemispheric
asymmetry is a consequence not only of the impoverishment of experimental and
conceptual approaches but is additionally a reflection of the multitude of factors which are
at work (Bertelson, 1981). Sergent (1983) has proposed that some resolution of this
problem may be achieved through the rigid control of incoming information. In addition
having acknowledged the multifactorial nature of many tasks it is equally important that
attempts are made to sample the effects of task upon task subcomponents through the
concurrent use of multiple measures rather than, as has been conventional, a single criterion
of task performance. With the appropriate fine grained analysis (c.f., Allen, 1983), it may
be possible to tease out the effects of a number of factors upon expressions of asymmetry
at a number of levels within a given task.
As Hammond (1982) suggests, if specialization of the two hemispheres does exist,
it is not for psychological but for physiological functions. The brain must deal with "neural
information" not with abstract categories. What exists at present is a problem whereby
Western philosophical traditions have sanctified what, with respect to hemispheric
asymmetries, has been a less arduous approach, pseudoanalytic explanations in terms of
convenient symbols or labels. The matter is succinctly stated by Rorty (1979, p. 237),
"That is to say that if physiology were simpler and more obvious than it is, nobody would
have felt the need for psychology".
Sergent (1983) makes an attempt to bypass the pseudoanalytic level of the local and
global model through an examination of the relationship between hemispheric asymmetries
and neural correlates extracted from the sensory areas. Certainly there is a pressing
requirement that neuropsychological findings be integrated with "cognitive psychology".
As outlined, a detailed analysis of an individual's responses with respect to the
characteristics of both the sensory input and task objectives may prove to be particularly
revealing.
This should not be regarded as a return to some form of associationist approach
which denigrates the role of cognition. Rather, it is an acknowledgement that there is little
to be gained, in terms of elucidating the processes underlying motor control, by equating
higher levels of processing with the symbolism which is the cornerstone of cognitive
psychology. As far as its role in the explanation of asymmetries is concerned, whether
manual or otherwise, it has been demonstrated that the construction of local and global
models operating at the symbolic level, far from providing illumination, has proved to be
almost entirely regressive. There are reasons to believe that this level of analysis is totally
inappropriate.
Searle (1984) finds no use for the level of symbolic representation which forms a
central role in cognitive science. This is a middle level between the physiological and the
phenomenological which reflects tenets of a mentalistic approach derived from the Kantian
model. Searle (1984) contends that two levels of explanation are sufficient and indeed
justified, the level of intentionality in the form of a "plain English" description of
behaviour, and a neurophysiological explanation of this behaviour. Offering objections of a
similarly fundamental nature, Dreyfus (1979) argues forcibly that it is non-sensical to
consider a third level of explanation between the physical and the phenomenological as a
coherent level of discourse.
The apparent rewards to be obtained by locating a psychological level between the
neurophysiological and the purely phenomenological have apparently sanctified the
adoption of an explanatory level for which the relationship between the other two is
certainly ambiguous.
Local and global models and the dichotomies they encompass may potentially be
considered as aids to description but not as means or levels of analyses. There is thus some
legitimacy in suggesting that a task which appears to emphasise the sequential processing
of material at a given level of description is accompanied by a left hemisphere superiority
on a certain response measure, or that a task involving the manipulation of complex spatial
relationships is performed more effectively, with respect to a given criterion, by the left
hand. It is quite another matter to subsequently conclude that the left hemisphere is
specialized for the sequential processing of information and that the right is superior for the
processing of spatial relationships.
It is also important that some consideration be given to the selection of response
measures. As an illustrative example, it may be useful to consider performance on a
repetitive finger tapping task It is usually found that the right hand performs in a superior 2 1
fashion, relative to the left, with respect to the criterion used to assess performance,
specifically the frequency of tapping. It has generally been concluded that this reflects a left
hemisphere superiority for the sequential control required to make rapid postural transitions
(c. f., Peters, 1980). Suppose however that it is determined that the criterion employed to
assess quality of performance will be the accuracy with which the spatial relationships
inherent in the task are perceived. Consider, in addition, that there is established a means of
measuring the accuracy of the perception independently of the measure of tapping
frequency, if a right hemisphere superiority is now found on this measure, can it be
concluded that the right hemisphere is superior on a tapping task? At present it is simply the
case that the response measures frequently employed are of an insensitivity such that the
conclusions drawn merely indicate assumptions of global superiority for various tasks.
Until there is examination of the processing subcomponents and asymmetries
therein, there can be no approach upon an adequate description of task asymmetries in
general and manual asymmetries in particular. Laterality effects obviously exist at a
multitude of stages within any task structure, any account of motor control processes will
be incomplete without consideration of these asymmetries and the factors to which they are
sensitiveh similar fashion, one goal at this juncture should be the detailed delineation of
the factors which are likely to influence asymmetry of performance at a number of stages,
and in this manner aim to establish some convergence with the realm of physiology.
CHAPTER 4
HANDEDNESS AND CEREBRAL SPECIALIZATION
4.1 PREFERENCE AND PROFICIENCY
It hardly seems worthy of comment, that most individuals are willing to express a
clear preference between the hands with reference to the performance of simple, or indeed
quite sophisticated, motor tasks. Yet, what appears self evident has in fact been critically
examined by a number of authors, notably, Barnsley and Rabinovitch, 1970; Benton,
Meyers and Polder, (1962); Palmar, (1964); Todor and Doane, (1978) and Heuer, (1987),
who concur in concluding that the relationship of hand preference to observed proficiency
is, at best, "moderate". Indeed, as Annett (1970a) outlines, although preference is generally
consistent with the writing hand, there can exist considerable diversity on simple tasks
requiring manipulation of the fingers, such as threading a needle or dealing cards.
Whilst Doane and Todor (1978) consider as problematic the observation by Satz,
Achenbach and Fennel1 (1967) that manual proficiency appears less consistent when
individuals are classified on the basis of hand preference rather than proficiency, this
heterogeniety is not unanticipated. There is certainly little justification for considering the
distribution of proficiency to be any more unidimensional than that for preference.
Although it is certainly the case that the widespread use of handedness questionnaires has
led to considerable misclassification in terms of proficiency, it is not clear that the use of
proficiency measures to establish proficiency is an entirely satisfactory alternative. Rather,
a problem arises when any simplistic measures of either preference or proficiency are taken
as indices of underlying functional or structural organization.
The problems related to this approach have probably been most clearly
demonstrated with regard to "left handed" individuals. Heuer (1987) has criticized the
concept of "handedness" as it fails to distinguish between hand preference and hand
superiority. It appears possible to subdivide the population of left handed individuals into;
unilateral left handers, "ambidextrals" and "ambisinistrals" (Todor & Doane, 1978) on the
basis of a number of performance measures. Unilateral left handers are taken to display the
characteristic "strong" dominance associated with right handers, ambidextrals displaying
enhanced performance on both hands, whilst ambisinistrals perform poorly with both
hands. This has generally been taken to indicate the partial or incomplete lateralization of
function which has also been observed, for left handed populations, on a number of
measures of higher cognitive processing (Levy, 1969; Bryden, 1965) and on the basis of
clinical evidence (Luria, 1970). There has arisen, as a result, a perspective which views left 2 3
handers, not as some unitary group but rather, as individuals located upon a continuum of
lateralization. It is clear however that there has been less enthusiasm in applying this
construct to right handers, for whom output functions are subserved by, what appears,
essentially the same neurological substrate yet for whom the effects of incomplete
lateralization are taken as less clear, and for whom the consequences are interpreted as, or
masked by, reflections of variations in manual proficiency rather than as varying patterns of
cerebral organization. As Flowers (1975) points out, the tasks generally employed as
proficiency measures involve widely differing aspects of controlled movement, it is
certainly not the case that asymmetries will be expressed equivalently in all instances.
Annett (1985) concludes that an individual's choice of hand relies on more than
relative proficiencies, depending also on a decision criterion which is subject to influence
by, among other things, external social pressure, though it is to be expected that few
contemporary subjects were exposed in their "formative years" to measures such as the
strapping of the "sinistral" hand behind the back. Annett's (1985) formulation is derived
from a unitary concept of skill (Heuer, 1987), a position which is certainly untenable. As a
variety of tasks is considered, correspondences based upon the combination of decision
criteria and relative proficiency themselves become equivocal. There are certainly tasks on
which the nonpreferred hand may be "superior" to the preferred one (c.f., Todor & Smiley,
1985).
Proficiency is an observable, a "reflection on the surface of motor systems" to
paraphrase Marin (1976), which is in turn not directly related to any one other variable such
as task complexity (Steingruber, 1975; Todor & Doane, 1978) but is subject to the
influence of factors operating at a number of levels. Thus, in any attempt to relate hand
proficiency to asymmetries of structure or function one must be sensitive to the presence of
a range of possible mediating factors. As has been stressed previously, laterality effects
expressed in terms of output measures are indicative, not merely of some structural
asymmetry, but also, of the imposition of task specific constraints and functionally higher
levels of motor control, upon both the neural substrate and upon output mechanisms. It
would thus appear that there exists a need, not only to establish the relationship existing
between measures of hand preference and those of hand proficiency but, rather more
importantly, to establish the mechanisms and structures which subserve each.
It appears to be common practice, in the literature, to directly attribute hand
proficiency or superiority on various tasks to the specialization of the contralateral cerebral
hemisphere (Heuer, 1987). Certainly, hemispheric specialization is likely to be a major
determinant of manual asymmetries, however, there are a number of considerations which
ought not to be overlooked. In the first instance, and as has been highlighted in the 2 4
previous section, the assumption of unitary task structures is unwarranted. The use of a
small number of dependent measures assigned by an experimenter can merely touch upon
the subtleties within the task gestalt and certainly may often exclude what the subject
perceives to be his "action goals". Secondly, it is necessary to consider the imposition of
functionally higher levels of motor control which themselves may or may not be
represented asymmetrically (Heuer, 1987). The following section will deal with a third
potential reason for caution, the relationship of the output of the two cerebral hemispheres
to the motorneuron pools located in the spinal cord. The situation is summarized by Heuer
(1987, p278) "the relationship between hand superiority and hemispheric specialization
poses a problem of enquiry". As there is no simple way to infer hemispheric specialization
from the performance of the contralateral hand, the reverse also holds true
4.2 THE CONTRIBUTION OF DESCENDING PATHWAYS
There is considerable anatomical evidence to suggest that there exist
"corticomotoneurona1" connections directly to the distal musculature of the hands,
pathways which are implicated in the independent movement of, in particular, the fingers
(Brinkman & Kuypers, 1973; Kuypers, 1978; Lawrence & Kuypers, 1968a; Lawrence &
Hopkins, 1976). These are pathways which may be distinguished from other parts of the
Cortico Spinal Tract, in having only monosynaptic connections, though it should be noted
that those pathways comprised of disynaptic and trisynaptic connections will not "cross" when otherwise "uncrossed", as a consequence of synapsing within the spinal cord. With
reference to the control of more proximal muscles, less direct "uncrossed" pathways may
also be involved (Gazzaniga, Bogen & Sperry, 1967; Zaidel & Sperry, 1977). Theorists
have, as a result, postulated that hand differences, arising from contralateral cerebral
specialization, may not exist for movements predominantly involving the proximal
musculature (Todor, Kyprie & Price, 1982), whilst many other studies of hand differences
rely upon the assumption that control of the hands is accomplished by the contralateral
cerebral hemisphere (Todor & Smiley, 1985). There have been recent demonstrations
(e.g., Kuypers, 1984) that considerable control may be exerted even without recourse to
corticomotoneuronal pathways, it will prove useful to consider the routes by which this
may occur.
Of the four descending pathways have been outlined by Kuypers (1981), the most
functionally substantial of which is the corticospinal tract. The majority of these fibres
cross to the contralateral side within the medulla, though a small number are thought to
cross below the brain stem, crossing at segmental levels via interneurons. Whilst
corticospinal fibres are responsible for the innervation of distal musculature, it also appears
that proximal muscles may be similarly controlled (Kuypers, 198 1). The notion of simple
and direct contralateral control is further eroded when one considers that the majority of
these pathways, with the possible exception of those innervating some finger musculature,
comprise many interneurons and collaterals, even for many finger movements, one cannot
assume that all pathways are crossed. It is, however, the functioning of the direct pathways
which apparently cannot be undertaken by other fibre tracts, lesions of the corticospinal
tract in neonates prevents the development of differentiated finger movements (Todor &
Smiley, 1985). Corticospinal innervation is also extended to proximal musculature, the
effects of which become most evident when task requirements emphasise the need for
contralateral control (Di Stefano, Morelli, Marzi & Berlucchi (1980); Todor, Kyprie &
Price, 1982).
The rubrospinal tract, originating in the red nucleus, which is itself located in a
region of the midbrain, provides a second crossed pathway which appears to be associated
with the control of hand movements. The maintainence of capacity for control of the hands
which survives following disruption of the corticospinal tract, is curtailed when the
rubrospinal tract is sectioned (Todor & Smiley, 1985). However, with regard to the
rubrospinal tract, it is at present unclear as to the nature of the connections above the level
at which this pathway appears to originate. The nature of the connections are such that a
pathway that appears crossed may in reality be uncrossed
A third potential means of control, for the proximal and axial muscles at least, is
provided by the reticulospinal and vestibulospinal pathways. These proceed ipsilaterally
within the spinal column, however the bilateral projections from this tract appear to
contribute little to the fine control of the hands (Todor & Smiley, 1985).
In addition, there appear to exist direct brainstem pathways which alter the
exciteability of the motoneuron pools. Kuypers (198 1, 1984) has identified these tracts as
some of those descending from the raphe nucleus. Whilst these pathways directly synapse
with the motoneurons associated with the distal musculature, they also possess collaterals
which project multisegmentally (Todor & Smiley, 1985).
One must acknowledge then that many movements may be primarily but not
exclusively under contralateral control. Although the corticospinal tract is largely comprised
of crossed fibres, it is an open question as to how the excitation of the motoneuron pools
innervated by this and other descending pathways is affected by functionally higher levels
of control. One need only note the effectiveness of the Jendrassik procedure in which the
exhibition of spinal reflexes is enhanced when an individual attempts to pull apart his
tightly clasped hands. It seems quite plausible that excitation of this kind may proceed
asymmetrically. Trevarthen (1984) has speculated, on the basis of research conducted by 2 6
Grillner and Zangger (1979) demonstrating that the spinal locomotor system may consist of
distinct left and right "generators", that a slower system in the left side of the human spinal
cord may account for some portion of exhibited hand differences. Certainly, one ought not
to dismiss the possibility that asymmetries exist below the cortical level.
4.3 DEFICIENCIES IN THk "STRUCTURAL" APPROACH
The impoverishment of any approach which posits that between hand differences in
"proficiency" are primarily due to a structured contralateral connection with a specialized
cerebral hemisphere has been illustrated by series of studies demonstrating that advantages
in terms of choice time are due to the effects of spatial compatibility rather than elementary
anatomical connectivity.
On the basis of "classical" research (e.g., Proffenberger, 1912), it had for some
time been considered that simple reaction times to "unstructured" lateralized stimuli were
determined largely by structural constraints, and therefore, that ipsilateral responses, for
example stimuli to the left hemiretinae requiring responses with the left hand, would be
concluded more rapidly than contralateral responses. This, in turn, relies upon the
assumption that ipsilateral responses can be mediated by intrahemispheric hemispheric
processes. Indeed, the latency difference between ipsilateral and contralateral responses has
been viewed as an estimate of interhemispheric transmission time (see Bashore, 198 1 for a
review).
Contemporary research has consistently demonstrated that the time taken to respond
in a choice situation can be potently affected by altering the relationship between the
location of the stimulus and the spatial position of the hand making the response (e.g.,
Simon, Hinrichs, & Craft, 1970; Wallace, 1971; Brebner, Shephard, & Cairney, 1972).
Compatibility effects do however appear to depend on other task constraints, Anzola,
Bertoloni, Buchtel & Rizzolatti (1977) confirmed the superiority of the ipsilateral response
for simple reaction time, yet demonstrated that when subjects were required to decide
which hand to employ depending on the spatial position of the stimulus, the faster response
was now accomplished by the hand which was in the same visual space as the stimulus,
regardless of whether the anatomical connections were contralateral or ipsilateral. The
required experimental manipulation was achieved by having the hands of the responding
individual "crossed" or "uncrossed" in front of the body. Anzola et al note that the extent to
which the effects of spatial compatibility may be expressed will be dependent upon the
"information content" of the stimulus, concluding that "when the task does not require the
necessity of making a decision, the directness of the anatomical connectivity between the
receiving hemiretinae and the responding hand prevails" (Anzola et al, 1977, p301). There
are clearly few instances in which an individual will proceed with a "motor act" in the
absence of "decision making" of any kind.
These findings have been extended by, among others, Cotton, Tzeng and Hardyck
(1980) and by Ladavas (1987) who demonstrated that spatial compatibility effects occur in
the absence of any overt correspondence between the spatial attributes of a stimulus and the
required response. Both investigations revealed not only that right hand responses are made
more rapidly to right hemifield presentations and vice versa, but also that the right hand
responds faster to "right up" and "left down" positions, and the left hand responds faster to
"left up" and "right down" positions, when stimulus lights appeared not only to the right or
left of, but also above and below fixation.
The diminishing importance of elementary anatomical connectivity between central
and peripheral regions as task constraints are imposed is paralleled by indications that
anatomical factors at the cortical level become less significant as "demands" are increased
Bashore (1981) has reviewed evidence which suggests that estimates of interhemispheric
transmission times 0 derived from sim~le reaction time tasks are well correlated with
electrophysiological measures. These measures involve the subtraction method
(Poffenberger, 1912) and assume that "transmission of simple sensory information and the
initiation of uncomplicated movements are mediated over fixed and reasonably well isolated
neuroanatomical pathways" (Bashore, 1981, p. 353). It appears however, that as task
complexity increases along a number of dimensions and moves away from simple reaction
time, estimates of M?T are considerably elevated Presumably, the "fixed"
neuroanatomical pathways are unchanged, yet the alterations in the cerebral activation
responsible for these increases remain unclear. They do appear to arise as a consequence of
the type of movement required for as a response (e.g., Krsiteva, Keller, Deecke &
Kornhuber, 1979). Investigations conducted by Di Stefano and co-workers @i Stefano et
al, 1980) produced indications of response hand advantages for abductive lever pulls but
not for key press responses in what was otherwise a simple reaction time study. Estimates
of M?T are also considerably altered by, the required complexity of decision making,
where the more complex decisions are presumably engaging more extensive "cognitive"
processes, and by structural characteristics of the stimulus itself (c.f., Sergent, 1983). In
concluding his review, Bashore (1981, p. 366) states, "intuitively, one is led to
hypothesize that intrahemispheric processing and interhemispheric communication are
integral functions of stimulus input, information processing requirements, and motor
output". As will become apparent, that which appears to critically modify IE-I'IT can be seen
to strongly influence manifestations of hemispheric asymmetry in general. In the sections
which follow, the relationships between these factors will be more closely examined. 2 8
CHAPTER 5
AN "INDIRECT APPROACH
5.1 THE SIGNIFICANCE OF THE STIMULUS INPUT By way of introduction to her synopsis of visual asymmetries, Sergent (1983)
outlines her grievance that, of the factors which might potentially influence the expression
of laterality, implicitly Bashore's three factors, the characteristics of the input have been
afforded only the most superficial consideration,
neuropsychologists emphasized the processing characteristics of the tasks required from subjects, and did not consider the characteristics of the stimulus as a major causative agent, but only as the instigator of behavior (Sergent, 1983, p. 482).
The author highlights the dependence of the developing brain upon sensory stimulation and
considers that, in terms of elucidating the relative competencies of the cerebral
hemispheres, one must, as a logical first step, examine the manner in which the quality of
the input varies as a function of the attributes of the entire visual system (Sergent, 1983).
This might be seen as constituting a description or explanation at the physical level and
should perhaps be viewed as complimentary to the approach of direct perception (Gibson,
1966) or direct action (e.g., Turvey & Carello, 1986). It is Sergent's view that only
through systematic control of the incoming information may asymmetries be assessed. For
incoming information one should perhaps read stimulus energy as it is this which Sergent
(1983) considers to be the determinant of the characteristics of incoming information. One
can see how this may be directly related to some quite fundamental tenets of the "ecological
approach", in particular, that the lawfully based flow fields of light, sound and touch
associated with an object give rise to a unified impression of that object, even though these
flow fields are considered to be different stimuli and of different energies (Turvey &
Carello, 1986). These flow fields are, however, characteristic of that object at that instant.
A different object will have associated with it different flow fields and, in each modality,
different "stimulus energies" thus giving rise to a different impression. Although there is
clearly a perceptual constancy in that, whilst the stimulus energies associated with an object
may change over time for example as a result of movement, the identity of that object is \
viewed as invariant. This does not preclude, of course, that more generally what are at any
instant different stimulus energies are associated with many objects and therefore give rise
to impressions of many objects. It seems to follow that ones impression, ones "perception"
is critically determined by the stimulus energy of that which is being perceived. Husserl
would have held that the constitution of space and objects begins with the "visual field", the
"visual sensations available when the eyes, head and body are at rest" (Scheerer, 1986, p.
164). The visual field identified by Husserl is essentially that envisaged by the Gestalt
psychologists or the "Gibsonians" (Scheerer, 1986), thus, if one proceeds with this line of
reasoning, it follows that the stimulus energies reaching even the static eye will have
consequences in terms of the received impression of space. Similarly, variations in
stimulus energies, other than those associated for example with a single object over time,
will lead to variations in the received impression of space. More importantly in this context,
asymmetrical reception, conversion and transmission of this energy by the nervous system
will result in an impression of space which, when assessed through the use of behavioural
measures, will itself appear asymmetrical. This follows in a manner which does not
compromise the necessity of employing two levels of discourse, physical and
phenomenological, as outlined by Dreyfus (1979) and Searle (1984). As such, the work of
Sergent (1983) may assume a broader significance, though perhaps not in the manner that
author had intended.
Clearly, there are practical, not to mention philosophical, difficulties in dissociating,
what in the language of cognitive science would be, cognitive and sensory operations,
however some attempts have been made to consider independently the asymmetries which
may arise at these pseudo stages. Sergent (1983) is probably misguided in her selection of
the point of demarcation of sensory from cognitive, nonetheless as an aid to exposition, it
will prove useful to adopt her heuristic that the "sensory processing" of the relevant stimuli
has been "neurophysiologically localized" in the visual sensory areas of the left and right
occipital cortex. Considering asymmetries in stereopsis, the perception of colours, and the
duration of iconic storage, Sergent (1983) dismisses elicitations of what are apparently
laterality effects, by concluding that these investigations may be sampling "elementary
cognitive visuospatial operations". Whilst questioning the possible ecological utility of an
asymmetrical sensitivity to the visual world, Sergent (1983) does acknowledge that further
research is required.
What is more puzzling is that which Sergent (1983) ascribes to be "hemispheric
processing beyond the sensory level" (p 492), or cognitive processing. The problems of
this approach are quite evident when one considers that included in the subcategory of
'state limiting variables' are those which are affected by variations in exposure duration,
luminance, stimulus size and retinal eccentricity or the introduction of visual masking.
Whilst the means of analysis may be dismissed, more interesting are the observations that
the direction and magnitude of observed asymmetries may be directly influenced by
duration and luminance and thus by stimulus energy (e.g., Sergent, 1982, c,d, & f). The 3 0
most important conclusion one may draw is that articulated by Sergent (1983, p 493, that
"the two hemispheres are not equally affected by variations in stimulus energy", though by
noting that it is the resulting 'visual information' upon which cognitive operations are
performed, the original sensorylcognitive distinction seems to have been quite dissolved.
Evidence has also been presented which appears to suggest that the effects of retinal
eccentricity and stimulus size are similarly asymmetrical (e.g., Polich, 1978; Sergent, 1982
b;c). It is not necessary to consider in detail what Sergent (1983) terms "process limiting
variables", suffice to say, the influence of what are apparently mediating cognitive
"operations" are treated elsewhere. There can however can be no objection to Sergent's
closing comments that "one cannot make unequivocal predictions about the outcome by
considering only the quality and characteristics of the input" (1983, p 506), a point which
has indeed been stressed throughout this review. Having given some consideration to
stimulus input, the following section will deal with the influence of what Bashore (1981)
would view as the "information processing requirements of a task
5.2 THE DUAL TASK PARADIGM
The practical manipulation of the information processing requirements has generally
involved tasks in which "cognitive operations" of various kinds are performed concurrently
with some kind of manual activity. Designed initially as a means with which to investigate
functional lateralization in a broader sense, the paradigm has been extensively employed to
examine the relationship between cerebral specialization and the proficiency of the preferred
and non-preferred hands. I Associated with the use of the Dual Task Paradigm are a number of assumptions,
both explicit and implicit (Todor & Smiley, 1985). Following Kinsbourne and Cook
(1970), interference or facilitation (though more usually the former) will occur if the two
tasks ("cognitive" and "motor") which are usually associated with the differential activity of
one hemisphere, are performed simultaneously. The rationale was extrapolated by
Kinsbourne and Hicks (1978) who assumed that the extent of this effect was determined by
the "functional overlap" within a given "controlling" hemisphere. It has also been
maintained that, for a given "dependent" task, (conventionally the manual task) the
interference resultant upon the concurrent (usually cognitive) activity may be unilateral,
bilateral of asymmetrically bilateral depending on the specific processing requirements of
that activity (Allen, 1983; Friedman & Polsen, 198 1; Hellige & Longstreth, 198 1). Clearly
the motor task is not bereft of cognitive components, and similarly, all cognitive operations
require for their expression some form of motor output and thus control, whether this is a
pointing response or overt vocalization. Indeed Lomas (1980) in work (to be discussed 3 1
shortly) which was controversial but insightful, drew attention to the need for consideration
of the intrahemispheric interference which may occur, and in particular the specificity of
this interference with regard to the level, within some stimulus to response continuum, at
which this interference may occur. In a similar vein, Todor and Smiley (1985) highlight
that not only is the assumption of exclusively contralateral control via crossed anatomical
pathways not supported, but also that movement related activity in the brain stem and
higher centres may proceed asymmetrically. Their qualifications are worth quoting at
length,
it should be noted that preceding a unirnanual movement using crossed corticospinal pathways, movement planning may have occmed bilaterally. This movement planning would have involved cortical areas such as the supplemental motor areas, frontal association areas, premotor area and subcomcal structures such as the basal ganglia and the cerebellum. Although the motor outflow may be more or less restricted to the motor area of a given hemisphere, it may reflect the converging influence of other intra and inter-hemispheric structures (p. 320-321).
In view of the multitude of factors to which the expression of laterality is likely to
be sensitive, it is perhaps of little surprise that use of the dual task paradigm has produced
"a rather complex pattern of results" (Bryden, 1982). In what might be regarded as the
prototypical experiment in what is now an extended collection of studies, Kinsbourne and
Cook (1971) required that subjects attempted to balance a dowel rod on the index finger of
their right or left hand. A control condition indicated that this task was performed in
superior fashion by the right nand. When a single interfering task was introduced, that of
speaking a sentence, a decrease in dowel balancing proficiency was noted for only the right
hand. The conclusion was drawn that the drop in right hand performance occurred as a
result of competition for "resources" within the left hemisphere, between speaking and the
manual task.
Attempts to extend the findings of unilateral interference to a variety of manual tasks
have produced results which are a good deal less clear. Lomas and Kimura (1976), in the
first of a series of three experiments replicated, at least for male subjects, the finding of
Kinsbourne and Cook (1971) in that concurrent speech was again found to disrupt right
hand dowel balancing. Humming was also used, as a task which was ostensibly mediated
by the right hemisphere, concurrent performance, on this occasion resulted in a depression
of dowel balancing proficiency for both hands. Considering the dowel balancing task to be
an inappropriate means of assessing the "motor generator", Lomas and Kimura (1976)
essentially replicated their initial experiment, altering only the nature of the motor task by
substituting a finger sequencing task for dowel balancing. Once again speaking decreased
preferred hand performance in right handers, though humming apparently exerted no 3 2
effects. The final experiment of the series used sequential telegraph key strikes in place of
the sequencing task. Concurrent speech now resulted in a bilateral decrement in the rate of
single finger tapping and a unilateral depression of right sided performance for sequential
whole arm tapping. The latter result seems to run counter to intuition, particularly in light of
the presumed presence of direct contralateral connections for only the most distal
musculature. Subsequent findings of bilateral interference have not been untypical,
concurrent cognitive activity which emphasizes verbal "abilities" has generally resulted in
depressed right hand performance (e.g., Bowers, Heilman, Satz & Altman, 1978; Hicks,
Provenzani & Rubstein, 1975; Thomton & Peters, 1982) whilst tasks apparently requiring
right hemisphere mediation have, on occasions, produced greater decrements for the left
hand (e.g., Benton, 1979; Hellige & Longstreth, 198 1). In other instances equivalent
decreases in proficiency have been observed for both hands (e.g., Summers & Sharp,
1979). This pattern of bilateral interference further challenges the view that movements are
subject to exclusively contralateral control (Todor & Smiley, 1985).
In an attempt to explain the manner in which the extent of right hand interference
accompanying speech is dictated by specific task demands, Kimura (1979) and Lomas and
Kimura (1976) have postulated that it is interference between seauential movements and the
production of speech which is responsible for the depression of right hand proficiency.
From this perspective, the unilateral interference observed in the second experiment of the
Lomas and Kimura (1976) study is seen as arising because the finger sequencing task did
indeed involve sequential movements whilst the tapping experiments of the final study did
not. This assumption is made explicit, "the term 'sequence of movement' implies that the
task does not consist merely in the repetition of the same discrete movement over and over,
as in single finger tapping" (Lomas & Kirnura, 1976, p. 31). The explanation seems
somewhat ad hoc, one would have difficulty envisaging any movement which is not in
some sense sequential. McFarland and Ashton (1978a), using six verbal and six non-verbal
tasks and a manual response which required successive pressing of two keys, observed a
pattern of results in many ways similar to those of Lomas and Kimura. Verbal tasks
resulted in unilateral right hand disruption, whilst in the non-verbal conditions, a bilateral
disruption ensued. By way of explanation for what have been less consistent effects,
namely that primarily visuospatial right hemisphere tasks result in unilateral left hand
deterioration, McFarland and Ashton (1978b) argue that
the neural structures of the right hemisphere, which mediate the visuospatial cognitive task, may overlap only those right-hemisphere structures which mediate the spatial components of left hand performance but not those structures which mediate the spatial components of right hand performance (P. 344).
3 3
It is important to note that with regard to the McFarland and Ashton formulation and
that of Lomas and Kimura (1976) which, in analogous fashion suggests that degree of
overlap may dictate the level of left hemisphere disruption, the distinction is functional
rather than anatomical or physiological. This is in spite of the liberal use of the term
"structure". The use of an explanatory construct such as "functional unit" has its obvious
limitations, by relegating the need for a physiological correlate, it is difficult to establish a
priori which tasks will interfere (Bryden, 1982). By adopting this perspective, one appears
to be quite at liberty to conclude that, since two tasks have interfered, there must have been
some overlap of functional space.
Whilst continuing to adhere to the view that the extent of interference is dependent
upon the degree of functional similarity, Lomas (1980) has proposed that consideration of
more specific functional overlap must be entertained. In Lomas's view, there has been a
general inability to demonstrate unilateral right hand interference with concurrent tasks
other than those associated with motor production of speech, included in this category are
both non-visual and visually guided motor tasks. It is further presumed that there has been
no appropriate demonstration of left hand deterioration in accordance with right hemisphere
interference. In an attempt to provide support for these assertions, Lomas (1980) employed
conditions in which individuals either could or could not have recourse to visual guidance
of axm and finger tapping, and obtained indications that concurrent verbal tasks interfered
only with motor tasks which did not utilize visual control. More contentious has been the
suggestion that it is not sequencing per se which is under left hemisphere control, subjects
were in each condition required to enact a sequence, yet interference was only evidenced
when the movement was not visually directed (Lomas, 1980). Rather, it was suggested that
the left hemisphere has, within its functional domain, a set of processes specifically
concerned with the control of movement transitions in the virtual absence of visual
information, this is the system which is in turn assumed to be implicated in speech
production (Lomas, 1980). Although this attempt at specificity is in itself admirable, the
generalizations made with respect to non-supportive literature are clearly too broad. In
addition, there is recent evidence that non-vocalized verbal tasks do have a disrupting effect
upon right hand performance (Ikeda, 1987). Although there exist interpretive problems, as outlined above, there have been
frequent indications that verbal tasks may unilaterally interfere with concurrent motor
performance and, admittedly less pervasive, data suggesting that the left hand may be
similarly disrupted by activity of an appropriate nature. In remaining consistent with what
has been a recurring theme, it is only likely to be through the subtle manipulation of
procedural variables, in conjunction with the use of behavioural measures of appropriate 3 4
resolution and sensitivity, that the mechanisms of interest will be elucidated The rate of
rapid finger tapping is one measure which has been extensively used as both an index of
neurological damage (e.g., Kimura, 1977) and as a central dependent measure in a large
number of experimental investigations. There are certainly a variety of measures which may
be utilized in addition to the global appraisal of tapping frequency. It has been pointed out
(Peters, 1980; Todor & K ~ P A ~ , 1980) that the finger tapping task involves, in addition to
sensitively timed "posture transitions", sequencing between and within "motor acts" (c.f.,
Lomas & Kimura, 1976).
The work of Brodie (1984) represents at least one attempt to provide a more fine
grained analysis of the tapping task substructure. The dependent measures used to assess
tapping performance were; Inter Tap Interval, which was the time between successive key
closures and thus a measure of the frequency of tapping, Dwell, the duration of key
closure, Slack, which is assessed as the period between successive key depressions (ITI - dwell time) and the Maximum Force exerted during key depression. Rather than being
required to tap as rapidly as possible, subjects were instructed to maintain one of a number
of set tapping frequencies. Individuals were initially trained to tap in time with an auditory
tone, in an attempt to equalize the performance of both hands prior to testing, a requirement
Bryden (1982) identifies as essential for effective use of the paradigm. In the test situation,
tapping was evaluated after the tone had been removed, concurrently with four tasks and in
a control condition in which tapping proceeded alone. The latter condition is only one of
two controls Bryden (1982) feels are necessary precursors to analysis, the other being an
evaluation of performance on the "interfering" task, p t h when performed alone and in the
dual task setting. This may provide some means of telling whether decrements arise purely
as a consequence of performing two tasks simultaneously (capacity interference) or due, as
is generally assumed, to structural interference. The absence of these controls in a large
proportion of the early dual task studies, along with the absence of a training or
"equalization" period, may have contributed considerably to the ambiguous nature of many
findings. The four concurrent tasks utilized by Brodie (1984) were; Read, reading of a short paragraph upon which subjects were later tested for retention, Geo, pre and post
tapping matching of geometric figures, m, a rapid five finger sequencing task and &, involving slow single finger flexion. Somewhat unexpectedly, neither of the "cognitive"
tasks, Geo or Read, had any appreciable effect upon tapping performance. That this may
have arisen as a consequence of training is not without significance with regard to the
interpretation of many early studies. Concurrent performance on both motor tasks resulted
in appreciable interference of left and right hand tapping, to which the dwell component
was apparently most sensitive. The pattern of asymmetrical interference was complex and 3 5
somewhat inconsistent, however, the data may be summarized as follows: performance of
the Prax task, with its emphasis on sequencing, seemingly required the involvement of the
left hemisphere when performed with either hand, however the interference was greatest
when tapping was performed by the right hand, as left hemisphere involvement was again
directly required. Conversely, the Flex test, somewhat tentatively, assumed to involve a
greater "spatial" component &d thus greater right hemisphere involvement, produced
interference which was "reversed" relative to that created by the Prax test. There exists
however an interpretive problem, as both tasks involve both hemispheres to some extent.
In line with the initial assumptions of the Dual Task Paradigm, a greater required
involvement of both hemispheres results in a greater potentiation of interference. Therefore,
in any comparison between two tasks, though both manual, where one task is assumed to
be primarily right hemisphere mediated, the other primarily left hemisphere mediated, the
extent to which interference is bilaterally expressed then depends upon the relative
involvement of the non-primary hemisphere in each task. The extent of this involvement is
obviously difficult to determine in advance, leading to problems equivalent to those which
beset explanations in terms of "functional space". Again there is no a priori method of
determining the level of involvement of the "secondary" hemisphere on the interfering task.
If there is bilateral interference it may be the case that the "secondary" hemisphere is highly
involved, alternatively there may be no "structural" interference between the dependent and
interfering task, the bilateral decrement may result rather from demands made upon an
overall "finite processing capacity" (c.f., Kahneman, 1973). It is also likely to be the case
that the relative involvement of each hemisphere is dictated, in part, by the overall task
structure, for example the Flex task may be primarily mediated by right hemisphere control
when performed in isolation, but is subject to increased left hemisphere involvement in a
dual task context (Anzola, Pulirneno & Rizzolatti, 1980).
The relative involvement of the "primary" and "secondary" hemispheres is a
function not only of the task context and the task itself but also of the level of practice on
the dependent task. Increasing the level of practice prior to testing reduces the potential of
finding interference effects (Bahrick & Shelley , 1958; Rodney, 1980). Practice trials on a
finger sequencing task,a day prior to testing, have been shown to obliterate the effects of
supposedly interfering tasks in a replication of the Lomas and Kimura (1976) study
(Rodney, 1980). As Bryden (1982) points out, it is important that the left hand be afforded
the opportunity to reach the same level of performance as the right hand, as a result any
effects of right hemisphere interference will be more faithfully exhibited.
As has been mentioned, bilateral interference may arise as consequence of
hemispheric interaction on the interfering task itself or as a result of overall capacity 3 6
interference created when two "difficult" tasks are performed simultaneously. Such is the
contention of Summers and Sharp (1979) who revealed disturbances on motor sequencing
which were equivalent for both verbal and spatial interfering tasks, despite indications that
these tasks involve opposite hemispheres. An adequate test of structural versus capacity
interference requires that there exist a means of manipulating the difficulty or capacity
demands of a task which is assumed to provide structural interference. There is often no
straightforward means of obtaining a metric for assessing task difficulty (Bryden, 1982).
One is again left without a means of specifying, in advance, relative capacity demands.
Rodney (1980) motivated it seems by pragmaticsm, employed two levels of difficulty for a
vocal (structural interference) task which were distinguished by the number of words his
subjects were required to repeat, one or six. The second structural interference task
required motor sequencing, the levels of difficulty in this instance were represented by one
or four finger sequencing. The dependent task was also a manual sequencing task It was
hypothesized that if structural interference was primarily responsible for depression of
performance, the motor interfering task would have a greater detrimental upon the
dependent task than the vocal interfering task. Alternatively, if capacity interference is
implicated, the more difficult tasks should produce the highest level of interference
regardless of the apparent potential for structural competition. Rodney (1980) observed
significant interference effects arising from both structural and capacity factors.
The work of Rodney (1980) thus provides no resolution of this issue, indeed one
should not presume that any delineation should be made. Once again attempts at
classification are precluded by obvious contextual dependencies. Similarly, the dual task
paradigm can provide little that explicates the functional nature of the lateralized system, at
least with respect to motor control (Bryden 1982). Bryden is probably misguided in his
assumption that asymmetries are not expressed in "one shot movements", indeed it is difficult to visualize the constitution of such a movement. As Bryden himself realizes
"movements in a sequence might be considered as a structured series of smaller motor
elements or features" (1982, p. 120). Every movement proceeds in a manner which at some
level can be described as sequential.
Albeit expressed in the unfortunate lingua kanca of current motor control, Bryden's
reflections are insightful. Posing the question of, what aspects of a movement series
exhibits left hemisphere "control", The conclusion is drawn that : the left hemisphere might be crucial for selecting and stringing the motor features together: The left hemisphere could be seen as a master programmer. Alternatively, producing a series of movements also requires on-line monitoring at some level of the current position of the limb ... The left hemispheric system, then, might be responsible for this on line feedback monitoring (Bryden, 1982, p. 120).
3 7
Bryden concludes that the resolution of this issue lies ahead, it is the case however that
there is no issue as such, if resolution is required it is of the manner in which these
potential processes coexist and transact. What seems certain however is that the dual task
paradigm can provide few of the requisite tools. Clearly explanations in terms of the
assignation of processing "space" for somewhat arbitrarily selected concurrent tasks in
which there is competition for capacity, can reveal little of the situation which pertains for
normal goal directed actions which, although potentially, comprising the same task
subcomponents, may impose few of the same artificial constraints.
5.3 APRAXIA AND RELATED DISORDERS
Some elementary support for the view that left hemisphere adopts the role of "motor
programmer" is provided by clinical evidence arising from experimental manipulations and
from case studies of apraxic conditions. The need for considerable caution in interpreting
these studies arises independently of the general problem of extrapolating from clinical
cases which was highlighted in section 2.1. Although there seems to exist a strong
correlation between indications of perceptual asymmetries derived from "normal" and
clinical studies, this correspondence is notably absent when comparisons are made with
regard to motor performance (Bryden 1982). It is unlikely that the two sources of evidence
are irreconcilable, nevertheless, when generalizing on the basis of clinical studies, some
restraint is clearly prudent.
The clinical l i teram does at least have the virtue of providing further confirmation
that asymmetries in motor performance are not solely accountable to anatomical
considerations. A series of studies, initiated by Wyke, demonstrated that the effects of
unilateral brain damage were not symmetrical with regard to a variety of motor tasks (e.g.,
Wyke, 1966, 1967, 1968, 1969). Whilst, in these studies, the preponderance of crossed
anatomical pathways appeared to account well for the effects of right hemisphere damage,
producing deterioration of left hand performance, equivalent disruption of the left
hemisphere resulted in bilateral decrements. This work provided the impetus for a further
series of studies conducted by, in particular, Kimura (e.g., Kimura & Archibald, 1974;
Kimura, 1977; Mateer & Kimura, 1977). It appeared from these experiments that though
patients with either left or right hemisphere damage had few problems in producing single
hand postures, those with left hemisphere trauma experienced considerable difficulty with
the production of sequences of those postures (Kimura & Archibald, 1974). Bilateral
depression of the ability to perform motor sequences, in association with left hemisphere
damage was also revealed by Kimura (1977).
It has also been demonstrated that this disruption applies not only to the brachial but
also to the oral musculature (Mateer & Kimura, 1977), leading the latter author to suggest
that the left hemisphere controls "sequences of movements" and that this side of the brain
contains "a system for accurate internal representation of moving body parts, important for
the control of changes in the position of both oral and brachial articulators" (Kimura, 1979,
p. 197). This position then closely equates with that of Lomas (1980). Certainly, one may
justifiably conclude that an intact left hemisphere is important for the generation of the
movement series designed by these workers. This does not entail that the role of the left
hemisphere in movement mediation is singularly that of movement programming, neither is
it clear, the extent to which the production of postures does or does not consist of
movement transitions and sequences. The level of analysis merely seems convenient.
The study of apraxia, generally considered to pertain mainly to the purposeful
generation of gestures, has provided what are potentially converging perspectives on the
role of the left hemisphere in movement organization. In concluding their historical review,
Faglioni and Basso (1985) note that most contemporary accounts of the praxic condition
afford a prominent role to the left prefiontal area, which may mediate "motor commands"
traveling to both sides of the body. By this viewpoint, it is prior to movement of limbs on
the left side of the body that commands are transmitted, h m the left prehntal area, via the
corpus callosum to the right premotor area and ultimately to the right motor area.
Alternatively, it has been suggested, that for right sided movements, commands are
dispatched directly to the left motor area (Faglioni & Bassi, 1985). This somewhat
restrictive explanation is itself inconsistent with a considerable number of case histories and
provides few details of the manner in which the relative contributions of "feedback
processing" and "movement programming" are sensitive to task demands.
The problems of extrapolating fiom clinical studies are more general, as Bernstein
shrewdly commented, almost five decades ago, The understanding of motor co-ordination, like many other scientific goals, has been achieved by a negative method - through observation of the phenomena of lack of co-ordination - and has been only gradually enriched by the accumulation of observations on pathological movement. Like all knowledge acquired by negative means it has constantly suffered, and suffers at present, h m the absence of accurate determinations (Bernstein, 1984, p. 213).
5.4 THE LEFT HEMISPHERE AS A "FEEDBACK PROCESSOR"
The view that the left hemisphere is uniquely specialized for the processing of
feedback information is itself not without support. Using a procedure differing somewhat
from that of the traditional dual task manipulation, Sussman, and his co-workers in a series
of studies (Sussman, 1971, 1979; Sussman & MacNeilage, 1975; Sussman & Westbury,
1978), has marshalled evidence which suggests that, not only may feedback control be
strongly implicated as a basis for motor laterality, but also that data which purportedly
demonstrates a left hemisphere advantage for movement programming may bear
reinterpretation.
The experiment employed by Sussman (1971) involved auditory tracking. Subjects
were required to match an electronically generated tone, of which the frequency was under
subject control, presented to the left or the right ear, to a target tone of varying frequency
which was presented to the subject's other ear. Individuals were provided with one of two
alternative means of controlling the "cursor" tone, either via a transducer responsive to
lateral tongue movements, or directly using the right hand. In the condition in which the
"mouth controlled cursor was presented to the right ear and the target to the left ear, a
greater degree of accuracy was observed than when the cursor was presented to the left ear
and the target to the right ear. In contrast, when the tracking was performed by the right
hand, no significant difference by ear was observed. It was hypothesized that the effect of
ear was associated with factors relating to motor control of the cursor. This appears
plausible, as the target tone, and presumably the target analysis, were the same for both
hand and "articulator" tracking. In an explanation which appears, only superficially,
analogous to that forwarded by Lomas (1980), it was suggested that both the pursuit task
and speech articulation involve the comparison of a controlled auditory output to an
"acoustic standard" and an "ability to control the motor system in terms of acoustic
consequences, presumably with concurrent use of somatic sensory information" (Sussman
& MacNeilage, 1975, p. 146).
As Bryden (1982) comments of the Sussman position, it is a central tenet that
feedback is evaluated in terms of a superordinate goal, ongoing motor activity is to be
compared to a standard. Indeed it is conceivable that the deficits witnessed in individuals
having suffered left hemisphere lesions, may arise from an inability to evaluate the relative
positions of elements within a sequence rather than a failure to generde sequences per se
(Bryden, 1982). Again it must be stressed that during ongoing movement in normals, both
processes are typically occuning, the respective involvement of each being significantly
influenced by task demands.
Although predominantly employed to examine the relationship between the
processing characteristics of the left hemisphere and the execution of motor performance,
the dual task paradigm may, with some caution, be used as a means through which the
features of right hemisphere processing may be discerned. As Hellige and Longreth (198 1)
point out however, the interpretation of lateralized motor interference is somewhat more 40
convoluted when it is the characteristics of the right hemisphere which are of interest.
Whilst it may be the case that the right side of the cortex has a unique competence for the
processing of "visuospatial" information, this superiority may well be masked by the nature
of the output measures, for example, responses taking the form of vocalization or of a
number of expressions of manual proficiency, may be subject to a large degree of
mediation by the left hemisphere.
In the second experiment of a 198 1 study, Hellige and Longreth required that their
subjects perform in what were essentially two conditions. In the first, individuals were
required to simultaneously tap a key with the index finger of either the left or right hand,
whilst, in the other hand, handling blocks in a way that apparently involved little cognitive
processing. Tapping rate was depressed equivalently for each hand. This may be
interpreted as indicating that the motor output requirements of the block manipulation,
putatively requiring some left hemisphere involvement, may have swamped any effects due
to the visuospatial nature of the task. Direct support for this interpretation is provided by
data arising from the second condition. When a spatial task including both cognitive and
motor components, the Block Design test of the Wechsler Intelligence Scale, was
substituted for simple block manipulation, there was greater disruption of left hand than of
right hand tapping. It seems the Block Design test required a degree of spatial processing
which was sufficient to be manifested as a depression of the rate of left handed tapping.
In a similar vein, there is evidence to suggest that visual detection tasks (analogous
in a very strictly limited sense to the Block Design test used in the Hellige and Longreth
(1981) study) exerts an asymmetrical influence upon motor output. Beaton (1979) had
subjects perform a bimanual sorting task comprising two individual components, one of
which was consistently the index of overall performance. Proficiency on this component
(nut sorting) was depressed to a greater extent when the yisual detection target was
projected to the right hemisphere. Detection to the left hemisphere did not
disrupt manual performance. Summers and argued that the movement
sequencing tasks, frequently utilized in this paradigm, are subject to control by both
hemispheres, as finger sequencing involves both the localization of limb segments in space
and the ordering of movement subcomponents within a series. The implications of this are
wide ranging. As Todor and Smiley (1985) highlight, the vagaries between visually guided
and unseen movements outlined by Lomas (1980) may have arisen due to the absence of a
visuospatial component in the latter, it is conceivable that the need to employ an
"internalized spatial position system" had some direct influence upon the decreased rapidity
of movement sequencing. In the sections which follow, further reference will be made to
this issue. 4 1
CHAPTER 6
CHARACI'ERISTICS OF THE MOTOR OUTPUT
6.1 REASONS FOR CAUTION
In turning to Bashore's final factor (section 4.3), the motor output, little further
need now be said regarding the exigency of emphasizing the functional unity of the
perceptual and motor systems (e.g., Lee and Thomson, 1982), yet it remains the case that
the extent to which an asymmetry is expressed is highly task dependent and relies, in large
part, upon the nature of the motoric components of that task As has been illustrated, this
dependence has exerted a highly constraining influence on the definition of what, for some,
may constitute an adequate account of the underlying mechanisms (c.f., Lomas, 1980).
Consequentially, accompanying attempts to satisfy the appetite for the specification of
mechanisms or processing devices, there have been numerous ad hoc assignations of
functional properties or arbitrarily defined capabilities. The level to which these
pseudoassumptions may descend is well illustrated by the suggestion of Todor and Srniley
(1985) that in a certain task "one might expect the left hand to opt for a strategy that
optimizes its performance capabilities" (p. 3 14). It would perhaps be more profitable to
consider the manner in which the characteristics of the nervous system give rise to the
expressions of manual asymmetries which are so highly task dependent.
As a research endeavor this approach obviously requires consideration, not only of
the characteristics of the nervous system per se, but also highly specific examination of the
nature of the motor behaviour to which the activity of the nervous system gives rise. It will
be argued that this dual sensitivity is an initial requirement. It has been customary for
accounts of manual asymmetries to give a superficial and highly selective account of the
nervous system as a whole, which, as has been shown, may be described at the
phenomenological level as a unitary instrument subserving goal directed action, yet this
almost incidental consideration alters appreciably the nature of the interpretations to which
observations give rise. In the sections which follow reference will be made to an extensive
body of work which, when examined in view of known properties of the neuromuscular
system, requires significant reevaluation.
6.2 TAPPING TASKS
There have been a large number of recent studies which have employed tapping
tasks as a means of assessing the characteristics of manual asymmetries. Two varieties of
this task can be distinguished. In one instance, the trajectory of the tapping finger is highly 4 2
restricted by, for example, attaching the finger directly to the tapping key (e.g., Todor &
Smiley-Oyen, 1987). There are as a result few accuracy demands and it is the speed of
tapping which is generally emphasized, though contemporary studies have also required
that subjects tap to specified frequencies. The other variety of tapping task is rather less
restricted, being more akin to a succession of aimed movements, and will not be described
at this juncture.
Traditionally, the primary dependent variable employed in this paradigm has been
the rather global measure of tapping frequency. Kimura and Davidson (1975) observed a
superiority for the right hand in terms of the rate of tapping, although Peters (1976) found
that this difference could be eliminated through extensive practice. It seems however that
the variability of left handed tapping, in terms of inter-tap interval (ITI) remained
significantly higher, even after 1300 trials, eliciting fiom Peters (1976) the less than
illuminating suggestion that the left and right hands differed in their mode of motor control.
Further work has confirmed that generally, on this task, the right hand taps more rapidly
and with greater regularity than the left (Peters & Durding, 1978, 1979a).
In something of an effort to expand upon his suggestion that manual asymmetries
arise fiom differing modes of control, Peters (1980) published a contentious three
experiment study which, in spite of what are manifestly fallacious conclusions, proved to
be a seminal study. The first experiment may be considered relatively insignificant in
producing an unconvincing illustration that asymmetries are not reflections of greater
strength for the right hand. This is somehow inferred from an indication of resistance to
fatigue.
In the second experiment of the series, a rather more sophisticated tapping
apparatus was used, permitting the evaluation of measures in addition to simple tapping
rate, specifically, the time taken to "reverse" a movement at the top and bottom of its travel,
and the period spent in the single up or down movements between reversals. The results
obtained appear to have indicated that, whilst a difference in terms of overall movement
duration and thus tapping rate was present, it was the portion of the movement in which
reversals occurred that represented the major contribution to this difference. This phase
Peters (1980) regards as the transition between flexor and extensor movements which, he
feels, reduces to the precision of "force modulation".
In the course of a third experiment, in which the maintainence of performance, in
terms of tapping rate, was presumed to require even greater precision of force modulation
(by further limiting the extent of digit excursion), it was hypothesized that the modified task
would lead to a proportionately greater decrement in left hand performance. The results
indicated that the performance of the non-preferred hand did indeed decline to significantly
greater extent when this task constraint was imposed.
Whilst it may indeed be convincingly argued that it is apparent precision of force
modulation which differentiates the hands, the factors which give rise to this asymmetry are
less well appreciated by Peters. The contention that "peripheral factors are relatively
unimportant in producing between hand performance differences" (Peters, 1980, p. 70)
clearly contrasts markedly with views within "contemporary" physiology, for example,
Bernstein (1984) who in expressing aspects of his "circular principle of control" stressed
the necessity of examining the nature of the "sensory feedback connections" (p. 344). Peters' confusion perhaps arises from a misconception of the nature of the movements he is
investigating, considering them to be "too short for the direct involvement of sensory
feedback" (1980, p. 70).
It does appear however that there are no voluntary movements which can proceed
more rapidly than the speed with which some form of sensory feedback can be mediated
(Bawa, note 5). The fastest voluntary movements may only approach the frequency of the
physiological tremor, which, with respect to the digits, is approximately 10 Hz. Some
researchers have suggested that peripheral kinesthetic feedback loops can be completed in
as little as 25-50 msec (Dewhurst, 1967; Evarts, 1973).
There has been a tradition of considering the initial phase of a movement to be
"ballistic" and as such unaffected by sensory feedback (e.g., Welford, 1971). Apparent
support for this position has generally been provided by analysis of the
electromyographical (EMG) activity during movements. The archetypal triphasic pattern of
activity is composed of an initial agonist burst, followed by activity in the antagonist and
finally a further separate agonist agonist burst. In practice, this pattern is difficult to
achieve. It was generally held that "open loop" control exists during the initial portions of
the movement (up to 100 msec.), as perturbations or blocks to the movement were found to
have no effect upon the initial agonist f ~ n g , as revealed by EMG (e.g., Hallett, Shahani &
Young, 1975; Wadrnan, Denier, Geuze & Mol, 1979). Certainly, exclusion was made of
any potential role of visual feedback during this period. However, when target position
lights are displaced, in this portion of movement, there occurs a very rapid modulation of
EMG, leading to the suggestion that a "closed visual loop" of some nature may be
operating (Gielen, van den Heuvel & Gon, 1983; c.f., Young, 1987).
The changes in the initial agonist firing pattern which had been seen to accompany
increased resistance to movement (Brown & Cooke, 1981; Lee, Lucier & White, 1981)
have been dismissed as merely reflexive and having no contribution to unconstrained
visually guided movements (e.g., Todor & Smiley, 1985). It has recently been suggested 4 4
however that long latency EMG responses are not only compensatory but permit the
alteration of ongoing control in light of peripheral conditions and in turn reflect the presence
of routes along which sensory information may flow to central structures (Requin, Semjen
& Bonnet, 1984).
Peters (1980) postulated rather that tapping: depends on central preprogramming (Schmidt, 1975) and any role of sensory factors in the guidance of such movements must be restricted to the evaluation and adjustment of the central motor program (p. 70),
and continues to maintain that feedback is utilized through "occasional probing" rather than
on a continuous basis (Peters, note 1). The work of Hary and Moore (1985) is cited as
partial support for the view that probing may occur where the speed of movement precludes
"continual sampling". Even if it were the case that voluntary movements could proceed this
rapidly, controlled motor output in the apparent absence of sensory input does not imply
that normal movements proceed in this fashion (Agarwal & Gottlieb, 1984). The point is
made succinctly by Reed (1982), who in restating Bernstein's position outlines: Bernstein suggests that the animal is in continual &-equilibrium with its environment, requiring that it not to stimuli, but rather that it all the time and that it constantly evaluate its actions with respect to ever changing current conditions, while at the same time modulate its activities so as to meet its needs and goals within the environment (p. 108).
It is thus seems that it is only on the most fragile basis that Peters (1980) may question the
role of sensory feedback in contributing to manual laterality effects and conclude that: the most likely source of asymmetries is in the process whereby the movement is preprogrammed and whereby the selection and activation of appropriate neurons occurs (p. 7 1).
What appear to be ballistic movements rnay,in turn, seem preprogrammed, yet even these
movements are sensitive to the "most recent peripheral states of the system" in a manner
which permits not only load compensation but also modulation of the "central command"
(Agarwal & Gottlieb, 1984, p. 568).
Peters major contribution with regard to the evaluation of movement asymmetries is
likely to have been the delineation of movement phases within tapping and the examination
of the extent to which asymmetries are expressed in these respective portions. From a
number of additional, well controlled, studies conducted during the past decade, a
consistent pattern of results has emerged. The more rapid rate of tapping for the preferred
hand is generally associated with shorter periods during which the tapping key is
depressed. That is, less time was spent in the phase of movement in which reversals of
movement took place (Todor & Kyprie, 1980; Todor & Smiley-Oyen, 1987), additionally,
there was less variability of the IT1 (Todor & Kyprie, 1980; Todor, Kyprie & Price, 1982).
The observation that it was differences of variability in the interval between force peaks
which reached statistical significance, rather than the differences in the variances of the
more global Ill (Todor & Smiley-Oyen, 1987) appears to lend partial support to the notion
that, the left hand is more variable in its regulation of force, and for Peters' (1980) view
that this hand can less precisely modulate force.
It is unfortunate that virtually all successive attempts to provide some explanation of
these results have included, from the onset, erroneous assumptions regarding the role of
sensory input, including suggestions that the latency of a movement reversal is "below the
minimal time estimated for the use of peripheral feedback" (Todor & Smiley, 1985, p.
3 16). This has given rise to conceptualizations of the bases of manual asymmetries which
are not only insensitive to the characteristics of the nervous system but also embrace a
confused notion of the presumed relationships between the components of that system.
Thus, "the left hand is less adept in establishing an effective framework around which to
regulate successive directional reversals" (Todor & Smiley, 1985, p. 316). Elliott (note 2)
has been somewhat more circumspect in proposing that the right hand advantage arises
from a superiority in "parameterizing" movement.
Although there has been a failure to elicit O ing asymmetries in a small proportion P of studies (e.g., Flowers, 1975), the majority of studles seem supportive of the view that
tangible differences do exist. What has been viewed as variations in force modulation
(Peters, 1980) or in regulation (Todor & Kyprie, 1980) arising from some aspect of
preprogramming, has also been considered as due simply to differences in the "intrinsic
variability or noise in the production of force time patterns" (Annett, Annett, Hudson &
Turner, 1979, p. 647). This position is descended directly from the Schmidt, Zelaznik and
Frank model of 1978 which encapsulates the prediction that "variability in force amplitude
varies inversely with the square of movement time and the variability of force duration
varies directly with movement time" (Annett et al, 1979, p.647). The task employed by
these authors was substantially different from the highly restricted tapping tests.
Individuals were required to transfer pegs, independently with each hand to appropriate
locating holes in a proximally placed board. Using a film record of performance, something
akin to a kinematic analysis was attempted though, as the data was sampled at 40 frames
per second, the results should be approached with some caution. Gross analysis revealed
that the preferred hand could more rapidly perform the movement series. More detailed
analysis of the film record indicated that the differences occurred primarily during the
"positioning" element of the movement rather than the "transport phase". Specifically, the
non-preferred hand more frequently missed the target hole, making on average 50% more
corrective movements. Indications from the patterns of latencies suggested to Annett et al.
(1979) that the corrections were themselves conducted on the basis of a "kinesthetic 4 6
feedback loop", and that the duration of these corrections did not differ significantly
between the hands. It seems that in the Annett et al. study at least, the non-preferred hand
was simply more variable. The movements to the target itself were assumed to be "open
loop" and were made at the equivalent rate for each hand. It was rather the accuracy of
these movements which diff& Whilst, as noted, the conviction that movements to the
board were open loop was misguided, it is of interest that Annett et al. (1979), following
Schmidt et al. (1978), consider that the variability arises from the characteristics of the
entire system, rather from neural, muscular or mechanical properties. This, however,
provides little illumination of how the variability arises or is asymmetrically expressed.
More recently, Peters (1987; note 1) has implied that "attention" may play an intervening
role in the expression of asymmetries and that this factor may be more important than the
variability of output as such.
One persistent problem for those seeking to examine the relative distribution of
manual proficiency in left and right handers is that members of the former population have
generally had more experience in performing movements with the non-preferred hand in
comparison to their right handed counterparts. One way in which this limitation has been
overcome is through the use of tasks which reduce the importance of practice levels, an
example being bimanual tapping. Although, as has been described, the maximal rate of
tapping varies between the hands, along a number of dimensions, examination of tapping at
submaximal rates reveals few such differences (e.g., Wolff, Hurwitz & Moss, 1977). It
appears to be the case however that when submaximal tapping for each hand is combined in
a dual task, asymmetries are once more revealed. In a "2: 1 task" in which one hand
completes two taps for one tap made by the other hand, individuals perform in superior
fashion when the preferred right hand makes the two taps (Peters, 1985, 1987). This
benefit is evidenced by tapping which is both more rapid and more regular. For Peters
(1987) this enhancement arises as a consequence of "a tendency of right handers to direct
attention preferentially to the right body half" (p. 97). In summarizing, Peters (1987)
considers that the essence of this bimanual task is the organization of movement into
"rhythmically different concurrent sequences" concluding that "both in the perception and
production of different temporal sequences there is the problem of how to focus attention
on the component parts of the sequence" (p. 98). This is not however a problem for the
nervous system.
6.3 THE NON-ROLE OF ATTENTION
The view that the differential distribution of attention may account for hemispheric
asymmetries in general has appeared previously in a variety of guises, most notably 47
Kinsbourne's "bidirectional, negative interaction" model (e.g., 1970, 1974a, 1974b). The
essential characteristic of Kinsbourne's model is that a mutual inhibitory balance exists
between the hemispheres with respect to attention or orientation and to consummation or
the motor act. A central balance or bisymmetry is taken however to be an improbable state,
and is "specifically programmed only if functional pressures make it necessary"
(Kmsbourne, 1978, p. 11). Thus, if an individual has "language ability" lateralized in the
left hemisphere, preparation for "verbal activity" will lead to "neuronal activation" of the
left hemisphere, resulting in a rightward attentional bias (Kinsbourne, 1978). This, it is
presumed, explains the asymmetries evident in both dichotic listening and tachistoscopic
presentations. In similar fashion, it is maintained that manual asymmetries arise as
bisymmetry is functionally disadvantageous. Rather than considering that the magnitude of
the asymmetries are dictated by the extent to which a function is lateralized, Kinsbourne
argues thusly: The extent to which attention swings contralateral to the more active hemisphere is determined not only by where the active processor is lateralized, but also by how hard it is working. How hard it is working is in turn deterrnined by how difficult the task is and how willing and able the subject is to rise to the challenge of the task. (p. 10).
This at least has the merit of encapsulating a sensitivity to the task and context specific
constraints, though again one must confront the problem of how "active processors" may
be reconciled with what appear to be examples of mutual inhibitory balances in simple
neural systems. There are more immediate problems for this formulation: there now seems a consensus that, although attentional factors undoubtedly play some role, they are certainly not the only factors responsible; strong asymmetries can be demonstrated even when attentional components are minimized or eliminated. In addition there have been some difficulties in replicating the original Kinsbourne data. (Allen, 1983, p. 83).
The pervasive nature of spatial compatibility effects offers a potent indication that to
maintain that attention is predominantly channeled to the right side of the body is severely
shortsighted As Peters (1987) acknowledges, there is really no clear indication of that to
which "attention" refers. In spite, or perhaps because of, this obvious limitation, there has
been a considerable volume of research, conducted primarily by Honda (e.g., 198 1, 1982,
1984) which has examined the issue of whether vagaries in the distribution of attention may
account for the observed superiority of the preferred hand In order to fashion some
appreciation of the context in which these formulations arose, it will prove efficacious to
briefly review the background to Honda's work.
It has for some time been apparent that the transaction between the organism and its
environment dictates the form of the percept to which the activity of the organism gives
rise. It is important to recognize that, as transactional, not only does the activity of the
organism, relative to its environment, structure perception, but in addition, the changing
characteristics of the environment relative to the organism alter the behavioural mechanisms
through which this perception takes place.
One explicit illustration of this may be derived from the research generated by
Rashbuss's (1961) conclusion that smooth pursuit eye movements are governed by both
the direction and velocity of "target" movements, whilst saccadic eye movements are
sensitive to the position of a target. Festinger and Canon (1965) undertook a series of
experiments which provide some illumination of the significance of this disposition. The
studies were conducted in a completely dark room. In one condition, a target light appeared
briefly within the subject's visual field, subsequently the individual was required to point to
the position at which the light had appeared. A saccadic eye movement was required to
fixate the target in this instance. In a second condition, the target light appeared and moved
slowly across the visual field before disappearing, some form of tracking was thus
required. Subjects were, in each case, required to indicate, by pointing, the final position of
the target. Performance when subjects had directed a saccade to the target was superior to
that when the position had been tracked.
By way of explanation, it was proposed that the superiority of saccadic eye
movements was due to the availability of efferent "signals" associated with the oculomotor
system. These concern the position of the eye within its orbit and were presumably utilized
in a manner which provided some indication of target position. In contrast, it has been
assumed that the use of the extraocular muscles responsible for tracking movements, does
not give rise to information which is amenable to the generation of a conscious perception
of eye position (Brindley & Morton, 1969). It should be noted however, that in order to
exert an influence upon the control of overt responses it is not necessarily important that
consciousness as such is invoked.
On the basis of observations that a superiority for the preferred hand was evident
only when the motion of the hand was visually monitored, Honda (1982) proposed that the
movement of the preferred hand is itself intimately associated with the oculomotor control
system. As an initial objection, the original premise is clearly inappropriate (c.f., Lomas,
1980), and as will become manifest, the conclusion is no more convincing.
In the first experiment of the Honda (1982) study, subjects were required to make
bimanual movements to symmetrically placed targets. Eye movements were recorded by the
EOG technique, whereby, electrical activity was monitored through electrodes applied to
the lateral, and upper and lower sides of the eyes. The findings indicated that, although
individually moved their eyes rightward, there was no between hand difference in
performance. Indeed, in many instances there was a complete absence of eye movements, 4 9
the nature of the task was such that subjects were able to successfully complete the
bimanual movement without recourse to eye movement. Theorists have previously outlined
the possibility that an elevated tendency to make rightward eye movements may be due to
reading habits (White, 1969). It is then possible that the increased frequency of rightward
motions was actually due to .this overlearned disposition. This issue should obviously be
resolved by means of a controlled cross cultural examination.
In order to go some way toward dealing with this issue, Honda (1982) included a
second experimental series in which target sizes were reduced relative to those used in the
first experiment. It was proposed that rightward eye movements, if indeed due to reading
habits, would be more prominent if digits were presented as targets and no aiming
requirement was included (Honda, 1982). The logic of this was fuzzy to say the least, there
seems no reason to regard the digits as anything other than targets in the previous sense,
certainly they did not occur in a context bearing any more resemblance to reading than the
aiming task. It seems that a more appropriate question would have been whether the
rightward shifts in aiming tasks are due to reading habits. The results arising from this
second manipulation indicated a rightward superiority for the digit task, though of a
magnitude smaller than that for the bimanual aiming task. Honda (1982) somehow
interprets this as demonstrating that a rightward superiority of eye movement was a feature
specific to tasks involving hand movements rather than simply looking at displays. A statistically significant between hands difference was observed, the performance of the
preferred right hand being superior to the left. Honda (1982) states that the implication of
this finding is that "subjects prefer to monitor the movements of the right hand than the left"
(sic) (p. 5 lo), and in turn that right hand movement control depends more on visual
feedback than the left. In view of the indeterminate nature of the rightward shift, the
conclusion that some preference is involved appears quite erroneous. It was further
postulated that the right hand's better performance in bimanual aiming tasks is based upon
the right hand's "superiority" of eye movements, yet this seems an almost trivial confusion
in an account which can suggest that, with respect to bimanual movements, "it seems quite
redundant to receive sensory feedback from both hands" (Honda, 1982, p. 512).
Nevertheless it may well be the case that the habitual rightward shift makes available, more
consistently, information regarding the right side of visual space, which in view of the
aforementioned spatial compatibility effects may contribute to a general superiority for the
right hand, as the hand most often also located in the right visual space. Similarly, it is
conceivable that the left hemi-field superiority witnessed for brief presentations of particular
types of stimulus material (see section 2.3), and most clearly revealed when required
responses are simple, short duration, single joint movements, may in turn be obscured 5 0
when responses are longer duration aiming movements extending into extrapersonal space.
In the latter instances, it would be supposed that, movements to stimuli presented to the
right of fmation would be favoured by the predominant rightward shift. It is not
immediately clear at present the effects this would have on behavioural indices such as
reaction time and movement ~ e .
Honda's (1982) suggestion that not only is the movement of the preferred hand
preferentially monitored but also that it is associated intimately with the oculomotor control
system was examined in rather more detail in a subsequent study (Honda, 1984).
Employing what was essentially the experimental paradigm utilized by Festinger and Canon
(1965), consideration was given to potential between hand differences in the magnitude of
the constant error of pointing responses. This factor was considered within the two
conditions in which subjects made either saccadic or smooth pursuit eye movements. It was
observed that for both hands, saccade eye movements were associated with superior
performance. Honda (1984) unfortunately confuses the level of statistical significance with
the nature of the effect, although there was no statistically significant pointing hand by
stimulus condition interaction, the author is led to claim that "the effect of the eye
movements on manual pointing is prominent on the preferred right hand than on the non-
preferred left hand" (p. 75). Clearly however, the prediction that the differential effects of
the two types of eye movements would be evident only for the preferred hand is not
supported. Honda (1984) continues to promulgate erroneous conclusions based on the
misinterpretation of the analysis, claiming "the left hand seems more independent of eye
position information than that of the right hand" (p. 86). In a spiralling confusion, Honda
assesses there to be a problem, suggesting that "eye position is not a dominant cue for
spatial localization of the left hand" (p. 86), and thus that there ought to be a search for the
missing dominant one. The explanation proffered seems quite independent of the pattern of
results obtained. It cannot be concluded that the behaviour of the left hand is not in some
way sensitive to the type of eye movements made. On the contrary, Honda's (1984) results
clearly indicate that saccadic eye movements were associated with superior performance by
the left hand. ! Of some interest was the observation that a statistically significant difference in
terms of the variable error of the pointing response was composed of a greater variability
for the right m. The magnitude of this difference appeared to be sensitive to the visual
half field in which the stimulus was presented. There was no obvious difference in variable
error between target presentation conditions.
It remains the case that conceptualizations of what constitute attentional asymmetries
remain so ill defined as to be of little practical worth. Similarly, there is no empirical 5 1
evidence, nor a priori reasoning to suggest that one should concur with Honda (1982) in
concluding that the right hand is preferentially monitored. However the view that manual
asymmetries result from a differential efficiency in the use of visual monitoring (Honda,
1982) has, in a number of manifestations, garnered considerably more support. In the
sections which immediately follow, attention will be given to those holding the perspective
that the magnitudes of manual asymmetries vary as a function of the differential efficiency
with which the cerebral hemispheres deal with feedback information, and in particular
visual feedback
CHAPTER 7
THE ROLE OF VISION IN THE REGULATION OF AIMED MOVEMENTS
7.1 A MINIMUM PROCESSING TIME FOR VISION?
It was Woodward who in 1899 provided perhaps the first quantitative support for a
view which might always have been intuitive, that the preferred hand performs rapid aim
movements both more rapidly and more accurately than the non-preferred hand.
Woodworth's subjects performed a series of line drawing movements either with their eyes
open or eyes closed, providing, in addition, one of the first rudimentary manipulations of
visual feedback. Perhaps contrary to expectations, it seemed that movements of particularly
short durations were unaffected by the presence or absence of visual information, leading,
in turn, to the suggestion that there was some minimum time required for the utilization of
information of visual origin.
Since this time a variety of authors have attempted to distinguish two distinct phases
within such movements: a ballistic transport phase during which the aiming hand is brought
to within the vicinity of the target, and a relatively short second phase permitting the
accurate final positioning of the hand (Pelisson, Prablanc, Goodale & Jeannerod, 1986).
The practical utility of this, merely descriptive, dissociation has had the unfortunate
consequence of sanctifying the implied physiological reality of the "motor program", as a
necessary and sufficient condition for the control of single movements. This rather
simplistic perspective has been portrayed most explicitly by Keele, in 1968, conceiving that a motor program may be viewed as a set of muscle commands that are structured before a movement sequence begins, and that allows the entire sequence to be carried out uninfluenced by peripheral feedback (p. 387).
Whilst it would require a considerable digression to provide any substantial coverage of the
"centralist" versus "peripheralist" debate, it remains sufficient to note that Keele's proposal,
as received, was somewhat less than cautious. Although it is impossible to provide any
substantial justification for a view which, although perhaps only an inelegant exposition of
the perceived need for the motor program as an explanatory construct (c.f., Requin,
Semjen & Bonnet, 1984), clearly belittled a large corpus of physiological research, it is
enlightening to examine the research which was conjunctive with Keele's 1968 review.
Most central and precipitative was the work leading to the publication by Keele and
Posner (1968) of an estimated minimum time for the processing of visual information.
Subjects were trained to move a hand held stylus approximately six inches to a small target,
taking either 150,250,350 or 450 ms to do so. On certain trials, randomly determined,
visual feedback was removed upon movement initiation, the mom lights were 5 3
extinguished. Performance was assessed in terms of the proportion of occasions on which
the target was "missed in each condition. Predictably, the probability of missing decreased
with increasing movement duration, importantly however, an advantage for conditions in
which visual feedback information, of hand and target position, was available, only
occurred for the longest three movement durations. When aiming movements were
performed in the 150 ms condition (actual average movement time was 190 ms) there were
no differences in performance between the two illumination conditions. As vision of the
hand and target appeared, superficially, not to facilitate movement control for movements
of 190 ms or less, Keele and Posner (1968) concluded that this was the minimum time
required to usefully incorporate visual feedback.
There are, however, a number of procedural considerations which constrain the
universality of these findings. As the potential availability of feedback was randomly
determined, it may have proved useful for subjects to adopt a strategy whereby it was
assumed that the movement would be made without visual feedback. This may have served
to diminish the apparent magnitude of the facilitation effects of visual feedback, and
purportedly, though less obviously, contributed to a "delay in processing" for No-Vision
trials (Zelaznik, Hawkins, & Kisselburgh, 1983).
These latter authors also suggest that the specification of target movement times
may have shifted subject's attention form considerations of accuracy. It is difficult to see
however that target, movement time bandwidths favoured initially by Zelaznik et al. (1983)
provide anything which more closely resembles an "ecologically valid" action goal than a
set movement time. Conventionally, when movement time appears salient for appropriate
task completion, it is in circumstances in which the temporal and spatial coordinates of the
body or of a body segment must be made coincident with the coordinates of an external
entity. It is unlikely that movement time per se is of interest to the organism (c.f., Lee,
1980). In natural reaching movements, were the emphasis is on accuracy, movement time
may more appropriately be examined as a dependent variable (Jeannerod & Prablanc,
1983). There are alternative and less artificially constrained ways in which the relative
contribution of visual information to task completion may be examined (see Section 7.2). In a more contemporary examination of this general issue, Zelaulik, Hawkins and
Kisselburgh (1987) have suggested that it is not the minimum time to process visual
information which has been sampled, but rather, the "minimum average momentum that the
visual feedback control system can overcome" (p. 183), reasoning that the time required to
complete a correction will vary as a function of the of the limbs resistance to perturbation
and thus as a function of its momentum and, for these purposes, velocity. Zelaznik et al.
examined this issue by considering combinations of three movement durations, 200,400, 5 4
and 600 ms, and three movement distances, 10,20, and 30 cm, yielding three possible
combinations for which the average velocity was 50 c d s , specifically 10-200,20-400,30-
600. Once again the use of movement time goals creates interpretative problems,
additionally, in view of the authors prediction that resistance to perturbation is a function of
momentum, one would have to question the use of average velocities rather than peak or
instantaneous velocity. hde& the results obtained by Zelamik et al. (1987) provide no
support for the speculation that required correction time varies as a function of limb
momentum, observed benefits in terms of spatial accuracy, for the 50 c d s combinations
varied as a function of movement time.
As noted, consideration should more importantly be given to the manner in which
the time required to make a correction may vary as a function of instantaneous velocity and
thus instantaneous momentum. The required manipulation may now, in principle, be
achieved through the use of microcomputer based motion analysis systems. It is possible
from these systems to obtain an, on line, estimate of the position of a limb segment in three
dimensional space. By referencing the final target location in terms of some coordinate
system and through "on line" computation of changes in displacement over time, it should
be possible at any instant to establish both "instantaneous velocity" and the time to target
contact. Through the use of appropriate external operators, visual feedback may be
removed at a predetermined peak or instantaneous velocity or at a predetermined estimated
time to contact.
Returning directly to the Keele and Posner study of 1968, a further, and
potentionally critical, limitation was the use of the probability of missing the target as the
central dependent measure. It is certainly the case that this is a rather insensitive measure,
certainly in comparison to spatial accuracy. The use of response measures of greatly
enhanced sensitivity, in a series of subsequent studies, has precipitated continuous
downward revision of the estimates of minimum visual feedback processing time from the
190 to 260 ms range suggested by Keele and Posner (1968).
An initial experiment conducted by Carlton (1979), which whilst not directly
manipulating visual feedback as such, was one of the first to employ a rudimentary
kinematic analysis as a means of demonstrating that "discrete" movement corrections could
be made more rapidly, in the basis of visual information, than the 190 ms limit imposed by
Keele and Posner (1968) or the 290 ms time later suggested by Beggs and Howarth
(1971).
The need to fashion some manipulation of visual feedback has spawned a number
of novel and innovative procedures. Using a closed circuit camera system, Smith and
Bowen (1980) delayed visual feedback during an aiming movement. Movement durations 5 5
ranged from 150 to 450 ms, across all movements, feedback delayed by 66 ms was
associated with an overshooting of the target which was not present in a no delay
condition. That this disruption was observed for even the most rapid movements
(approximately 160 ms) lead Smith and Bowen (1980) to conclude that the processing time
was actually less than 100 ms
Carlton (1981a) also using aimed hand movements, noted changes in the kinematic
profile which occurred within 135 ms of the appearance of the individual's hand from
behind an occluding screen, providing, it seems, converging support for the revision made
by Smith and Bowen. In a second experiment, Carlton (1981b) provided an indication that
the relative importance of vision of the hand and vision of the target may, in turn, be highly
task dependent. Subjects were required to make, short amplitude, aiming movements using
a hand held stylus in five conditions of visual feedback; ambient lighting present, ambient
lighting removed, vision of the target only, vision of the stylus only, and vision of both the
target and the stylus in the absence of ambient lighting. In terns of procedure, there were a
number of associated limitations, a movement time goal of 330 rns was established and
performance was scored in a manner similar to the Keele and Posner (1968) study, as a
proportion of target misses. Somewhat predictably, the poorest performance was for I
movements made in the complete absence of visual infoxmation. A similar level of
performance was witnessed for instances in which only target information was present. On
the other hand, movements made with vision of the stylus only, with vision of the target
and stylus, and with ambient lighting present were performed equivalently. Whilst the
results of this study do not speak directly to the issue of feedback utilization latencies, it
does provide evidence, that ir, this instance, useful utilization of visual information was
limited to assessing the position of the stylus and, by implication the hand, relative to
external space, rather for comparison of the relaqve positions of stylus and target.
Information regarding the position of the target alone proved no more useful than no visual
information whatsoever.
Issues pertaining to the relative importance of visual information emanating fiom
various sources, in relation to task specific constraints, will be examined in more detail in
section 7.2. At present, it is sufficient to note that estimations of the time required to
usefully incorporate information of a visual origin will vary as a function of the nature of
that incorporation within the context of a goal directed act. Similarly, the overt behavioural
consequences of this incorporation may, in turn, vary considerably with task demands
(c.f., Pelisson et al., 1986). If, on the one hand, it may be proposed that the time taken to
usefully employ visual information is highly task dependent, the reverse of this argument
might be used to as forcefully suggest that, in particular, apparently short latency feedback 5 6
processes (e.g., Smith & Bowen, 1980; Carlton, 1981a) may merely be due to procedural
artifacts.
In an attempt to verify that, for movements of short duration, visual feedback can
"aid motor control via processes not associated with intermittent error corrections" and to
demonstrate that the substance, if not the magnitude, of these effects was robust, Zelaznik
et al. (1983) performed a series of experiments within the Keele and Posner (1968)
paradigm which to some extent removed the procedural ambiguities associated with the
initial study. The first of four experiments addressed three of the potential limitations of the
Keele and Posner procedure; in place of the rather insensitive dependent measure
"proportion of target misses", the spatial accuracy of movements was monitored, as
previously mentioned, movement time bandwidths were used in place of movement time
goals, and additionally, trials occurred in blocks of visual feedback present or visual
feedback removed for which subjects could presumably then select the "optimum strategy"
for each condition, or within blocks in which, like the original study, the availability of
feedback was uncertain. The results indicated that the exclusive use of "uncertain
conditions" in the Keele and Posner (1968) study may have directly served to reduce the
effectiveness of the visual feedback manipulation. Zelaznik et al. obtained indications that
visual feedback processing times may be in the region of 100 ms Potentially however, the
depressed performance in conditions in which visual feedback is removed upon movement
initiation may be due to the sudden change in the visual environment as ambient lighting is
removed rather than due to the absence of visual information per se. In a direct attempt to
examine this possibility, Zelaznik et al. (1983, exp 2) reversed the protocol through making
ambient light and thus visual information available upon initiation of the movement. A pattern of results paralleling those for experiment 1 were obtained, leading the authors to
conclude that the "critical factor in determining accuracy is the presence of vision during the
movement, not a constant environment " (p. 227).
A third experiment dealt with the possibility that the benefits seen in the presence of
visual information might be due to some unknown artefact, through an attempt to establish
some movement time below which visual information no longer appears beneficial. The
results of experiment 3 indicated that movements of 70 ms duration appeared unaffected by
visual feedback. A further, albeit unlikely, possibility exists that visual feedback is not
useful during the movement with which it is associated, but rather that information, for
example of terminal location, is beneficial for subsequent trials, in terms of what, for
Schmidt (1975) would constitute an updating of the "recall memory" or motor schema. In
experiment 4 of the Zelaznik et al (1983) series, information regarding the feedback status
of the forthcoming trial was provided prior to each individual movement and feedback 5 7
conditions were alternated over trials. The results strongly favoured the explanation which
posits that visual information is utilized during individual vision trials. Beaubaton and Hay
(1986) have given further consideration to the balance between "corrected ongoing
responses" and "amended delayed responses", and the manner in which this may vary as a
function of both movement time and the nature of the feedback which is available.
More recently, Young (1987) has attempted, to both c o b the potency of the
effects elucidated by Zelaznik et al. (1983), and to examine whether increasing the salience
of monitoring the target position, in terms of satisfactory task completion, alters the relative
importance of hand and target information (c.f., Carlton, 198 lb). In addition to the
manipulation of ambient and target related visual information, the target was potentionally
subject to perturbations coincident with movement initiation. Movement times in two
conditions approximated to 165 and 330 ms respectively. In both instances, movement
endpoints were biased in the direction of target motion, providing further evidence that
visual information may be utilized more rapidly than assumed by Keele and Posner (1968).
This biasing persisted when either target information or ambient lighting was removed 50
ms after the initiation of movement, suggesting that visual information pertaining to target
motion can be disseminated during the first 50 ms of movement. Young (1987) proposes a
possible feedforward role for this information, clearly vision of the responding hand and
target in the first 50 ms of motion could not be used as a substrate on which to evaluate
changes of relative position, and thus feedback based corrections, at least in central vision
(unless the movements were particularly slow, and both target and hand were in the central
field of vision, which does not appear to be the ase). 1 There was a failure to replicate the findings of Zelaznik et al. (1983) in that, for
stationary targets, no differences in spatial accuracy existed between ambient lighting
present or target continuously present, and ambient lighting removed and target illumination
removed. However, it should be noted that there was in all instances a lag of 50 ms
between movement initiation and the removal of visual information, therefore, in this task
at least, the initial information regarding hand, and target position and motion, was
sufficient upon which to base the accurate completion of the movement.
Accurate in this instance merely implies that the level of inaccuracy between
conditions was equivalent as assessed by the behavioural indices used by Young (1987).
This does not bear entirely upon the issue that the activity of the nervous system may be
differentially affected by the relative variations in the optic array or that in a functional sense
the satisfactory completion of a goal directed act may be contingent upon the availability of
visual information in a fashion which evades detection by conventional behavioural or
indeed kinematic measures (c.f., Pelisson et al., 1986). 5 8
As previously noted, and as remarked by Zelaznik et al. (1983, p. 218) "estimation
of visual feedback processing time is not just an empirical adventure", clearly it has a
bearing on issues pertaining to the centralist versus peripheralist debate, and in particular
with respect to the concept of the "motor program". More importantly, in this context, it is
central to consideration of the manner in which, at least visual, feedback is involved in
modifying the outcomes of ongoing movement.
It has for some time been assumed that the alterations made during the course of a
reaching movement arise only during the latter phases of the movement, during which
portion, it is assumed, the position of the seen hand may be compared with target position
(e.g., Keele & Posner, 1968; Beggs & Howarth, 1972; Crossman & Goodeve, 1963).
There has, however, been continuing debate as to the way in which this latter phase,
supposedly mggered when with the eye fixating the target the retinal image of the hand
approaches the fovea, was subject to alteration as a result of required or desired movement
precision. Crossman and Goodeve (1963) and Keele (1968) suggested that aiming
movements consisted of an initial propulsive or "ballistic" movement which, it was
assumed, consists of some error and a second phase comprising as many error corrections
as were necessary to achieve target contact. It was also assumed that these corrections were
of a constant duration, therefore as greater precision was required or movement amplitude
increased, it was held that overall movement time would increase as a consequence of a
greater number of error corrections. The problems inherent should by now be apparent, no movement or movement segment can be truly "ballistic" if it is held to p e e d in the
absence of concurrent "feedback" of any nature, it is always likely that some facility exists
for "correction" on the basis of proprioception and/or efference copy (Goodale, 1987). If a
weaker definition of ballistic is adopted whereby it is to apply only to movements which
proceed in the absence of visual feedback, provision may be made only, at the present time,
for movements of rather less than 100 rns A second problem accompanies any suggestion that the initial propulsive phase
contains error. One ought to enquire, error with respect to what? For the organism itself,
errors can only be assessed with respect to the achievement of task goals, to suggest that
one may have an error in a movement subcomponent, regardless of the movement
outcome, by implication presupposes the existence of some recipe of action, which, if not
specifying the entire course of the action ought then to specify the "ideal" situation at that
point. As Greene (1972) forcibly argues, it is highly unlikely that such recipes exist. As if
to relieve investigators of the burden of addressing either of these issues, the studies
examining rapid aimed movements have rarely demonstrated the presence of more than one
corrective response (Carlton, 1979). In response, Keele (1981) has embraced the Howarth, 5 9
Beggs and Bowden (197 1) model, wherein, as accuracy demands are increased, movement
time is increased such that a greater proportion of the movement amplitude is covered and
the hand is brought closer to the target before corrective adjustments are made( see also
section ). In contrast, Carlton (1979) observed that modifications of required precision had
little effect upon the initial impulse, rather the increased time necessary to complete "more
difficult" tasks appeared to be due to an increase in the decelerative portion of the
movement. These observations are fundamentally in agreement with indications obtained
by Annett, Golby and Kay as long ago as 1958.
7.2 THE MULTIDIMENSIONAL CONTRIBUTION OF VISION
How arbitrary is this segmentation of aimed movements? In the following sections,
a line of reasoning will be developed which in its essence owes a great deal to the French
INSERM group of experimental neurophysiologists. The resulting perspective may
potentially reconcile apparently conflicting indications of both early and late accommodation
to increased accuracy demands. It will be suggested, following Pelisson et al. (1986) that
updated information relating to target position is conventionally utilized throughout the
entire duration of the movement. Further, aimed movements may be continuously regulated
with respect to variations in the optic array, whether these be due to target motion, target
size (associatively required precision) or the presence or absence of target, hand or ambient
illumination, and in conjunction with information from non-visual sources. This vista will
then be used as a basis on which to conduct further examination of potential asymmetries in
visually guided reaching.
Although, in most cases, the goal of a movement is defined by visual parameters,
visual information constitutes merely one input contiguous with an ongoing attempt to
match the final position of the limb to the position of a target which, although initially
defined in terms of visual space, must be referenced to egocentred, multimodality,
extrapersonal space. In what might be taken as a pragmatic approach to establishing the
invariances associated with limb movements, motor-physiologists have not only typically
restricted consideration to one degree of freedom movements (Bizzi & Morasso, 1982),
neglecting the three dimensional movement through space which characterizes our daily
lives, but have "ignored the manner in which visual information contributes to the
spatiotemporal topology of goal directed movements" (Fisk & Goodale, 1985, p. 160). On
the other extreme, and as has already been extensively catalogued, a large body of
researchers have attached paramount importance to the role of vision and in particular to
visual feedback as an error signal, and have rarely entertained consideration of non-visual
factors let alone the import of these factors to an organism which must deal with a single, 6 0
unitary external world. The role of vision is itself multidimensional, and it is this absence
of a single function for vision (such as error correction) which has provided the clearest
indication of the necessary role of non-visual factors and thus of the nature of the "system"
as a whole.
Prablanc, Eschallier, Komallis and Jeannerod (1979) demonstrated that the
relationship between movement time and the "index of difficulty" (the ratio of movement
amplitude and target size as described by Fitt's Law, see Section 8.1) characteristic of
visually aimed movements, is preserved when the movement is conducted without vision
of the reaching hand It seems therefore that the accuracy of the movement (as defined by
the index of difficulty) is governed not only by visual feedback of the hand but also by
proprioceptive information and conceivably "efference copy" associated with the motor
output. Whilst one may find it necessary to dispute that "such information must have been
compared to some internal representation of the target" (Prablanc, Pelisson & Goodale,
1986, p. 294), on the basis that it is not necessary to posit a "mental" representation, nor as
a result that a comparison takes place, it is clearly the case that visual information regarding
the position of the target has an action guiding function for movements which are themselves not seen.
With a view to extending these findings, Prablanc et al. (1986) manipulated the
availability of target information for movements which were again made in the absence of
visual feedback. Four target conditions were utilized; in the fist condition the target LED was extinguished upon movement initiation, in condition 2 the target was removed 120 ms
after completion of the initial eye saccade toward the target, this enabled the subject to make
a corrective saccade aligning the eye with target position before the target disappeared, the
third condition permitted vision of the target throughout the movement, whilst in the final
manipulation, which may be usefully contrasted with condition 2, subjects were directed to
complete their eye movement and thus obtain accurate foveal information regarding the
target before initiating a movement which caused the target to be removed Movement
accuracy, in terms of constant error, was found to increase from conditions 1 to 3, accuracy for condition 4 was between that for conditions 2 and 3. Generally then, there
was an improvement which paralleled the duration of target display. In discussing these
trends, Prablanc et al. (1986) reason thusly, " 'open loop' movements (open loop with
respect to visual feedback about the relative positions of the target and the moving limb) are
far from being uncorrected or ballistic. Indeed, the prime effect of target duration on
pointing accuracy suggests that visual information about target location is somehow used to
control the movement during its execution." (p. 300).
It also appears that the particular nature of that visual information assumes some
significance, accuracy for condition 4 in which foveal information was available was
superior to condition 1 in which it was not, yet accuracy for condition 3 for which foveal
information was present throughout the greater part of the movement was superior to
condition 4. The advantage for condition 3 over condition 2 appears to indicate that the
extra-retinal signal regarding eye position is, in this instance, not a sufficient basis on
which to guide the reaching movement (c.f., Mather & Fisk, 1985). In concluding,
Prablanc et al. (1986) speculate that ongoing corrections conducted on the basis of, visual
target information, and that which is motion related but from non-visual sources requiring
very little time, are characteristic features of normal reaching movements. The results of a
closely related study (Pelisson et al., 1986) provide converging evidence that this may
indeed be the case.
Under consideration were the nature of aiming movements, made without vision of
the responding limb, to targets which had been perturbed during eye saccades. It is a
property of the oculomotor system that initial saccades tend to undershoot the target, this
has some heuristic value, if more accurate information about target position is subsequently
made available, it is better to have underestimated than overestimated, particularly with
respect to the amplitude of a required movement (Prablanc et al., 1986). The initial saccade
is rapidly followed by a second which brings the target into foveal vision. Generally, if a
target if a target is displaced by a few degrees of visual angle or about 10% of saccade
amplitude during the first saccade, the movement of the stimulus is not available to
conscious perception. It appears that the perceptual system "assumes" that the discrepancy
has arisen from an inaccurate first saccade. Pelisson et al. (1986) induced target
displacements on randomly determined trials, and in a manner consistent with previous
investigations (e.g., Mackay, 1970; Brooks & Fuchs, 1975; Matin, 1982), subjects were
unable to identify the trials on which perturbations occurred. It is a further characteristic of
responses made to targets presented in the visual periphery that EMG onset in the brachial
and ocular musculature is virtually simultaneous (Biguer, Jeannerod & Prablanc, 1982).
Although initiated together, the initial saccadic eye movement is the first completed as the
eye has smaller inertial forces to overcome. Therefore, it is the case that displacements of
the visual target during or after the initial saccade necessarily occur after the response
movement has been initiated. The most significant results obtained by Pelisson et al. were
that there were no differences in localization errors between trials on which the target had
been displaced and those on which the target had remained stationary.
As with the experiment conducted by Prablanc et al. (1986), it appears the case that
modifications of the response movement could be accomplished in the absence of visual 6 2
information relating the relative positions of the limb and the target. The results, in addition
appear to rest comfortably with those obtained by Megaw (1974), subjects were able to
successfully modify ongoing movements when targets were displaced by a few degrees
within 100 ms of movement initiation.
In the Pelisson et al. (1986) study, the responses were subjected to a kinematic
analysis, however. Most significantly, the modifications of the reaching movements
manifest in the accuracy measures were not revealed as inflection points in the velocity
profile. Acknowledging that it is in practice impossible to distinguish between early discrete
mmcations which have been filtered by the inertial properties of the limb, and
"continuous control", the authors argue in favour of a " 'pseudo-continuous' corrective
mechanism acting upon the hand trajectory: the new target position information is not only
used near the end of the movement but earlier as well" (p. 3 10).
It is also of particular interest to note that the mechanisms responsible for the
regulation of the motor response were somewhat more acuitous than the perceptual
equivalents, individuals were never aware of the change in target position. Converging
evidence that there exists a plurality of visuomotor mechanisms is provided by patients
suffering hernianopia, and having lost vision for one half of the visual field usually
following a unilateral lesion within the visual cortex. These individuals are generally never
consciously aware of visual stimuli presented to the hemifield opposite to the lesion or on
occasions to that side of extrapersonal space. The presence of some tangible sub-cortical
visual functions has been revealed through having subjects guess as to the locations of
visual stimuli (Jeannerod & Biguer, 1982). When target stimuli are presented in a
purportedly "blind" area, hand pointing (Weiskrantz, Warrington, Sanders & Marshall,
1974; Perenin & Jeannerod, 1975) and eye movements (Poeppel, Held & Frost, 1973)
revealed that individuals were able to locate that of which they were not consciously aware.
The same phenomenon has been observed by Perenin and Jeannerod (1978), patients made
movements toward targets presented in the blind hemifield which, in terms of accuracy,
were equivalent to those made by controls.
7.3 THE CONTRIBUTIONS OF CENTRAL AND PERIPHERAL VISION
Whilst all aimed reaching movements are in some sense goal directed, ultimately
each movement will be directed towards some particular aspect of the visual world,
whether this is an object to be lifted and placed or simply a spot which must be contacted.
The highly function specific termination to many reaching movements has lead a number of
theorists, most notably Trevarthen (1968) and Schneider (1969) to propose that there exist
at least two segregated visuomotor pathways, subserving "shape" and "spatial location", 6 3
what in Paillard's (1971) expressions are "l'espace des lieux" and "l'espace des formes".
For the present purposes, consideration of modifications of the reach which are sensitive to
the intrinsic properties of the target object, such as weight or shape, (c.f., Marteniuk,
Leavitt & MacKenzie, 1987) will be simplified to consider merely those visual mechanisms
responsible for reasonably precise guidance of a single finger or hand held stylus to a
"punctiform" of point target. 'The "classical" perspective (e.g., Keele & Posner, 1968), as
has been extensively outlined, holds that most reaching movements are too rapid to permit
utilization of visual feedback in anything but the final portion of the deceleration phase.
Whilst visual feedback may certainly be mediated more rapidly than initially presumed,
there is clearly also some initial portion of the movement during which visual feedback
cannot be used as a basis for discrete modifications. As revealed however, the nature of the
visual information available during this period has profound effects upon the movement
outcome. The high velocity and segmental pattern of muscle activation during this initial,
otherwise termed ballistic, portion of the movement has extensively been viewed as a
reflection of preprogrammed unmodifiable motor commands (c.f., Glencross, 1977). Yet it
does appear that the initial phases of "error correction" are based upon "central monitoring
of the commands for movement", and may take the form of suppression of muscular
activity prior to overt manifestations of motion in the movement which is ostensibly being
corrected (Cooke & Diggles, 1984).
What is seemingly an independence of visual reafference and a clearly modifiable
progression may potentially be reconciled if one accepts the tenet of Jeannerod and Biguer
(1982) that at least some portion of the visual information available during this portion of
the movement enters the "space channel". It is suggested that not only are "space" and
"object" channels subserved by specific areas of the brain, but also that the motor output
with which these channels are intimately associated is similarly specialized and distinct.
Thus, 'space' channels have a completely different function: to match the final position of the moving limb with the position of the target within extrapersonal space. For this purpose, central visual processing can be limited to the computation of a set of spatial coordinates establishing the location of the object with respect to the body. This requires that the central 'map' used to relate each point of the visual field to the visual system encode body-centred coordinates, rather than retinal coordinates only. Both the position of the eye with respect to the head and the position of the head with respect to the body must be taken into account in determining the direction of the arm movement. In addition,eye, head, and body movements may themselves become part of the act of reaching when it is directed at an object within a peripheral part of the visual field (Jeannerod & Biguer, 1982, p. 388).
This latent computational approach does not seem to be the most assuring basis on
which to proceed, as it stresses the motor or action guiding function of internal
representations rather than of perception per se, nonetheless it does contain elements which
are germane. Certainly, it does appear that central and peripheral vision do provide some
what distinct contributions to the regulation of aimed hand movements. Paillard (1982) has
ex~ressed his conviction that. I
to some extek peripheral vision can be regard ed... in charge of the transport of the hand form its initial position toward the target with the computing of the appropriate trajectory, and central vision might be considered as providing ... the cues necessary to achieve the precise and smooth landing of the hand on the target (p. 367).
One may perhaps dismiss the suggestion that one "appropriate trajectory" is subject to
computation, clearly there are an infinite number of movement trajectories which may lead
to the satisfactory accomplishment of the task goal, it is not even a requirement that they all
have the same end point, as in this conceptualization, "precision" and "smooth landing"
itself variable is superimposed upon the movement trajectory. Neither is it apparent that any
"computation" as such need occur. However, there is evidence that central and peripheral
vision do assume distinctive roles within the regulation of aimed movements.
Paillard's interpretation has inherent, the assumption that the vision of the limb,
which has been shown to be useful when provided only during the initial phases of the
movement, is processed only in the peripheral visual field Conti and Beaubaton (1976)
manipulated conditions such that visual feedback could be provided through various
portions of the movement, such as the first third or the second half of the trajectory. The
salient findings were indeed that visual feedback, even when provided during the initial
phases of a movement leads to an improvement in performance. This is assumed to be
vision of the trajectory which should be distinguished from vision of the target, for
example with respect to target motion. There is also converging evidence which indicates
that adaption to prismatic disturbance of vision may be subserved by distinct mechanisms
for the restoration of order to peripheral and central vision respectively (Brouchon-Viton &
Jordon, 1978). In completing his (1982) synopsis, Paillard reasons that movement cues,
and in particular "self moving stimuli" or vision of ones own body segments, are
predominantly processed in the periphery and are intimately associated with motion relative
to a visual axis established by foveal grasp of the target (c.f., Paillard, 1980). It is this
relativity to both the visual axis defining visual space and to the motion of the body, which
as has been stressed, must be with reference to egocentric and non-modality specific space,
which suggests that peripheral vision is allied to the regulation of the movement trajectory,
at least in its initial stages.
CHAPTER 8
ASYMMETRIES OF RAPID AIMED MOVEMENTS
8.1 THE ROLE OF FEEDBACK PROCESSING
One might enquire how the functional characteristics of the visual guidance
mechanisms may also be related to asymmetries in the regulation of movement. The
observation that a reaching movement is perturbed when concurrent vision from the
peripheral field is removed was made by Trevarthen as long ago as 1974. Significantly, his
results indicated that : peripheral vision of the movement of either arm is governed, at least in the commissurotomy subject, more by the right hemisphere, while coincidence of eye and hand in fmation of the point target is governed more from the left hemisphere (p. 253).
As noted, one traditional perspective emphasizes that manual symmetries are a function of
the differential efficiencies with which visual feedback is processed. In particular, the
preferred hand is ostensibly associated with a neural substrate which may more effectively
use this visual feedback to effect error corrections apparently required for accurate
responses (e.g., Doane & Todor, 1978; Flowers, 1975; Todor & Doane, 1978). With
respect to this approach, it has been customary to link the performance of each hand with
the assumed processing characteristics of the contralateral hemisphere. In terms of the
dichotomy presently favoured by theorists of motor control (e.g., Todor & Doane, 1978),
the left hemisphere is allegedly superior for the processing of information in a sequential
manner, whilst the right hemisphere can deal more effectively with the parallel processing
of information (c.f., Cohen, 1973).
Although this dichotomy of function has a certain intuitive appeal, the left
hemisphere and thus the right hand will demonstrate a superiority in dealing with the
sequential input of sensory feedback which is supposedly characteristic of certain aimed
movements, the situation is a good deal more complex than this characterization might
suggest. As outlined in section 3.2, the notion of dichotomous hemispheric specialization is
some way from being accurate or indeed pragmatic and, as revealed in the preceding
section, the role of visual information in the regulation of movement is itself highly
complex and very much dependent upon task constraints.
Consideration will be given to a series of experiments instigated initially by Flowers
(1975), all of which have in common highly constraining initial assumptions, both implicit
and explicit, as to the nature of both the integration of visual information and the
asymmetrical organization of the nervous system. These studies are worthy of
reexamination, as it is through consideration of their specific task parameters that the results
of these studies may be accommodated within the body of literature already discussed.
Flowers (1975) hypothesized that there existed during aimed movements a "corrective
mode of control". It was further held that the use of the Fitts reciprocal tapping task
permitted the appropriate manipulation of the required level of control and that, if between
hand were observed to vary as a function of these demands, it could be concluded that: "the
essential dexterity difference between the preferred and non-preferred hands is in the
sensory or feedback control of movements rather than in motor function per se" (p. 39).
It should be noted that the "Fitts Tapping Task" differs si@icantly from those considered
in section 6.2. It is a central tenet of the Fitts task that the requirement for the use of
feedback may be manipulated through the modification of very specific task parameters
(c.f., Peters, 1980). In Fitts' "classic" exposition, it was postulated that the number of
possible alternatives which could be made "correctly" for any given response determines
what is regarded as the level of "precision". Using a small target, relatively few responses
would suffice as correct, whereas with a larger target, a number of responses drawn from a set of similar movements would be scored as correct. Similarly, as the amplitude between
successive targets is manipulated, the precision covaries. The two measures were combined
in producing an index of difficulty (ID) such that:
(1) Id = log2 2 x am~litude of movement (bits)
width of the target
Within the framework assembled within an information processing perspective,
Fitts (1954) suggested that the speed with which movements were made was governed by
the capacity limits of an individual's motor system and by the "information" required to
make any one movement. The above relationship can alternatively be expressed in terms of
movement time, as follows:
where A is the amplitude of the movement, W is target width, and the constants a
and b are obtained empirically.
There has generally followed, the implicit assumption that for cases in which the
amplitude between, and size of targets is futed, the resultant speed of movement may be
taken as some measure of the capacity of the "hand system" to make the response (e.g.,
Flowers, 1975). It was proposed that at low ID values, individuals are responding with 6 7
ballistic movements, by which the intended meaning is presumably unmediated by sensory
feedback, whilst for more "difficult" combinations it was necessary for subjects to adopt
the use of some form of corrective procedure.
The problems inherent in a distinction between ballistic and non-ballistic (in
circumstances in which normal vision is maintained) are overlooked by (Flowers, 1975),
for whom the data decides. Statistically significant differences between the hands, in terms
of movement duration and proportion of target misses, were observed at ID values 4 and
above, for subjects classified as strongly lateralized. Flowers (1975) was led to conclude
that the preferred hand advantage for "non-ballistic" movements was due to the "lower rate
of information transmission" for the non-preferred hand, which would presumably then be
revealed on any task permitting rapid processing of concurrent sensory feedback and thus
all movements. That these experiments failed to elicit indications of asymmetries in
movements having ID values smaller than 4, and for simple tapping (which is clearly in
itself in conflict with a considerable body of research) is perhaps indicative of the relative
insensitivity of the response measures employed. Indeed, it is notable that Flowers (1975)
concludes that the data are in agreement with the findings of Woodworth (1899) in that "left
hand movements become ballistic if they take 750 ms or less, while right hand ones only
become ballistic at 400 ms" (p. 50). In a partial replication of the Flowers study, Todor and
Doane (1978) failed to obtain right hand superiority in conditions apparently requiring
greater feedback control. Performance was assessed in terms of hits per ten second
interval, a measure which is similarly impoverished, providing few indications of the
manner in which the hands differ.
8.2 THREERECENTSTUDIES
Perhaps the most adequate examination of the proposal that asymmetries are due to
differential efficiency of processing feedback information was that conducted by Todor and
Cisneros (1985). Employed were single aiming movements, again within a Fitts-type
paradigm, for which it was possible to manipulate required precision. The use of an
accelerometer and subsequent identification of acceleration changes led the authors to
partition the movement into four specific phases. These were time to peak positive
acceleration (TI), time h m T1 to acceleration reversal ( 7 3 , time from T2 to peak deceleration (T3, and time h m T3 to target contact (T4). Todor and Cisneros suggest that
aiming movements of this nature, with durations greater than 400 ms may be partitioned
into further distinct stages. Uncontroversially these are a distance covering phase, and a
"homing in" or "error correction" phase corresponding to T4.
The results appear to indicate that the largest hand differences were exhibited in the
latter phase, these in turn increased in magnitude with the demand for precision. This, the
authors believe, demonstrates an enhanced ability for the preferred hand in executing error
corrections. It should be noted however that there was a right hand advantage for the final
portion of the distance covering phase. This portion, T3, was, presumably, what the
authors would have regarded as a programmed aspect of the movement. Thus it appears
that the asymmetry is not entirely associated with differing facilities for error correction in
the T4 stage, since the preferred hand exhibits some form of advantage prior to this point.
It is perhaps unfortunate that there was no means of assessing the variability of
movement in the initial phases TI, 7'2 and T3. Todor and Cisneros (1985) performed more
detailed analysis of the movements themselves which indicated that when greater accuracy
was required, the hand was brought closer to the target before correction was made. This
was generally correlated with a higher initial velocity maintained for a longer period of
time. Whilst these observations would initially appear to conflict with the predictions based
on the Schmidt, Zelaznlk, Hawkins, Frank and Quim (1979) model of output variability,
there is no reason to suppose that the pattern does not reflect an initial superiority of the
preferred hand. The evidence from the T3 stage appears to suggest such a superiority, and
indeed it seems counterintuitive to suggest that the advantage apparently enjoyed by the
right hand for making corrective movements arises as a result of a less satisfactory initial
movement. It does rather appear that the adoption of a two stage characterization serves to
obscure rather than to illuminate.
Turning to even more recent evidence, Roy and Elliott (1986) employed a pointing
task for which a range of movement times were obtained. Dependent measures were a 'self
paced' movement time measure 'movement time', and 'radial error', consisting of
amplitude error (i.e., error in the direction of the target) and directional error (i.e., error
perpendicular to amplitude error). Both directional and amplitude error can themselves be
expressed in terms of constant and variable error.
The rationale appears to have been that with increased movement speed, and thus
reduced movement time, there would be less visual information available upon which to
base corrections. There is an obvious problem with this, in that, shorter movement times
are likely to be associated with higher initial velocities resulting from greater force
production. This may in itself result in a decremental effect on performance which is
expressed asymmetrically, for example as a consequence of the relative inability of the
non-preferred hand to achieve precision of force modulation. This has, on occasions, been
assumed to be an aspect of programming. Roy and Elliott predicted that examination of the
speed-accuracy trade-off function should reveal a steeper slope for the left hand as a result 6 9
of its reduced efficiency of processing visual information. However, any such difference in
the slope of the function could equally well have been said to reflect the increasing effects
of force variability as movement speed increases.
A second level of manipulation was employed whereby ambient lighting was
removed upon presentation of the stimulus. This was assumed and indeed appears to be a
more direct means of manipulating the amount of visual information which is available
during movement. The authors again predict that removal of visual information should
affect right hand performance to a greater degree than left hand performance which, in line
with Todor and Dome (1978), they suppose is dependent on a preprogrammed mode of
control.
In broader terms, it seems implausible to suggest that the movements of each hand
are conducted on bases which are qualitatively different. This is particularly so in the
context of conventional movements in normals for whom "information" is available to both
hemispheres on what is essentially a continual basis. Certainly, it may be the case that one
hemisphere will demonstrate relative benefit for the processing of material of a given
nature, but this will in itself be a matter of degree. It does not seem to be the case that the
movements of the left hand should depend only upon a preprogrammed mode of control.
As noted in section 3.1, it is widely held that hemispheric asymmetries might best be
considered a continuum rather than as some bipolar system. In contrast to the view
forwarded by Roy and Elliott (1986), a more comprehensive model would lead to an
alternative way of evaluating these effects, specifically, that the removal of visual feedback
would reduce the relative level of enhancement for the preferred hand. Though it must also
be assumed that in many circumstances, for example those for which the necessity of
dealing with complex spatial relationships does not exist, the right hand will already exhibit
a "baseline" superiority, regardless of visual feedback processing demands, due to what
has previously been regarded as a presumed greater precision of force modulation.
The results obtained by Roy and Elliott (1986), in terms of radial error, c o n f m
expectations for this task that the right hand was more accurate than the left, that accuracy
increased from shortest to longest movement times and that performances when ambient
lighting was present were superior to those when it was removed. As movement speed
increased, there was a relatively greater decrement in accuracy for the non-preferred hand.
In terms of the speed accuracy function, the left hand had a "steeper negative slope".
Significantly , however, the difference in slope between the two hands did not vary
between illumination conditions, indicating that the differences were perhaps not due to
variations in the efficiency with which the visual information was processed. There was, as
noted, an effect of illumination per se, this appears to suggest the almost equal expression 7 0
of this effect over both hands. This is unexpected, though it is possible, as Roy and Elliott
(1986) themselves indicate, that the dependent measure radial error, which is a composite
of both constant error and variable error, was not entirely appropriate for these purposes. It
is conceivable that the composite measure obscured a right-hand advantage in terms of
variable error for movement execution.
With a view to examining the issue further, Roy and Elliott (1986) conducted a
further series of experiments in which subjects were required to conduct movements in less
than 200 ms This was assumed to preclude the possibility of visual corrections during
movements, though as considered in the previous chapter, visual feedback "loops" may
operate over latencies much shurter than 200 ms. Dependent measures were, in this series,
reaction time, movement time and radial error. With respect to the reaction times obtained,
it is notable that a feedback by hand interaction was obtained. Subjects required less time to
program left hand movements in the illuminated condition than in the other three
permutations. This is interesting and may reflect the enhanced ability of the right
hemisphere to integrate and assimilate the spatial relationships associated with the task, a
difference which would presumably not be evidenced for the non-illuminated condition.
Movement time analysis indicated a main effect for hand and a hand by feedback
interaction. This effect was also evident in the initial experiment but failed to reach
statistical sipficance. Specifically, the right hand advantage in movement time was
greatest in the illuminated condition. This would appear very much in line with suggestions
that the left hemisphere exerts its superiority for the processing of feedback information at
this stage.
In terms of radial error, a right hand advantage was again observed, though there
was an absence of any interactions. Again the compound nature of this measure may have
obscured a number of effects. Of course, the right hand advantage on an overall measure of
pointing accuracy is not unanticipated. Roy and Elliott (1986) suppose that the right hand
advantage observed in this instance could not have been due to differential feedback
processing as there was no significant difference in pointing accuracy between the two
illumination conditions. The difference between illumination conditions, although not
statistically significant, was in the direction anticipated.
In view of the hand by illumination interaction for movement time, one would have
to question the use of the compound radial error as the final arbiter of this issue. In
addition, it is possible that some of the between hand differences could be accounted for in
terms of non-visual feedback processing, once more assumed to proceed in latencies of less
than 200 ms Rather than excluding processes mediated by feedback, as the authors
suggest, the evidence appears to indicate that both types of processes may be occurring. 7 1
Although in this study it was not possible to examine the decomposed error scores, the data
are not incompatible with a global model which emphasizes the plasticity of the contribution
made by visual information, and as such considers as a primary determinant of the
asymmetrical manifestation of this contribution, the task constraints associated with various
forms of goal directed action.
In a follow up of the 1986 study, Roy and Elliott gave further consideration to the
relative contributions of, inherent differences in the variability of force output, and possible
variations in the utilization of visual feedback Roy and Elliott (in press) viewed anchoring
of movement time for movements of various amplitudes as a way of varying the force
required to complete the movement and thus a means of examining between hand
differences in force variability.
As noted previously, the use of target movement times is probably more
controversial than the more usual yet questionable use of target latency bandwidths. Setting
a task goal which is incompatible with other task constraints may well induce the use of
strategies resulting in variation in the type of movement made in each case. Thus, different
movements are being made rather than what is essentially the same movement differing
only in the amount of force initially produced. Obviously behavioural indices such as error
scores can provide no assurance that the movements are indeed equivalent as these
constitute the dependent measures of primary interest. In the absence of ratification of the
equivalence of response profiles through the use of, potentially, kinematic analyses, one
cannot distinguish between variations in accuracy which arise as a consequence of
variegated initial production of force or from movements which are, in essence dissimilar.
Roy and Elliott considered that "long" and "short" movements made from two
home positions, 25 and 35 cm from a single target, constituted an adequate manipulation of
required force, when subjects had been "trained" to complete the movements within time
ranges of 150 to 249 ms, 250 to 349 ms and 350 to 449 ms As the authors themselves
point out, the use of a single target and a single starting location in the 1986 study
represents a considerable limitation. It is indeed likely that this lead to "less reliance on
visual information" (Roy & Elliott, in press). Nonetheless, it hardly seems that the use of
two spatially distinct starting locations is a great improvement. There are some indications
that when sets of a very few targets are employed, superimposed manipulations such as the
removal of visual feedback have substantially reduced effects (c.f., Guiard, Diaz &
Beaubaton, 1983). The use of a single target position is in itself problematic, the concept of
"equifinality", that a movement pattern can be reached "reproducibly" from a variety of
initial conditions, is a pivotal assumption in contemporary approaches to the study of motor
control (Kay, Kelso, Saltzman & Schoner, 1987). If this principle is upheld, one must
consequently question the legitimacy of second order manipulations such as that of
illumination.
In the study itself, three conditions relating to visual feedback were utilized, ambient
lighting removed upon movement initiation, ambient lighting removed 10 sec. prior to
movement initiation, and ambient lighting present throughout the movement. Of import
with regard to the suggestion that the hands differ in terms of the variability of force output
is the measure of variable error. The results in this instance indicated that there was no hand
by movement distance interaction for this measure. Thus, if one assumes that movement
distance was indeed an indicator of required force, one must similarly conclude that there
was no difference in the variability of force production between the hands. Perhaps it is in
this regard that the significance of using a single target with two starting positions should
be viewed, rather than with respect to the use of visual information. That is, the use of a
single target location may have served to depress what were, potentially, between hand
differences in variability.
Examination of constant error perpendicular to the direction of movement indicated
that a three way interaction of hand, vision and distance had occurred Although it seems
that the trend was for right hand movements to be biased to the left, and for left hand
movements to be biased to the right, when ambient lighting was removed, no indication of
the trends in the absolute variation in this error was provided, thus it is difficult to establish
whether the "performance" of each hand was differentially affected in a summary fashion.
Variable error perpendicular to the direction of motion was affected by visual condition in a
manner similar to variable error in the direction of motion.
It is interesting to note that although robust variations in variable error were
obtained as a result of manipulating visual information, there was no concomitant effect
upon movement time. This is unusual. Although the view that discrete corrections based
upon visual information take a constant amount of time, has all but dissolved (see section
7. I), it has generally been observed that the removal of visual information, when this is
assumed to be acting as some substrate for the modification of ongoing movement, is
accompanied by an decrease in movement time.This was not the case in the Roy and Elliott
(in press) study.
The authors themselves suggest that the consequences of the 10 sec. delay interval,
in terms of accuracy, may have been due to the deterioration of a "visual representation".
They also perhaps fail to appreciate that this could also have been the case in the condition
in which vision was removed upon movement initiation. When a 10 sec. delay was
present, visual information regarding the target was absent for 10 seconds plus the duration
of the movement. When movement initiation triggered the removal of ambient lighting, 7 3
visual information of the target was absent for the duration of the movement as the target
was not illuminated independently. Thus the effects of the visual condition could have been
due to a bilateral deterioration of the information relating to target position in extrapersonal
space over the interval between the last instant at which target information was available
and movement completion, rather than the removal of the information upon which
corrections are made, which is that process which may proceed asymmetrically.
The importance of considering the delays between the instants at which information
upon which "successful" movement, in part, depends is last available, and the execution of
the movement itself, whether this interval be "filled" or "unfilled", has been highlighted by
Carson (1987b). It does appear that for the movements considered by Roy and Elliott (in
press), completion did not only rely upon the presence of visual information during
execution, particularly when a single target position was employed and movements were
essentially two-dimensional, that is the plane on which the target was located was the plane
on which the movement started. As Roy and Elliott (in press) recount, the differences in
accuracy between the traditional "lights-on" and "lights-off' conditions were small even for
the longer movement times, and diminished as movement times decreased. In line with the
suggestion outlined above, for shorter movement times, the difference in the duration for
which target information was available between movement initiation and completion
represented a smaller difference between visual conditions than for longer movement times.
The seemingly striking effects of the visual information manipulation, most evident for the
10 sec. delay condition could, in all cases, be viewed as being due to a deterioration of
information relating to target position over time. Therefore the experiment does not
constitute an adequate test of the hypothesis that manual asymmetries are not due to
dissimilarities in output variability, or rather that they are due to the ability of the "right
hand system" to more accurately modulate force (Roy & Elliott, in press).
In a second reported experiment, ostensibly conducted as a means of further
examining this hypothesis, Roy and Elliott had subjects make aimed movements as rapidly
as possible, assuming that this would increase any differences which existed between the
hands in terms of force variability. This experiment also differed in that four additional
starting positions were used, which allowed movements with either hand to be initiated
from either the right or the left of the midline. It is clear however, that as a single target
location was again used for all conditions, it was merely the starting position which was
varied. Therefore, the problems associated with the use of planar movements to a single
target location were not alleviated. Roy and Elliott obtained no indications of a hand by
movement length interaction which, in their terms, would have provided evidence
pertaining to a force variability interpretation, though perhaps in this instance, examination 7 4
of the variation by movement time (i.e. using post grouped movement times as an
independent variable) for a given movement length would have provided a more
appropriate test of this issue. In the absence of confirmatory kinematic profiles, one may
only assume that movements made over the same movement length are equivalent, however
it would seem reasonable to conclude that shorter movement times are concomitant with
greater initial production of force.
There was one notable deviation from the results obtained for experiment 1, in that
for constant error in the direction of movement, something of a hand by visual condition
was obtained. Both hands had a tendency to undershoot the target, which for the right hand
was equivalent across all visual conditions, and for the left hand similar for the vision and
no-vision conditions but greater to a statistically degree when there was a 10 sec.
premovement delay. Roy and Elliott (in press) feel this suggests that "when using the left
hand, subjects do not encode as clear a representation of target location, or they have less
access to this representation, or this representation decays more rapidly" (p. 19). If there is
indeed some visual representation of target position (itself unlikely) which is subject to
decay, the advantage of the right hand seems remarkable and unexpected in light of the
"known" superiority of the right hemisphere with respect to "spatial information". There
seems no reason to believe that the information pertaining to target position, initially
present, is not made available to each hemisphere to an equivalent degree. There can be
little dispute that the salient "information" is subject to decay over relatively short intervals,
however the decay is probably not modality specific (Carson, 1987b). If Roy and Elliott
have in mind some visual representation of an iconic nature (e.g., Sperling, 1960), it is
highly unlikely that such "storage" will persist for anythmg longer than a second after the
display has been terminated. If the decay was of a "representation" of a target in space, one
would assume that the deterioration would not be directionally specific, yet, no hand by
visual condition interaction was obtained for constant error perpendicular to the direction of motion. As the alteration in the magnitude of constant error was specific to the direction of
movement and was an extension of a bias already existing, one is led conclude that the
deterioration, in this instance, may be more closely associated with the motor output "side"
of a perceiving-acting continuum. Certainly, the suggestion that the left-hand / right-
hemisphere may less clearly "encode a representation of target location" is less than
convincing.
CHAPTER 9
THE SIGNIFICANCE OF SPATIALITY IN MOVEMENT
9.1 ARGUMENTS FOR ECOLOGICAL VALIDITY
One might do well to again consider that the superiority of the right hemisphere for
the manipulation of spatial relationships is virtually uncontested. Yet, remarkably little
consideration has been given to the implications of this with respect to motor activity. In the
final section of this review, the role of the right hemisphere in mediating purposeful, goal
directed action will be examined. The perspective to be adopted leads necessady to a
highly task orientated view of manual asymmetries and to an accompanying and continuing
rejection of the force variability / feedback processing dichotomy.
It is clear that the studies which have been conducted thus far have been noticeably
impoverished with regard to the complexity of the spatial relationships with which subjects
have had to deal. Often the movements have been two dimensional, in the sense that the
target and starting position lie in the same plane, any movement into the third dimension
may, in addition, be described in terms of a second plane perpendicular to the targetlstarting
position surface. The "tapping" movements typical of the Fitts' paradigm are perhaps the
clearest example of this type of restricted movement. In any attempt to provide a complete
account of manual asymmetries, and as Guiard et al. (1983, p. 11 1) point out, "one should
turn to more adequate tasks from the right hemisphere's viewpoint".
The "more adequate" task employed by Guiard et al. was a fast unimanual
movement to a visually presented target, located in what was assumed to be body centred
space, and for which vision of the movement itself was obscured. Guiard et al. were
probably misguided in considering their movements to be open loop and ballistic, clearly
this could not have been the case. The actual motion required was a form of reaching,
requiring motion in three dimensions, certainly it could be described as progression into
space. Their interpretation of results rested heavily upon the assumption (Granit, 1972;
Paillard & Brouchon, 1974) that constant error (i.e., averaged, signed, measurement
error) may be taken as indicative of the accuracy of "central programming", reaction time
then being considered an index of the speed of programming. Variable error (i.e., standard
deviation of scores about the constant error) is assumed to reflect the accuracy of motor
program execution, whilst movement time is the rapidity with which this is accomplished.
The results obtained by Guiard et al. (1983) indicated a superiority in terms of
constant error for movements made by the left hand and for targets presented to the left
hemi-field. The authors attribute these effects to "the superiority of the left hand in 7 6
movement programming" (p. 113), in the absence of concurrent visual feedback. Although
not reaching statistical significance, the data obtained for movement time and variable error
indicate a trend toward a right hand advantage. In terms of the interpretation of error scores
favoured by Guiard et al., this suggests a superiority of the right hand for movement
execution.
Carson (1987a) was perhaps being naive in suggesting that these findings can most
easily be reconciled if one assumes that the superiority of the right hemisphere for
manipulating spatial relationships exerts its effects during "programming" of the
movement, whilst the assumed left hemisphere advantage for sequentially processing
feedback information becomes salient during movement execution. It is obviously not the
case that the influence of visual information can be parcelled out in this fashion. One might
ponder, however, whether that which is assumed to be a left hemisphere advantage for
processing feedback may be induced in circumstances in which sensory feedback is
diminished, in this case through the absence of vision of the responding limb. The trend
toward a right hand advantage which was evidenced for movement time and variable error
may have arisen as a result of the processing of information from non-visual sources. It has
elsewhere been noted (Paillard & Brouchon, 1974) that in the appropriate circumstances,
proprioceptive information arising from active positioning of a target finger might actually
lead to performance superior to that when visual information was provided.
9.2 CONVERGING EVIDENCE
Some further support for the view that manual asymmetries, in particular as an
expression of right hemisphere characteristics, may be seen across modalities is provided
by the work of Roy and MacKenzie (1978), in which examination was made of
asymmetries in the "blind" reproduction of spatial positioning movements. Using a thumb
positioning task, in what was termed the "kinesthetic modality", the authors were able to
elicit a marked left hand advantage in terms of constant error scores. Again this would seem
to emphasise that the right hemisphere advantage for localizing positions in space, exerts its
most obvious effects in terms of "processes" reflected in constant error scores. It should
also be noted, however, that a left hand advantage for variable error was also present. This
would initially appear to conflict with any model which predicts a right hand advantage,
expressed in terms of variable error, as a consequence of a left hemisphere role in the
concurrent processing of sensory feedback. However, the movements employed by Roy
and MacKenzie were "slow positioning" for which accuracy was the sole criterion. With no
requirement for speed, it is likely that the presumed superiority of the "right hand system",
for the processing of contiguous sensory feedback, was not exhibited to the same extent. 7 7
Examination of the effects of limiting the presentation of visual information to a
single visual field, and thus initially to one hemisphere, has been greatly enhanced through
the use of innovative procedures, such as the use of suitably occluded contact lens (Sivak,
Sivak & MacKenzie, 1985). In this manner, it has been possible to examine, in normal
subjects, the consequences of unilateral presentation of information over the course of an
extended movement. MacKenzie, Sivak and Elliott (1987) had subjects make relatively
slow positioning movements, with an unseen responding hand, to continuously displayed
target lights. All movements were made under monocular conditions, that is, one eye was
occluded. Through the use of appropriately positioned and partitioned contact lens', it was
possible to ensure that all but peripheral vision from a single visual field was effectively
blocked In a control condition, unrestricted central and peripheral vision of the entire
visual field was permitted.
Subjects were required to move an unseen knob to the perceived position of the
target light. The movement itself was highly constrained. The knob moved over a surface
directly beneath the target display board, and was free to move only in two dimensions.
Error measures were computed on the basis of the discrepancy between the XY coordinates
of the final position of the knob and the position of the target light. Constant error scores
were calculated in the direction of movement, and perpendicular to the direction of
movement. A measure of radial error was also assessed. As there were only two trials for each unique combination of conditions, there was no practical means of providing a
measure of variable error.
The most notable characteristics of the results were that directional constant errors
were of greater magnitude for the left hand when movements were made into both the left
and right visual fields. An overall tendency to overshoot the target seemed to be
independent of the hand making the response. In striking contrast however, subjects were
appreciably more accurate when making responses to the left section of the target board,
and when visual information was projected solely to the left visual field. This effect is again
consistent with the large body of research which suggests that there exists a right
hemisphere advantage for the localization of targets in extrapersonal space (e.g., Semmes,
Weinstein, Ghent & Teuber, 1963; Grusser, 1986; Hannay, Varney & Benton, 1976). The
trend seemed unaffected by the hand making the response (MacKenzie et al. 1987).
Also worthy of comment is the finding that localization accuracy did not vary
between the control and vision occluded conditions. Thus it appears that the availability of
foveal information contributed little to terminal accuracy in this task. This is not entirely
surprising, given that, vision of the responding hand was not available, therefore there was
no opportunity to conduct correction procedures on the basis of visual information of 7 8
relative target and hand positions as the hand never entered foveal vision. The main role of
vision in this procedure, would appear to have been as an aid to the localization of the target
in extrapersonal space, which, if Paillard (1982) is correct, is accomplished primarily
through peripheral vision. It would then be of little surprise to note that the presence or
absence of foveal vision had little effect in this instance. It is this use of, what is
essentially, peripheral vision which may have contributed to the extent of the left hemi-field
and left external space advantage for target localization (c.f., Trevarthen, 1974).
The initial requirement for an estimation of the target position in extrapersonal space
may have represented a truly spatial task, as reflected by the left hemi-field advantage. The
actual execution of the locating movements was so highly constrained however (2-
dimensional movement on a planar surface) that no attenuation of a right hand advantage
would have been anticipated. In order to establish whether the right hemisphere advantage,
in terms of the localization of targets in extrapersonal space, impinges upon the
characteristics of the motor output, it is a prerequisite that tasks be considered which permit
an adequate expression of this presumed superiority (c.f., Guiard et al., 1983). As stressed
repeatedly (e.g., section 7.2), the utility of available visual information is multifaceted, and
is most clearly revealed when movements which have some ecological validity, such as
reaching and pointing, are examined. Conjunctively, the cooperative interaction of the
cerebral hemispheres is likely to be most faithfully reproduced when movements which
bear at least a passing resemblance to "natural acts" are adopted.
9.3 AN EVOLUTIONARY PERSPECTIVE
In a thought provoking review, MacNeilage, S tuddert-Kennedy and Lindblom
(1987) have called for a reconsideration of the evolution of the observed patterns of human
handedness. Drawing on an extensive collection of, previously disjointed, studies they
suggest that the evolutionary precursors of asymmetries in human upper limb action may be
detected through phylogical studies of extant subhuman primates (Glezer, 1987).
Challenging the pervasive view that the development of language is responsible for the
laterality seen exhibited in human handedness, MacNeilage et al. argue that the
development of manipulative functions lead to a subsequent left hemisphere specialization
which eventually served as the substrate for language acquisition. Central to this position is
the claim that there was a pre-existing right hemisphere specialization for the regulation of
reaching movements.
The authors suggest that the vestiges of this evolutionary progression may still be
observed in a number of species of monkey which continue to exhibit a left hand
superiority and preference for reaching, accompanied by a right hand advantage for
manipulative functions. They suggest that, in monkeys left handedness for reaching may reflect a spatiomotor specialization of the right hemisphere related to the right hemisphere's visuospatial specialization in humans ... some residue of the spatiomotor specialization we attribute to prosimians and monkeys may be present in humans (and) is suggested particularly by the study of Guiard et al. (1983) (p. 258).
It is possible that the clearest exhibition of these trends indicates that "the initial
specialization may have been primarily for the use of visual information in preprogramming
of ballistic reaching movements" (p. 259). If one considers "preprogramming" to rather be,
the use of visual information prior to the initiation of movement, and that a "ballistic" action
is one which occurs in the absence of visually based corrections when the reaching hand is
in foveal vision, this perspective may be quite readily accommodated within the task
orientated approach which has been outlined.
Pivotal in consideration of the role of the right hemisphere in the regulation of any
movement is specification of the inherent spatial element. Peters (note 1) is probably not the
first to highlight that certain activities which, although obviously containing an important
spatial element, such as writing, are performed exclusively by the preferred right hand. It
may often be the case however that "motion of the right hand typically finds its vital spatial
references in the results of motion of the left hand" (Guiard 1987, p. 277). This is a
behavioural strategy which is certainly present within an infant's first weeks. Bresard and
Bresson (1987) recount their observations that a 5-month-old child will always fmt put its
left hand onto the edge of a tray, before using the right hand to retrieve objects from its
surface. Indeed, the study of the initial reaching movements made by human neonates, a
more explicitly ontological approach, has provided converging evidence that the initial
preference may be for reaching with what will eventually become the non-preferred left
hand (MacNeilage et al., 1987; Vauclair & Fagot, 1987). That this is initially not only a left
sided preference, but indeed also a superiority in terms of "open-loop" reaching for objects,
has been revealed by de Schonen and Bresson (1984). It is this initial left handed
superiority and preference which may provide some further indication of the cooperative
role of the right hemisphere in the regulation of goal directed action.
Clearly the magnitude or, more importantly, the functional significance of this
contribution is highly dependent upon the task structure. In view of the mutual
constrainment of action and perception, the extent to which this potential is realized will be
some function of both received environmental operators, be these visual or non-visual, and
the nature of the action they afford.
CHAPTER 10
EXPERIMENT 1
10.1 r n O D U C T I O N
Having conceived of observed manual asymmetries as reflecting a multitude of
transacting factors, it is perhaps only with some difficulty that one might specify the manner in which the relative involvement of these factors contributes to the outcome of goal directed action. As has been continuously stressed, tasks may not be considered
unidimensional, other than with respect to a unitary task goal, and as such, the
multidimensional nature of the asymmetries arising from the specific behaviour of the nervous system, and indeed from the behaviour of the organism per se, might only be
adequately revealed through a multidimensional examination of task execution rather than
through the use of some global measure. As a consequence, the possibility exists that the
"direction" of asymmetries across a "collection" of "appropriate" measures need not itself
be consistent and may vary as a function of both internal and external environmental
influences and "physical" constraints upon the "system". More tangibly, through the
experimental manipulation of task constraints, it should be possible to exert an effect upon
the behaviour of the nervous system such that, in moving toward the same action goal in
every instance, the asymmetrically balance may be altered in a way which may be detected
through the use of appropriate measures. As an illustrative example, consider that by utilizing tasks, some specific
components of which apparently require the involvement of the right hemisphere to a
greater extent, it should be possible to elicit manifestations of this differential involvement
upon appropriate dependent measures. More importantly, levels of this factor ought to
produce quantitatively different effects upon the dependent measure of interest, even
though the global task structure remains essentially unaltered
Tasks which have previously been considered unitary and often, as a consequence,
to "favour" the processing characteristics of one cerebral hemisphere over the other, may
potentially be subject to intervention of this hypothetical factor of interest on a subset of a
collection of experimental measures, reflecting an influence due to that particular factor
which does not directlv transcend all levels of the responding system. For example, by altering the levels of "spatial complexity" associated with a task, the appropriate dependent
measures will presumably reflect a covarying right hemisphere involvement. That is, the
extent of, for example, a right hand "advantage" should decrease as the requirement for the
processing of "spatial material" increases. Tasks which, for their satisfactory completion, 8 1
require that individuals deal with complex spatial relationships appear then to be the most
likely candidates for inducing the required shift (c.f., Guiard et al, 1983). From the point of view of examining manual asymmetries, it is important that some
degree of interaction may be demonstrated. In the past this has been taken to mean that it is
sufficient to specify one task as "spatial" and another as "non-spatial" and as such that a dependent measure such as simple reaction time should directly reveal differential
hemispheric involvement. A problem arises however, when one attempts to use these tasks
as a basis on which to examine manual asymmetries. One must foremost ensure that a
"natural compatibility" persists. One simple approach has been to specify that a reaching
task is more spatially complex than, for example, a tapping task and associatively that the
reaching task may more favour the left hand than the right. Obviously a problem exists in
that these tasks are dissimilar in other respects, and revealed laterality effects may not justifiably be attributed to this one, though perhaps theoretically "alluring", factor of spatial complexity. Consideration of the location of a target in either two or three dimensions may
be one correlate of spatial complexity, yet the use of such a manipulation necessarily
invokes discontinuities of the response movement. Whilst highlighting again, in line with successive theorists (e.g., Gibson, 1950;
Lee & Thornson, 1982), the functional unity of the perceptual and motor systems, it is
essential that for any adequate examination of the role of "spatio-perceptual" asymmetries
with respect to motor control, spatial complexity can be manipulated independently of, for instance, target location.
That this equivalence has proved d=cult to achieve is perhaps one reason why the
issue of spatial complexity has received relatively little consideration in this regard. As
discussed, Guiard et al. (1983) employed a single task structure which, it was postulated,
required processing of a spatially complex nature. However no consideration of levels of
complexity was entertained. It is thus necessary to establish some means of equating the
motor response, whilst manipulating the complexity of the perceptual processing that an
individual must accomplish, in order to establish the desired tenninal location of that
response in extrapersonal space.
Such an approach might initially appear to place the onus of asymmetrical influence
of spatial complexity upon the initial establishment of target position, rather than in terms of
a differential influence exerted during the course of the response movement as such.
Indeed, Fisk and Goodale (1985) argue that "the kinematics of the reaching movement are
most closely related to processing differences in neural systems associated with motor
output after the spatial location of the target has been specified by other neural systems" (p.
177). However, in emphasizing the importance of the action/perception coalition (Turvey, 8 2
1977), and while continuing to maintain that particular measures may be uniquely sensitive
to effects exerted at particular locii within the nervous system, it is unlikely that something
as broadly defined as the "kinematics of movement" will be unaffected by factors relating to
the localization of a target in space. It is misleading to equate the apparent temporal
distinctiveness of external events with associated characteristics of the responding system, the process is, at least in some sense recursive (c.f., Hofstadtter, 1979).
Those tasks which have previously been employed to sample the supposed right hemisphere superiority for the manipulation of spatial relationships have conventionally been "static" rather than "kinetic". As detailed in Chapter 2, these have included intra and
inter-modal matching of patterns. Young and Ellis (1979) were able to demonstrate a left
hand, and presumed right hemisphere advantage for the tactile detection and enumeration of braille dots arranged in a complex spatial pattern, whilst no such between hand differences were evident for dots presented in "predictable" straight line patterns.
In an effort to examine a spatial task of a kinetic, and thus ecologically valid, nature
Eals (1987) gave some consideration to asymmetries in the perception of "apparent movement". The task required prediction of the final location of a dot array from a series of
brief presentations simulating a pattern transition. Individuals were required to predict the spatial array which would occupy the next position in a sequence. Response accuracy was
found to be superior for those sequences presented briefly to the left hemi-field and thus
initially to the right hemisphere. It appears that the right hemisphere demonstrates superiority both of the appreciation of an initial spatial array (Young & Ellis, 1979) and extrapolation to a spatial and temporal displacement of that array (Eals, 1987). In a related
vein, Freyd (1983) has suggested that "mental representation of movement is a fundamental
organizing principle for human perception and cognition" (p. 575) and has provided
evidence that individuals are adept at representing implied motion when static stimuli are
observed, in turn providing convergence with the "classical literature" in which apparent
motion may be perceived from what may only be a flickering of two or more stimuli
separated in time. In normal subjects, the relationship of brain lateralization to perceptual and
visuospatial capabilities is conventionally investigated through the use of brief,
tachistoscopic presentations of stimuli to either the left or right hemi-field (Eals, 1987). In
circumstances in which the duration of presentation is brief, subjects are unable to initiate
an eye movement prior to the offset of the stimulus.
The rational for using such presentations is based upon the anatomical
characteristics of the visual system, specifically that projections from the temporal
hemiretinae to the visual cortex are contralateral, whereas those from the nasal hemiretinae 8 3
are ipsilateral. The consequences of this arrangement are that stimuli presented to the Right
visual field have direct access to the Left Hemisphere and those presented to the Left hemi-
field have an equivalent relationship with the Right hemisphere (Sergent, 1983).
The assumption that cerebral processing of the stimulus related information
commences in a single hemisphere, that contralateral to the receiving hemifield, may be
justified only in circumstances in which the stimulus is presented to either the left or right
of fixation and outside of foveal vision, that is presented at least one degrees eccentric of
the point of fixation, and when stimulus duration is 150 ms or less (Sergent, 1983). If
these conditions have been met, there is some justification in concluding that "perceptual
superiority" of the contralateral hemisphere is at least partially reflected in appropriate dependent measures (Eals, 1987).
In this initial experiment, an attempt was made to examine the assumption that the localization of a target in extrapersonal space, which is itself conceived of as a process involving spatial manipulations, is accomplished in superior fashion by the right
hemisphere. As a corollary of this, it was hypothesized that the more complex the spatial
manipulations required, the greater would be the magnitude of the right hemisphere advantage. That is, some degree of interaction was anticipated.
There is of course evidence that factors relating to "spatiality" pervade all aspects of goal directed action, and are not constrained to act upon what might have previously been
termed, initial perceptual processing. Spatial compatibility effects (see Chapter 4.3) are one striking demonstration of this tendency. This study also amounted to a partial replication of the Fisk and Goodale study of 1985, in which was conducted a detailed analysis of the
spatial and temporal organization of unrestricted reaching to punctiform targets. The findings revealed that the difference in the time taken to initiate contralateral movements
was, on average, 23 ms longer than the time to initiate ipsilateral reaching movements. This
Hand by Visual Field interaction was also reproduced for movement time. although the
amplitude of the movement was equivalent for ipsilateral and contralateral reaches. Thus,
this experiment represents the first part of a preliminary attempt to examine the influence of
spatial factors (broadly defined) upon the characteristics of visually guided, goal directed
reaching.
10.2 METHODS
SUBJECTS
The subjects were eight, naive, male volunteers, each of whom were paid $10.00
for their participation. Individuals were all classified as right handed, on the basis of a
previously administered Edinburgh Handedness Inventory (Oldfield, 1971). All subjects
had normal or corrected to normal vision.
APPARATUS FOR DATA COLLECTION
The subject was seated facing a 50 cm by 50 cm display panel. The distance from
the subject's eyes to the screen was approximately 50 crn. The panel was constructed in a
fashion such that a 21 by 21 array of red LED'S (centres spaced by 2 cm in the horizontal
and vertical directions) was not normally visible behind a translucent perspex sheet.
Activation of a single LED resulted in the back projection of light onto the perspex sheet,
which was viewed by the subject as point source of light. The position of the central light
of the array represented the fixation point, and was located directly in front of the subject at
eye level.
The forefinger tip of the subject's responding hand was covered with a rubber
thimble, upon which was mounted a small membrane microswitch. Both the microswitch
and the LED array were interfaced with an Apple IIe microcomputer, in which was installed
a Mountain Hardware Apple Clock. The status of the microswitch (open or closed)
constituted a single bit input. When the thimble was placed upon the starting platform,
located on a virtual line between the centre of the display panel and the subject's midline,
40 cm from the surface of the panel and 40 cm below the fixation point, the microswitch
was in a closed position. On leaving the starting position, upon movement initiation, the
status of the switch was reversed, and remained so until the subject made contact with the
surface of the display panel.
The control program was a combination of Applesoft BASIC and 6502 assembler
language. The assembly routines were used for the low level control required to achieve
millisecond timing of LED display onsets and offsets, and of changes in switch status (c.f.,
Gardiner, Franks & Goodman, 1987).
The only illumination provided in an otherwise light free, black walled room, was a
single 100 watt lamp, located behind the display panel to reduce both shadows and
reflections upon the perspex sheet. 8 5
The figure presented below gives some indication of the arrangement of significant
components.
FIGURE 10A Schematic representation of the position of the subject relative to the display
panel.
MATERIALS
The test materials consisted of four, five point symmetrical patterns, comprising
linear, quadratic, cubic and quartic functions (Appendix A). In the experimental
application, the first four dots in the sequence were presented, subjects were required to
predict the position of the final dot. For a given collection of four patterns, the position of
the final dot was equivalent, that is the x-z coordinates (where z represents the vertical axis)
of the last dot were always the same for all patterns. Indeed the x coordinates of each
pattern were also always equivalent, referring to a positive or negative temporal
progression along the x axis. Thus, although the global characteristics of each pattern were
distinct, the differences were accounted for entirely by the values of the respective z
coordinates for dots one to four. It was the non-presented final position which constituted
the target, and as can be seen, although the pattern and presumed "complexity" were
different in each case, an appropriate response, that is a movement to the extrapolated final
position, would be identical in all instances. Eight "phantom" final positions were
employed in this experiment, four to each side of the vertical midline of the panel, and
associatively, four to each side of the horizontal midline of the panel (Appendix B). As the
subject fixated at the intersection of these axes, there was a corresponding variation of
target position by visual field. It was possible for the target sequence to appear in one of
four possible orientations, all terminating at a single final point (referenced to one of eight
positions on the board corresponding to what would have been the final dot in the
sequence). In actuality, and for a given target position, a single display orientation was
used.
In accordance with the correspondence of the final target position to the panel
reference axes, each of the eight targets could be specified in terms of; the Visual Field to
which the display was presented, left or right, the position of the target relative to the
virtual horizontal line passing through the fixation point (hereafter referred to as the
Midpoint) i-e., above or below, the position of the target relative to the virtual vertical line
passing through the futation point (hereafter referred to as the Target Eccentricity).
Two values of Target Eccentricity were employed in this experiment, the final target
position being either 60 mm or 120 mm outward from the fixation point, representing 6.84 and 13.50 degrees of visual angle respectively. With respect to the relation to the Midpoint,
targets were 6.84 degrees of visual angle above or below the fixation point. In all instances
the orientation of the patterns was such that with respect to the vertical virtual axis, patterns
terminating in a target 6.84 degrees eccentric commenced 13.50 degrees eccentric and
progressed towards the axis, whereas for targets 13.50 degrees eccentric, the first dot in
the display was 6.84 degrees eccentric, with the sequence progressing away from the axis.
Patterns were thus always displayed in the visual periphery.
Five independent variables were utilized; the Hand making the response, the Visual
Field in which the pattern was displayed, the Pattern displayed, the position of the
"phantom" target in Relation to the Midpoint, and the Target Eccentricity. A repeated
measures design was employed, with levels of Visual Field, Pattern, Relation to Midpoint 8 7
and Target Eccentricity randomized within blocks by Hand, a 2 x 2 x 4 x 2 x 2 factorial
design.
In their physical realization, pattern sequences took the form of sequential
illumination of the appropriate LED'S such that; the first light was on singly for 37.5 ms,
the first and second lights were phased, so as to appear to be on simultaneously, for 37.5 ms, the first, second and third lights were phased for 37.5 ms, and finally all four lights
were presented simultaneously for 37.5 ms. Thus the appearance was of a spatial and
temporal progression, of duration 150 ms
Subjects were given no advance indication of the nature of the patterns, other than
to be informed that the patterns were symmetrical about the third dot, though not
necessarily forming a mirror image. The subject's were also told that the order of
presentation would help them to establish the position of the final dot.
PROCEDURE
Prior to the session itself, subjects were instructed on the importance of maintaining
fixation until the presentation of the stimulus sequence. Subjects were encouraged to lift
their finger from the starting position if they became aware of making an eye movement.
This had the practical effect of terminating that particulat trial, any such trials were
subsequently repeated.
Test sessions consisted of four blocks of 96 trials, two blocks with each
responding hand, preceded by two blocks of 32 practice trials. The subject was asked to
use his index finger, upon which the thimble was placed, to point to the place on the
display panel which he felt corresponded to the position which would have been occupied
by the final dot in the sequence. Individuals were asked to respond as quickly and as
accurately as possible. Each unique combination of, the Visual Field in which the display
was presented, the Pattern displayed, the position of the target relative to the Midpoint of
the panel, and the Eccentricity of the Target, appeared on three occasions within each
block. For half of the blocks, subjects responded with their left hand and for the other half
with their right hand. The order of hand use conditions was counterbalanced across
subjects. All trials began with a microcomputer generated tone initiated on the experimenters
command. Upon subsequent closure of the thimble microswitch on the starting platform,
the central fixation light was illuminated. Subjects were required to fixate upon this position
until, after a variable interval (of between 500 and 3000 ms) the fixation was extinguished.
Simultaneously the onset of the pattem display was initiated. Random foreperiods reduce 8 8
the probability of anticipatory responses and ensure against eye movements which have
been timed to coincide with stimulus onset. A trial was terminated when the subject
completed his movement to the display panel. There was a break of approximately five
seconds between trials. Reaction Times, assessed as being the time from the onset of the pattern sequence
until movement initiation (when the microswitch was first opened), and Movement Time,
the time between movement initiation and contact with the surface of the display panel, were automatically calculated for each trial, and were saved to floppy disk on the
completion of each block. The test session lasted approximately 70 minutes. Subjects were permitted rest period between blocks, as required.
10.3 RESULTS
MEDIAN REACTION TIMES
Median Reaction Time measures were obtained from each cell of the 64 unique
combinations of five factors, each summary median value was derived from six trials. A
repeated measures ANOVA was performed, this was a 2 x 2 x 4 x 2 x 2 factorial design
with Hand (Right, Left), Visual Field (Left, Right), Pattern (1,2,3, & 4), Relation to
Midpoint (Lower, Upper) and Target Eccentricity (Inner, Outer) as factors.
Although a left hand advantage, in terms of reaction time, was exhibited by six of
the eight subjects, and there was an overall "left hand advantage" when the results were
collapsed across all subjects, this effect failed to reach statistical significance. It is likely
that the characteristic response pattern of one of the "right hand advantaged subjects,
D.T., in particular, had some considerable influence on this distribution. This subject
exhibited a right hand response latency which was, on average, 64 ms more rapid than that
for the left hand. As it was, the apparent effect of Hand was some way from reaching
statistical significance, F(1,7) = 0.55, p > 0.05.
TABLE 10.1 Median Reaction Time (ms) as a Function of Hand and Visual Field
Hand
Visual Field
Left Right mean
Right
Left
mean 406.5 402.3
HAND BY VISUAL F L D
RIGHT
VISUAL FIELD
FIGURE 10.1 Median Reaction Time (ms) Hand by Visual Field
Although there was a marginal advantage for sequences presented to the right visual field,
this was, similarly, not of statistical ~ i ~ c a n c e , F(l, 7) = 2.21, p > 0.05. There were, in
addition, no statistically significant main effects for the factors pertaining to the relationship
of the target position to the central fixation point, there was no main effect associated with
Midpoint or with Target Eccentricity. It is of interest however that there was apparently
some relatively consistent effect on reaction time associated with the pattern which was
initially presented, this supposed effect marginhy failed to reach statistical significance,
F(3,21) = 3.02, p = 0.053. Although it is inappropriate to conduct formal post hoc
analysis on these data, inspection of the means provides some indication that responses to
pattern 4 were appreciably slower than those to the other sequences. Indeed the trend in
terms of latency was consistent across both responding hands.
TABLE 10.2 Median Reaction Time (ms) as a Function of Hand and Pattern
Pattern
Hand 1 2 3 4
Right 403.3 402.0 408.2 422.8
Left 395.0 391.8 397.5 414.4
mean 399.3 396.9 402.9 418.6
These trends in the means are illustrated by Figure 10.2.
HAND BY PATTERIV
RIGHT HAND LEFT HAND
FIGURE 10.2 Median Reaction Time (rns) Hand by Pattern
There was an absence of any higher order interactions of statistical significance for the
reaction time measure.
MEDIAN MOVEMENT TIMES
As for the Reaction Time measures, Median Movement Times were obtained from
each cell of 64 unique combinations. A 2 x 2 x 4 x 2 x 2 factorial design repeated measures
ANOVA was performed Hand, Visual Field, Pattern, Relation to Midpoint and Target
Eccentricity were again factors. Preliminary analysis revealed no main effects associated
with the factors of Hand, F(1,7) = 0.23, p > 0.05, or of Visual Field, F(1,7) = 0.03, p > 0.05, but did indicate the presence of a statistically sigmficant interaction of these factors,
F(l, 7) = 61 .25, p c 0.0001. As is clearly represented, both in Table 10.3 and Figure 10.3, right hand responses were made more rapidly when directed toward patterns
presented in the right visual field, than those made when sequences were presented in the
left visual field, similarly ipsilateral left hand responses were of shorter duration than left
contralateral responses.
TABLE 10.3 Median Movement Time (ms) as a Function of Hand and Visual Field
Visual Field
Hand Left "T mean
Right 421.5 404.3 412.9
Left 411.1 427.6 419.4 I
mean 416.3 416.0
HAND BY VISUAL FIELD 430 -I
RIGHTHAND LEFTHAND
RIGHT
VISUAL FIELD 1
FIGURE 10.3 Median Movement Time (ms) Hand by Visual Field
The analysis also revealed the presence of a statistically significant main effect associated
with the factor of Pattern, F(3,21) = 3.37, p c 0.05. This did not appear to parallel the
trends in Reaction Time indeed, although post hoc pairwise analysis of means using the
Tukey HSD procedure (Table 10.4b)revealed.the only pairwise difference that reached
statistical sigruficance was that between patterns 1 and 4, inspection of the pattern of means
indicates that the median movement times to targets specified by pattern 1 were appreciably
shorter than those specified by other patterns. Figure 10.4 provides a graphical portrayal of
this effect. Comparison of equivalent values for Reaction Time would suggest, as
outlined,that it was responses to pattern 4 which were noticeably slower to be initiated in
that case.
TABLE 10.4 Median Movement Time (ms) as a Function of Hand and Pattern
Pattern
Hand
Right
Left
mean 395.4 419.6 420.9 428.6
TABLE 10.4b Median Movement Times (ms) Differences among Means
HAND BY PATTERN
RIGHT HAND LEFT HAND
FIGURE 10.4 Median Movement Time (ms) Hand by Pattern
There was also provided confirmation of intuitive expectations that movements of
greater absolute amplitude, those made to above the midpoint, would be associated with
longer movement times, F(l, 7) = 52.57, p < 0.0002. There was however no main effect
associated with Target Eccentricity, F(l, 7) = 0.27, p > 0.05, nor an appreciable interaction
of these factors nor indeed any strong evidence for any other second order interactions.
TABLE 10.5 Median Movement Time (ms) as a Function of Hand and Relation to
Miduoint.
Midpoint
Hand Lower Upper mean
Right 399.3
Left 407.8
mean 403.5 428.7
HAND BY RELATION TO MIDPOINT
LOWER UPPER
MIDPOINT
FIGURE 10.5 Median Movement Time (ms) Hand by Relation to Midpoint
In addition, there was observed a statistically significant higher order interaction,
that of Hand by Field by Target Eccenmcity, F(l, 7) = 8.18, p c 0.05.
TABLE 10.6 Median Movement Time (ms) as a Function of Hand. Visual Field and
Target Eccentricity.
Target Eccentricity
Inner Outer
Visual Field Left Right Left Right mean
Hand
Right 413.9 412.1 429.2 396.5 412.9
Left 412.2 423.5 410.0 43 1.7 419.4
mean 413.1 417.8 419.6 414.1
As is revealed by inspection of Figure 10.6 the interaction reflects a tendency for
movements made by both hands to be completed more rapidly when directed at Outer, more
eccentric, targets only when responses were ipsilateral, that is when the responding hand
was moving into "its own" visual space. When movements were conualateral, movement
times were shorter for Inner targets.
HAND BY VISUAL FIELD BY ECCENTRICITY
"201
R.H. (L.V.F.) R.H. (R.V.F.) L.H. (L.V.F.) L.H. (R.V.F.)
390 ! I 1 . INNER OUTER
ECCENTRICITY
FIGURE 10.6 Median Movement Time (ms) Hand by Visual Field and Target
Eccentricity.
10.4 DISCUSSION
Somewhat contrary to expectations, Median Reaction Time measures provided few
consistent indications as to effects arising as a consequence of experimental manipulations.
With regard to the slight left hand advantage in terms of reaction time, the trend is thought
provoking rather than by any means conclusive. The measures of reaction time obtained
were characterized by large bekeen and within subject variability which is likely to have
contributed somewhat to the absence of statistically sigdicant main effects. Although the
magnitude of the overall left hand advantage was largely attenuated by the right hand
advantage exhibited by subject D.T., one cannot assume that the distribution, in this single
case, was due to anything other than chance. Conjunctively, although the trend toward an
overall right visual field advantage for reaction time is counterintuitive, beaxing in mind the
supposed nature of the stimulus materials, the tendency was not of sufficient consistency to
make it worthy of detailed comment.
In contrast, the main effect associated with the stimulus pattern, albeit marginally
failing to surpass the conventional criterion value for statistical sigdicance, does appear
notable. The increase in response latency associated with Pattern 4 was symmetrically
expressed across responding hands, which might, in itself, initially suggest that the
increase was not necessarily due to an increased complexity of spatial processing as suph.
It is perhaps of salience that the constituent points within Pattern 4 were more spatially
disparate than those within other patterns. The increased reaction times may reflect the
additional time required to assimilate information from a greater area of the visual field,
though this is somewhat conjectual. A perhaps related, and certainly likely, possibility
exists, that the nature of certain patterns were such that the global characteristics of the
sequence were not readily apparent. There was some concern that subjects were, rather
quickly, resorting to guesses as to the nature of each pattern, thus effectively negating the
intended manipulation of spatial complexity. In these circumstances it is possible that,
having formulated some heuristic as to the presumed terminal position of a particular
sequence, "secondary" factors such as the spatial distribution of display points assume
greater significance. Indeed, post-experimental debriefings indicated that this may indeed
have been the case. Rarely did subject's assignations of, in particular, patterns 3 and 4
correspond to the actual sequences.
It had previously been suggested by this author that a sufficient target display series
would be that which elicited an interaction with either responding hand or visual field of
presentation. On reflection, it seems there may be no justification for the post hoc selection
of tasks considered appropriate for a particular application simply because they were
successful in producing some form of asymmetrical response. Rather, there may only be a 100
priori specification of that which, it is hypothesized, represents some manipulation of a
variable of interest, in this case spatial complexity. Or rather more specifically, the spatial
complexity of sequence, the assimilation of which is a prerequisite for the establishment of
a target position in space. Assessment of the appropriateness of this specification is then, at
least in part, an empirical enquiry. Therefore, it does appear that only by making subjects
explicitly aware of the relative complexities of the patterns might one be relatively assured
that, at least, the desired manipulation was being applied, and thus that the desired enquiry
could be realized.
In terms of reaction time measures, there was no indication of spatial compatibility
effects, ipsilateral and contralateral movements were initiated with equivalent latencies.
There was thus a failure to replicate the findings obtained by Fisk and Goodale (1985), who employed a somewhat similar reaching task, or the results reported by, among others,
Anzola, Bertoloni, Buchtel and Rizzolatti (1977) using a less complex reaction time
paradigms. One cannot make appeal to the high variability of the reaction time measures
obtained in this experiment, there were few indications that a spatial compatibility effect
was present. The trends in means indicated that left hand contralateral reaches were, if
anything, initiated more rapidly than ipsilateral reaches.
The responding hand by visual field interactionj which was clearly evident in the
dismbution of Movement Time latencies, appears entukly consistent with previous work 1
highlighting what are assumed to be spatial compatibility effects (e.g., Fisk & Goodale,
1985). Ipsilateral movements were completed a good deal more rapidly than contralateral
equivalents. It should be noted, of course, that the me absolute distance between the
starting position and the target was equivalent for of response.
It is of some interest to note that, in this instance, responses made by the right hand
were not appreciably, or at least consistently, more rapidly completed than lcft hand
movements. Similarly responses made into one half of v i s u W y centrcd space were not
favoured over responses made into the other.
The main effect for Midpoint is, in itself, of little interest, other than in providing
some substantiation of intuition that the time required to complete a response was at least
some correlate of movement amplitude. However, a consideration of the apparent
interaction of Hand by Visual Field by Target Eccentricity, provides some convergence
with evidence for the presence of a spatial compatibility effect, in that it is clearly not the
case that movement duration is solely determined by the absolute distance between the
starting position and the target. In this case, iusilateral responses made to outer targets
consumed less time than those made to inner equivalents, although this tendency was less
clearly expressed for the left hand. Contralateral movements made by either hand were of 10 1
longer duration when made to the more eccentric outer targets. As with the Hand by Visual
Field interaction, the variations in movement time cannot be attributed simply to the
distance through which the responding hand is required to move in completing the
movement. The possible factors underlying these effects will be considered in greater detail
in discussing the results of Experiment 2. It did appear that, in this experiment, the manipulation of spatial complexity,
through the use of various display patterns, was ineffectual, as subjects were rarely actually
aware of the nature of each pattern, and imposed their own best guess as to pattern
characteristics. As has been discussed however, asymmetries in motor behaviour may only
adequately assessed through the use of a range of dependent measures, each of which is
assumed to have at least some, rather specific sensitivity. Reaction Time and Movement
Time may only be regarded as gross indicators, in this context.
Experiment 2 was designed to implement this multifactorial examination of
asymmetries in pointing behaviour, and also conducted with a view to more forcefully
imposing manipulations of spatial complexity, examining in greater detail what were
presumed to be spatial comp~tibility effects and considering, in particular, the related
contribution of information of a visual origin to the regulation of goal directed pointing
movements.
CHAPTER 11
EXPERIMENT 2
1 1.1 INTRODUCTION
In that it is the case that behavioural asymmetries are the manifestation of a complex
transactional process, it is similarly apparent that action, or overt behaviour itself, is merely
one aspect of an animal-environmental mutualism or synergy (Kugler, Kelso & Turvey, 1982). One other pivotal input to this synergy is the perception of the space into which any
action will be directed In emphasizing the exigency of the perception of space, one can detect the central role this function must assume in both the ecological approach to perception and action, and themore mechanistic computational approaches. Thus one can
find some initial cause to emphathise with Morasso and Tagliasco (1986) who,in detailing the requirements for a "global motor control system" outline that movement must be:
the final result of a plan of interaction of the body with the environment. The initial prerequisite for the Central Neural Controller is the representation of space, of the objects, of their mutual relationships as well as their relationships with the self (p. 243).
One must surely distinguish however, between representing ones surroundings or external space, and some extant geometric representation which is the "initial step for the motor
control system and thefinal step for the vision system" (Morasso & Tagliasco, 1986, p. 243, this author's italics). Indeed there are immediate contradictions in an approach which also encapsulates the proposal that:
such a representation communicates bidirectionally with the motor and visual systems ... does not drive the motor system in a purely top-down fashion but is refined and updated by it during the motor performance (and that) the visual system does not generate the geometric representation in a purely bottom-up fashion but is driven by it, in a recursive way, taking into account at each instant the current representation (p. 243).
A number of theorists have expressed fundamental objections to this form of
computational representation of the environment, or more specifically that of space. Turvey
and Carello (1986) argue that theories and experiments focussing on space as a
mathematical consmct within the context of a Cartesian Coordinate system are, whilst
appealing, unlikely to reflect that which organisms must percieve as functional
characteristics of, or affordances within, their environment. In a similar vein, the
"ecological perspective" exorcises the requirement for a third class of terms or level of
discourse (c.f., Searle, 1984), encapsulating intermediate "pseudoconstructions" such as
representations and programs, which a computational approach must include as a means to
mediate between the animal and the environment (Kugler, Kelso & Turvey, 1982).
103
The term recursive can in itself be misleading when referenced to the modification
of a representation rather than the global characteristics of an action per se. Alternatively, if
one adopts a sensitivity to the nature of the environmental information which defines the
goal of any relatively discrete action, then it is legitimate to enquire as to the effect the nature of this information will have on the characteristics of the resultant action. For
example if the same action goal is specified by a variety of environmental operators, or
more specifically stimulus energies, differences in the optical flow fields, is the appropriate
goal directed action likely to exhibit the same characteristics in each case? It is in this sense that the potentially recursive nature of goal directed action assumes practical significance. The environmental information specifying an action goal may exert its effects throughout
the time course of and upon all behavioural manifestations of, the responding system, that is its influence may recur. As with the examination of behavioural asymmetries it is a
requirement that there be a multifactorial examination of goal realizing action. The
assumptions underlying this approach may be usefully contrasted with the conclusions
drawn by Fisk and Goodale (1985), that the spatial location of the target is specified by "neural systems" prior to those.more specifically associated with "motor output" and as
such that this specification is not reflected by kinematic measures. Of course, this recursion, if it indeed occurs, may or may not be expressed asymmetrically, this is then a related topic of enquiry.
In the experiment 1, and as discussed, there could have been little assurance that the
task goal was indeed the same in every instance, as subjects appeared to make their own best estimation as to the nature of the presented pattern. By insuring a prior familiarity with
the relevant patterns it should be possible to maintain an equivalence of task goals across
sequence presentations, in addition to applying the desired manipulation of spatial complexity. Application of the same task goal in each instance, through the continued use
of identical target locations, allied with manipulation of "complexity", may represent the
required systematic variation in environmental information, the forms of which afford
specification of a single goal.
In considering the multidimensional contribution of visual information to the
regulation of goal directed action, it is of course not the case that this contribution is limited
to the initial or continuing specification of the target goal (see Chapter 7). Whilst Prablanc,
Pelisson & Goodale (1986) provided indications that, even discontinuous, visual
information regarding the position of the target has action guiding functions for movements
which are not seen, Conti and Beaubaton (1976) have shown that visual information
pertaining to the position of the hand is utilized through various portions of the movement
trajectory. There has, however, been no prior investigation of whether vision of the 104
responding hand, during the course of movement, continues to have a regulating function
when the position of the target is never actually seen but is merely implied. There is
certainly evidence that speed accuracy relationships are maintained in the absence of vision
of the hand (Prablanc, Eschallier, Komallis & Jeannerod, 1979), whereas Pelisson,
Prablanc, Goodale and Jeannerod (1986) suggest that modification of movement
trajectories in the absence of ambient illumination may be on a "pseudocontinuous" basis.
Clearly the nature of movement regulation in the absence of motion related visual
information or indeed the potentially non-discrete utilization of such information is still a
subject of enquiry.
Through manipulation of the ambient illumination present throughout the course of
the pointing movements previously described in Experiment 1, and by employing the
appropriate dependent measures, it may be possible to obtain some indication of the manner
in which visual information is utilized, both in defining the goal of that action and when
pertaining to the motion of the responding hand, in the regulation of ongoing movement.
The required manipulations are relatively straightforward to apply. As outlined, patterns
may be displayed which define the position of a punctiform target, the action goal, without
there being the need to explicitly represent that target, providing the individual is aware of
the implied relationship. The presence or absence of motion related visual information may
be controlled simply by altering the ambient illumination which is present throughout the
course of the movement. The appropriate dependent measures will obviously be a
multifactorial collection of behavioural and kinematic indices which, it is assumed, reflect
various aspects of the regulatory processes. These measures should perhaps be considered
as diagnostics rather than as representing specific stages within a "motor control process"
(c.f., Guiard, Diaz & Beaubaton, 1983). Thus Constant Error cannot simply be considered
as indicative of the accuracy of "central programming" any more than Variable Error may
be a reflection of the accuracy of "motor program execution". As discussed in Chapter 10,
the execution of a movement unaccompanied by identifiable and discrete modifications does
not imply the existence of a motor program.
There can be no doubt that measures such as Movement Time, Reaction Time,
Constant Error and Variable Error are salient pointers. The ultimate objective should
perhaps be to relate systematic variations on these measures rather more directly to the
presumed physiological substrate without recourse to a "third class of terms". Thus one can
permit descriptions at the phenomenological level, which may potentionally be related to
descriptions at the physical level without the need for intermediates.
Brooks (1974) has discussed how the relative continuity of movements may be
assessed, or diagnosed, through the measure of Zero Crossings of the Acceleration Profile, 105
if there is obtained a kinematic description of the movement. This then represents a formal
distinction between what are otherwise similar movements. Continuous movements have
been described as those which are "quick and smooth" exhibiting only one velocity peak, to be contrasted with discontinuous "equivalents" revealed by the presence of multiple
crossings of the zero line by the acceleration trace, for which "motor commands for successive steps are supposed to be given only after intermittent referral to peripheral and central information" (Brooks, 1974, p. 306). In an attempt to provide an explanation at the
physicaVphysiological level, it has been suggested (Eccles, 1967) that continuous control,
or rather regulation, is subserved by cerebellar influence associated with spinocerebellar
and cerebrocerebellar projections.
However, descriptions of the characteristics of the overt movement, at the
phenomenological level need not be related directly to some physical or physiological correlate. Thus it is quite legitimate to describe variability in the terminal locations of motor
acts, over the course of a series of trials, without there being the need to ascribe this
variation to some manifestation of a motor program. It has been suggested (e.g., Annett,
Annett, Hudson & Turner, 1979) that the difference between the hands lies in the
variability of, or noise associated with, the initial production of force. This may, in theory,
be simply tested. By examining the Jerk, the third derivative of displacement, associated
with an evolving movement, that is the rate of change of acceleration with respect to time
and therefore an index of the variability of Force, prior to peak velocity (if this may be
described as "initial"), one may obtain a direct measure of this variability. This measure
may of course be assessed independently for each hand Similarly, a measure of the
variability of the time taken to reach peak velocity may provide an indirect indication of the
variability in the production of force. The peak velocity attained is likely to be some
measure of the magnitude of the initial impulse. Examination of the overt characteristics of movement may therefore provide useful information as to the nature of the regulation of such movement, without there existing the need for recourse to some "third level"
collection of terms.
The experimental strategy was to have subjects perform goal directed pointing to
phantom targets, the position of which had been implied by the presentation of a stimulus pattern (the nature of which was known to the subjects), when these movements were
performed to a variety of target locations, by either hand, with and without the presence of motion related visual information during the course of the movement. The overt
characteristics of these movements would be examined in tenns of a collection of dependent
measures with a view to obtaining some indication of the manner in which these
movements were regulated. In addition to the measures of Reaction Time and Movement 106
Time employed in Experiment 1 and the kinematic indices described above, Radial Error, a
measure of the absolute discrepancy between the terminal location of the movement and the
target position, Constant Error in the direction of the X and Z axes of the display board and
associated measures of Variable Error were to be assessed.
11.2 METHODS
SUBJECTS
The subjects were eight, naive, male volunteers, each of whom were paid $10.00
for their participation. Individuals were all classified as right handers on the basis of the
Edinburgh Handedness Inventory (Oldfield, 197 1) which had been administered
previously. All subjects had normal vision.
APPARATUS FOR DATA COLLECTION
In this experiment, all reaching movements were recorded using a WATSMART motion analysis system. This is a 3-dimensional digitizing system which, by means of post
hoc reconstruction of 2-dimensional coordinates from two cameras, provides 3-
dimensional coordinates of reference markers.
Data acquisition requires the attachment of small infrared light emitting diodes to the
surface of the object or, for example body segment, the motion of which is of interest. The
marker LED's are pulsed in sequence at very high frequencies, as each marker is briefly
active, it is registered by custom cameras as a point source of light. These point sources are
digitized as 2-dimensional coordinates, and stored for subsequent transformation.
The display panel was that used in Experiment 1, a single modification was the
addition of 2 WATSMART infrared LED's which were aligned with the vertical (z), and
horizontal (x) axes of the panel. These were mounted upon the upper horizontal and left
vertical surfaces of the display panel, such that one marker was positioned to lie along the
same x coordinate axis as the central fsation point, while the other was located to lie along
the z coordinate axis of this point.
Subjects responded using a hand-held stylus, constructed from the plastic outer
barrel of a 10 ml syringe. On the tip of this stylus was placed a membrane microswitch,
mounted upon which was a rubber coated metal tip approximating the shape of a cylinder,
of dimensions, height 4 rnm and diameter 2 mm. A single WATSMART LED was attached
7.5 rnm above and behind this tip. The stylus was covered with a tape wrapping which also
served to indicate the part of the stylus to be gripped. The stylus microswitch was
interfaced with an Apple IIe microcomputer in the manner previously described. Infrared
LED's were interfaced with the WATSMART system.
The WATSMART cameras were positioned 1.8 m apart, 2.5 m from the ground , and a perpendicular distance of 2.1 m from the display panel with the respective viewing
108
fields converging between the subject and the panel. From the subject's perspective, the
cameras were positioned over his right and left shoulders. During the practice trial blocks,
it was verified that the stylus tip could be "seen" by both cameras when located at the
starting position and at the termination of the movement. The camera positions were
calibrated immediately prior to each experimental session.
The Apple IIe microcomputer was used as an external trigger to initiate and
terminate the collection of data by the WATSMART system. Connected via the annunciator
ports of the Apple, and controlled by a 6502 assembler subroutine, WATSMART data
collection was triggered by the initial change of status of the stylus microswitch when the
subject initiated a movement and was halted upon the next change of switch, when the
stylus tip made contact with the display board. In all trials, data was sampled at 400 Hz.
Ambient illumination within what was an otherwise completely dark, black walled
room, was provided by a Kodak Carousel 600H projector, containing a 300 watt light
source. The beam of light produced was diffused and projected onto the ceiling above the
subject. The projector was shuttered by a, normally open, Lafayette Instruments Model
No. 43016 shutter, activated by a Lafayette 4301 1/16 Shutter Control. The control device
was interfaced with the microcomputer in a fashion such that a logic pulse triggered shutter
closure with a response time of less than 2.5 ms The generation of the appropriate logic
pulse was controlled by a 6502 assembler subroutine and was specified by a single
parameter modifiable at the beginning of each trial block. For trial blocks for which ambient
illumination was to be removed throughout the course of the movement, closure of the
shutter was triggered by the first change of the status of the stylus microswitch, that is
when subjects initiated their movement, and was reopened 2 seconds after initiation. For
conditions in which ambient illumination was available during the entire course of the
movement, the shutter remained open upon movement initiation.
Subjects were positioned relative to the display panel in a fashion similar to that for
Experiment 1. The starting platform was modified slightly to accommodate the stylus, but
remained in the same position relative to the subject's body axes. Control of stimulus
presentation and the timing of "critical" events was again controlled by the Apple IIe
microcomputer, and in all other respects the apparatus was identical to that used for
Experiment 1.
Eight pattern demonstration sheets were prepared, these comprised two example of
each of the four possible display sequences, aligned in a variety of orientations. Constituent
dots were clearly marked, and labelled according to the order in which they would normally
occur (Appendix 2). Acetate template sheets corresponding to each demonstration sheet and
containing the appropriate target at the centre of an "area of presumed correctness",
approximately ten times the diameter of the represented display dot, were prepared for the
purposes of comparing subject's responses with extrapolated target positions.
The experimental test materials were identical to those used in Experiment 1.
PROCEDURE
As a means of ensuring that subjects fully comprehended the nature of the test materials, all individuals, having been shown prototypical examples of each pattern, were
required to mark on the pattern demonstration sheets the position that they considered corresponded to the final dot in the sequence. These responses were then scored. Correct
responses were those for which the mark was positioned inside the superimposed area of correctness. No subjects made incorrect responses on any of the demonstration sequences.
Test sessions comprised of four blocks of 96 trials, two blocks with each responding hand, and for each hand one block in each visual condition. That is, on half of the blocks ambient illumination was removed upon movement initiation, on the other half it was not. The order of hand use and visual conditions was counterbalanced across subjects. Each test session was preceded by four blocks of 32 practice mals, each block
corresponding to one combination of hand and visual conditions.
Subjects were instructed to hold the stylus in a manner such that the Watsmart LED
was always upright and was not obscured from either camera. In all other respects the
procedure was identical to that for Experiment 1.
DATA REDUCTION
Following the experimental sessions, and for each trial,the two sets of "raw" 2-
dimensional linearized camera data were converted to three dimensional cartesian
coordinates, by Direct Linear Transforms, using a Northern Digital CONVERT program.
ASCII files of these three dimensional coordinates were then generated using a
DUMPDATA routine. The ASCII fdes were stored to disk for subsequent analysis. 110
A custom designed program was employed to 'pick' from the ASCII files, the final
frame of data for each trial. This information then provided the x, y, and z coordinates of
the stylus and the x and z panel axis markers for the final frame of collection. This
information was saved to a single output file for each 96 trial block.
From the 3-dimensional coordinate data for all frames on each trial, instantaneous
vector velocity was computed by means of a two point central difference algorithm. The
velocity "curve" was smoothed with a 12.5 ms triangular window, and a custom "peak
picker"' program was used to establish the highest peak velocity, record its value and frame
reference. Instantaneous vector acceleration was computed, again by means of a two point
central difference algorithm and smoothed with a 12.5 ms triangular window. A custom
program was used to identify positive to negative, and negative to positive, crossings of the
zero "acceleration profile". from the time of highest instantaneous velocity to the
termination of the movement. A criterion 'windowing' filter was employed, such that
deviations from zero had to equal or exceed positive or negative 0.02 ms-2 to be registered as a "crossing". A fuller account of this procedure and a discussion of results is included in
Appendix C. The appropriate data concerning, the reference and stylus coordinates on the final
frame, total movement time as calculated from the Watsmart collection duration, peak
velocity, positive, negative and total number of crossings of the zero acceleration profile
were then merged with corresponding files containing the reaction time and movement time
data, obtained directly from the controlling micfocomputer, and information specifying the
board coordinates of the target position and presentation pattern on each trial. From this
information, simple calculations provided the x and z coordinates of the terminal position of
the stylus position relative to the target position. It was thus possible to calculate the
magnitude of "error" in both dimensions for each trial.
For each trial, data was available relating to; Reaction Time, Movement Time,
discrepancy between the stylus tip position and target position, along the x and z axes at
contact, Peak Instantaneous Vector Velocity reached during movement, the time taken to
reach Peak Velocity, and the number of zero crossings of the acceleration profde from the
time of Peak Velocity until the termination of the movement.
The calculation of Jerk profiles was precluded as it was a consistent characteristic of two
subjects, and an occasional characteristic of the remaining subjects, to make movements
which immediately after initiation involved a rotation which obscured the LED mounted
upon the stylus. Although this rotation generally lasted less than 50 ms it occurred with
sufficient frequency to make impossible the calculation of instantaneous Jerk during the
period of greatest theoretical significance with respect to this variable, that is from movement initiation to peak velocity.
11.3 RESULTS
MEDIAN REACI'ION TIMES
Median Reaction Time measures were obtained from each cell of the 128 unique
combinations of the six factors, each median value being derived from three trials. A repeated measures ANOVA was performed, the design was a 2 x 2 x 2 x 4 x 2 x 2 factorial
with Hand (right, left), Visual Condition (illuminated, non-illuminated), Visual Field (left,
right), Pattern (1,2,3, & 4), Relation to Midpoint (above, below) and Target Eccentricity
(outer, inner) as factors.The analysis revealed no main effects of statistical significance, nonetheless the trends may in themselves prove to be revealing.
TABLE 1 1.1 Median Reaction Time (ms) as a Function of Hand and Visual Field
Visual Field
Hand Left Right mean
Right 359.8 361.5 360.7
Left 347.9 351.1 349.5
mean 353.8 356.3 355.1
The left hand advantage, which although exhibited by six of the eight subjects and
evident across visual fields, did not reach statistical significance, F(l, 7) = 1.41, p > .05.
Similarly, there was no consistent effect of visual field, F(l, 7) = 0.76, p > .05. The
relationship of the means for each combination of these factors is graphically illustrated by
Figure 1 1.1 as follows:
HAND BY VISUAL FIELD
Q RIGHTHAND LEFTHAND
-a
LEFT RIGHT
VISUAL FIELD
FIGURE 1 1.1 Median Reaction Times (ms) by Hand and Visual Field.
The apparent effect of Hand upon Reaction Time was also reproduced across the
Pattern factor. It is evident that that the left hand advantage was expressed across all levels
of Pattern.
TABLE 11.2 Median Reaction Time (ms) as a Function of Hand and Pattern.
Pattern
Hand 1 2 3 4
Right 359.8 357.8 362.2 362.9
Left 348.2 344.9 352.3 352.6
Once again the trends are well illustrated graphically, for example as follows:
HAND BY PATTERN
FIGURE 11.2 Median Reaction Times (ms) by Hand and Pattern.
370 -
360 - - z Y
What was apparently a consistent trend across display patterns, failed to reach
statistical significance, F(3,21) = 2.02, p > .05.
There was also no appreciable difference between median reaction times in the
illuminated (355.6 ms) and non-illuminated (354.6 ms) visual conditions, F(l, 7) = 0.02,
p > .05.
Right Hand Left Hand
A single higher order interaction reached statistical significance, this was the
interaction of illumination by field by midpoint by target eccentricity, F(l, 7) = 16.34, p < .005. The interaction accounted for considerably less than 1% of the total variance, and
was theoretically uninterpretable.
5
MEDIAN MOVEMENT TIMES
In a fashion identical to that for Reaction Time, Movement Time measures were
obtained from the median of each cell of 128 unique combinations. A repeated measures
ANOVA was again performed, using a 2 x 2 x 2 x 4 x 2 x 2 factorial design, with Hand
(right, left), Visual condition (illuminated, non-illuminated), Visual field (left, right),
Pattern (1, 2,3, & 4), Relation to Midpoint (above, below) and Target Eccentricity (outer,
inner) as factors.
TABLE 11.3 Median Movement Time (ms) as a Function of Hand and Visual Field
Visual Field
Hand Left. Right mean
Right 421.7 409.8 415.7
Left 439.4 460.5 449.9
mean
There was a clear main effect upon Movement Time associated with the hand with
which the response was made, when using the right hand, subjects were consistently more
rapid, F(l, 7) = 15.13, p < 0.01. This effect was not expressed equally across the visual
fields in which the target position had been located, there was a clear, and statistically
significant, hand by visual field interaction, F(l, 7) = 14.16, p < 0.01. This relationship
between the respective means is clearly represented by figure 11.3:
HAND BY VISUAL FIELD
VISUAL FIELD
470 - 460-
450 - h
2 440- V
2 430-
420 - 410 - 4 0 0 .
FIGURE 11.3 Median Movement Times (ms) by Hand and Visual Field.
/ RIGHTHAND LEFTHAND
1 I I d
Subjects completed movements more rapidly when moving their right hand into the
right visual field, and the left hand into the left visual field, than when maldng contralateral
movements, that is, when the right hand was moving toward targets located in the left
visual field and the left hand to targets in the right visual field.
LEFT RIGHT
Median Movement Time was also observed to vary considerably as a function of
the displayed pattern, F(3,21) = 11.18, p < 0.0001. Movements made in response to
pattern 3 were of longer median latency than those made to the other patterns, post hoc
analysis, in the form of pairwise comparison of the means using the Tukey (HSD)
procedure, indicated the presence of statistically significant pairwise differences between
the mean Median Movement Time to pattern 3 and all other Pattern conditions (Table
11.4b). This effect was expressed equivalently across both hands and visual fields, no two
or three interactions with pattern reached statistical significance. The relationship is
illustrated the pattern of means displayed in Table 11.4 and by Figure 11.4.
TABLE 11.4 Median Movement Time (ms) as a Function of Hand and Pattern.
Pattern
Hand 1 2 3 4
Right 379.6 402.5 480.2 400.6
Left 413.1 432.2 520.0 434.4
TABLE 11.4b Median Movement Time (ms) Differences amone Pattern Means
HAND BY PATTERN
* right hand + left hand
F'IGURE 11.4 Median Movement Times (ms) by Hand and Pattern.
The factor expressing the relationship of the target location to the midpoint of the target
display board, and consequentially the absolute distance from the starting position to the
target ("upper" targets were a greater distance from the starting position than "lower"
targets), was also associated with a statistically significant main effect, F(l, 7) = 10.85, p
< 0.05.
It is of little surprise that subjects took longer to complete movements covering greater
distance, those made to "upper" targets above the midpoint. This effect was also expressed
equally across both hands and both visual fields. There were no statistically significant
interactions with these factors.
TABLE 11.5 Median Movement Time (ms) as a Function of Hand and Relation to
Mid~oint..
Hand
Midpoint Left Right mean
Lower 437.7 402.7 420.2
mean 449.9 415.7 442.8
HAND BY MIDPOINT 470 -
LEFTHAND 4 RIGHTHAND
450
h
V
410
400 UPPER LOWER
MIDPOINT
FIGURE 11.5 Median Movement Times (ms) by Hand and Relation to Midpoint.
Importantly, there was no effect of Visual Condition upon movement time (mean
illuminated = 436.2 ms, mean non-illuminated = 429.5 ms), F(1, 7) = 0.06, p > 0.05.
Subjects completed the movements with equivalent latencies, irrespective of whether
ambient lighting was or was not available during the course of the movement.
1 2 0
Two higher order interactions were of statistical significance, a four way interaction of Visual Condition by Pattern by Midpoint by Target Eccentricity, F(3,21) = 5.05, p < 0.01, and the five way interaction of Hand by Visual Condition by Pattern by Midpoint by
Target Eccentricity, F(3,21) = 3.16, p < 0.05. Combined, these interactions accounted for a great deal less than 1% of the total variance and are theoretically uninterpretable.
RADIAL, ERROR
Radial Error scores were subjected to a similar six way factorial, repeated
measures, analysis of variance (ANOVA), Hand by Visual Condition by Visual Field by Pattern by Relation to Midpoint by Target Eccentricity.
A main effect of statistical significance was obtained only for the factor of Visual Condition, F(l, 7) = 7.99, p c 0.05. This effect was consistent across hands and indeed all other factors. Individuals were more accurate, that is responses were terminated closer to the target position on an absolute scale, when ambient lighting was present throughout the duration of the movement.
TABLE 11.6 Radial Error tmm) as a Function of Hand and Visual Condition.
Hand
Visual Condition Right Left mean
Jlluminated 34.5 34.8 34.6
Non-illum 44.4 45.7 45 .O
mean 39.4 40.2 39.8
HAND BY VISUAL CONDITION
FIGURE 1 1.6 Radial Error(mrn) by Hand and Visual Condition.
Significantly, there was no main effect associated with the hand making the response. The left and right hands were equally accurate, F(l, 7) = 0.32, p > 0.05. Similarly, there was an absence of main effects for Visual Field, F(l, 7) = 0.80, p > 0.05,
for Pattern, F(3,2 1) = 0.49, p > 0.05, and for all other factors. There was however, a single.higher order interaction which reached statistical
significance. This was the interaction of Hand by Visual Field by Midpoint, F(l, 7) = 11.22, p < 0.05). The relevant means are displayed in Table 11.7, the relationship is then graphically represented in Figure 11.7.
TABLE 11.7 Radial Error (mm) as a Function of Hand. Visual Field and Relation t~
MidDoint.
Hand
Right Right Left Left
- -
Field Left Right Left Right
Lower
upper
mean 40.9 38.0 42.0 38.5 '
- - -
HAND BY VISUAL FIELD BY MIDPOINT
R.H. (LV.F.) RH. (RV-F.) LJI. (LV.F.)
0 LJI. (R.V.F.)
37 ! I I i
LOWER UPPER
MIDPOINT
FIGURE 1 1.7 Radial Error(mm) by Hand, Visual Field and Relation to Midpoint
It is the case that for contralateral movements made by both hands, the magnitude of error
is greater for upper movements. Conversely, for ipsilateral left hand movements, that is,
instances of left hand responses to targets located in the left visual field, accuracy increases when targets are above the midpoint, whereas for ipsilateral right hand responses, t h q is a weak trend toward increasing accuracy for lower movements.
X CONSTANT ERROR
Mean Constant Error scores in the direction of the X axis of the target board were calculated for each of 128 cells for each subject. The mean scores were subjected to a six
way factorial analysis of variance (Hand by Visual Condition by Visual Field by Pattern by
Relation to Midpoint by Target Eccentricity). In this instance, positive error scores correspond to responses made to the right of
the target positions, in the right visual field and to the left of targets in the left visual field (from the subject's point of view), whilst negative values of course represent the reverse.
TABLE 11.8 X Constant Error (mm) as a Function of Hand. Visual Condition and Visual Field.
Hand
Right Right Left Left
VisualField Left Right mean Left Right mean
Illuminated - 19.8 -20.1 -19.9 -20.4 -18.2 -19.3
- -
mean -23.5 -16.7 -20.1 -24.4 -16.7 -20.5
As is quite evident, subjects exhibited a strong tendency to make movements which were
"undershoots" when made into either the left or right visual fields. Specifically, movements made into the right'visual field were generally terminated to the left of the target, and
movements made into the left field were to the right of the target position. This trend was
expressed across all combinations of the six factors. There was no apparent effect due to
the hand making the response, F(1,7) = 0.22, p > 0.05, for Visual Field, F (1,7) = 0.67,
p > 0.05, or for visual condition, F(1,7) = 0.72, p > 0.05, or interactions therein.
There were a number of higher order interactions however, all involving either the factor of
Target Eccentricity or Relation to Midpoint
TABLE 11.9 X Constant Error (mm) as a Function of Visual Field. Relation to Midyoint and Target Eccentricitv,
Visual Field
Left Left Right Right
Midpoint Lower Upper mean Lower upper hkan
Eccentricity
Inner
Outer - 18.6 -22.4 -20.5 - 12.3 -7.7 -10.0
mean -22.5 -25.4 -24.0 -17.7 -15.6 -16.7
Visual Field interacted with Relation to Midpoint, F(1,7) = 7.38, p < 0.05, and with
Target Eccentricity, F(1,7) = 15.07, p < 0.01. Illustrated most clearly by Figures 11.8 and 11.9, the first of these second order interactions reflects a tendency for movements made to upper targets in the right visual field to be associated with a smaller degree of undershooting than upper targets in the left visual field. By contrast, movements made to lower targets were associated with a smaller degree of undershooting when targets were presented to the left visual field
VISUAL FIELD BY MIDPOINT
LV.F. R.V.F.
UPPER
RELATION TO MIDPOINT
FIGURE 11.8 X Constant Error (mm) by Visual Field and Relation to Midpoint.
Considering the interaction of Visual Field and Eccentricity, it seems that, while outer
targets were marginally less undershot, this trend was more clearly expressed for targets
presented in the right visual field.
VISUAL FIELD BY ECCENTRICITY
El L V F . -10 - R.V.F.
a w
4 U X
FIGURE 11.9 X Constant Error (mm) by Visual Field and Target Eccentricity.
There was also a interaction involving these three factors, F(l, 7) = 6.23, p < 0.05, illustrated most clearly by Figure 11.8. This appears to indicate that the decreasing
magnitude of Constant Error associated with Outer targets in the right visual field, and described above, was predominant when the targets were also presented above the
midpoint.
VISUAL FIELD BY MIDPOINT BY ECCENTRICITY 0 -
LV.F. (LOWER) LV.F. (UPPER)
0 R.V.F. (LOWER) -10 - R.V.F. (UPPER)
a w
4 U X
I
INNER OUTER
ECCENTRICITY
FIGURE 11.10 X Constant Error (mm) by Visual Field, Relation to Midpoint and Target
Eccentricity.
There was a single, statistically significant main effect, that associated with the factor of Pattern, F(3,21) = 6.91, p < 0.005. The trends in the means across levels of Relation to
Midpoint , and of Target Eccentricity are represented in Table 1 1.10.
TABLE 11.10 X Constant Error (mm) as a Function of Pattern. Relation to Midpoint and Target Eccentricity,
Midpoint
Eccentricity Inner Outer mean Inner Outer mean
Pattern
mean -24.8 -15.5 -20.1 -26.0 -15.0 -20.5
The movements made in response to the presentation of pattern 2 were associated with a
slightly greater directional bias than those made to the other patterns. Pairwise, post hoc
comparisons of the means, using the Tukey (HSD) procedure, indicated the presence of
statistically signrficant pairwise differences between the mean X Constant Error for pattern
1 and that for pattem 2.(Table 11. lob). These trends are also evident in Figure 11.11.
TABLE 11. lob X Constant Error (mm) Differences amonP Pattern Means,
PATTERN BY RELATION TO MIDPOINT
-26
0 1 2 3 4 5
PA'ITERN
FIGURE 1 1.1 1 X Constant Error (mm) by Pattern and Relation to Midpoint.
There were also present, higher order interactions associated with the factor of Pattern, with Relation to Midpoint, F(3,21) = 4.0, p < 0.05, reflecting a tendency for the degree of undershooting to targets specified by pattern 3 to be less when the targets were presented
below the midpoint, and with both Target Eccentricity and Relation to Midpoint, F(3,21) = 4.97, p < 0.01. Illustrated most clearly by Figure 11.12 below, this three way interaction
appears to indicate that the two way interaction of Visual Field and Relation to Midpoint is
accounted for primarily by responses to pattern 3 made to the less eccentric targets,
presented below the midpoint.
1 3 1
PATTERN BY MIDPOINT' BY ECCENTRICITY /
0 7
FIGURE 11.12 X Constant Emr(mm) by Pattern, Relation to Midpoint and Target Eccentricity.
Z CONSTANT ERROR
Mean Constant Errur scores in the direction of the Z axis of the target board w& calculated in a fashion similar to that for X Constant Error. A repeated measures ANOVA was again performed, ths design being a 2 x 2 x 2 x 4 x 2 x 2 factorial with Hand (right,
left), Visual Condition (illuminated, non-illuminated), Visual Field (left, right), Pattern (1, 2,3,4), Relation to Midpoint (upper, lower) and Target Eccentricity (inner, outer) as factors. Positive error scores correspond to responses made to above the target position, whilst negative values represent errors made to below the target position.
TABLE 11.11 Z Constant E m (mm) as a Function of Hand and Visual Condition.
Hand
Visual Condition Right Left mean
mean -1.0 4.1
Casual inspection of the mean values presented in Table 11.1 1 belies the fact that there were in fact no statistically sigmficant main effects for Hand, F(1,7) = 3.79, p > 0.05, or for Visual Condition F(1,7) = 1.01, p > 0.05, or for an interaction thereof, F(1, 7) = 0.60, p > 0.05.
Although there wen no apparent main effects for Field, F(l, 7) = 0.14, p > 0.05, nor for Pattern, F(3,21) = 0.40, p > 0.05, there was a statistically si@cant Visual Field by Pattern interaction, F(3,21) = 6.58, p < 0.005, and a Hand by Visual Field by pattern interaction, F(3,21) = 3.85, p < 0.05.
As illustrated by Figure 11.13, movements made in response to Pattern 2, presented to the right visual field were, to a greater d e p , texminated above the target position.
TABLE 11.12 Z Constant E m h r n ) as a Function of Hand. Visual Field and Pattern,
Hand
Right Right Left Left
VisualField Left Right mean Left Right mean
-
mean -1.4 -0.6
VISUAL FIELD BY PATTERN
n LEFI' 4 RIGHT
FIGURE 11.13 Z Constant Errortmm) by Visual Field and Pattern
It would appear that the Hand by Visual Field by Pattern interaction reflects an inordinate tendency for responses made by the left hand in response to pattern 2 to be completed above the target position when the pattern was presented to the right visual field. . ,
HAND BY VISUAL FIELD BY PATTERN
FIGURE 11.14 Z Constant Error (mm) Hand by Visual Field and Pattern.
lo-
5 - a V
4 5 0 -
rn R.H. (L.V.E) 4 R.H. (R.V.F.) E L.H. (L.V.F.) 4 L.H. (R.V.F.)
There was obsmed a large main effect associated with the factor expressing the relationship of the target to the Midpoint, F(l, 7) = 64.28, p < 0.0001. This expresses a tendency to make responses which t h a t e above the target position when the target was below the midpoint, and to make responses erring below target position, when this position
is above the midpoint. This factor interacted &th only one other factor, that of Pattern, F(3,21) = 12.79,
p < 0.0001. As reflected in Figure 11.15, the tendency to undershoot upper targets and to overshoot lower targets is less visibly expressed in response to presentation of pattern 3.
The factor of Midpoint apparently did not interact with Target Eccentricity, F(l, 7) = 0.72, p > 0.05, and there was nothing resembling a statistically sigdicant main effect for Eccentricity.
TABLE 1 1.13 Z Constant Error (mm) as a Function of Pattern. and Relation to Miduoint
Midpoint
Pattern Lower U P P ~
mean
MIDPOINT BY PATTERN
FIGURE 11.15 Z Constant Emr(mm) Pattem.by Relationship to Midpoint
VARIABLE ERROR
In order to establish a meaningful measure of variable error, the cell structure for error values in the X direction was collapsed over the factors of Midpoint and Target Eccentricity, thus providing twelve values within each unique cell, from which a measure of vaxiability could be c a l c u l a ~ The Variable Esror scores thus obtained were subjected to a Four Way Repeated Measure ANOVA, the &sign being 2 x 2 x 2 x 4 factorial with Hand (left, right), Visual Condition (illuminated, non-illuminated), Visual Field (left, right) and Pattern (1,2,3, & 4) as factors.
TABLE 11.14 X Variable Error (mm) as a . .
Function of Hand. Visual Condmon and Visual Field.
Right Right Left Left
Visual Field Left Right mean Left Right mean mean
-
Visual Condition
Illuminated 19.1 17.1 18.1 17.2 17.6 17.4 17.8
Non-iUum 18.9 20.1 19.5 20.1 18.9 19.5 19.5
mean 19.0 18.6 18.6 18.2
Analysis of variance revealed no statistically significant main effects for Hand, (mean, right hand = 18.8 mm, left hand = 18.4 mm), F(1,7) = 0.68, p > 0.05, for Visual Condition, F(l, 7) = 2.48, p > 0.05, or for Visual Field, F(l, 7) = 0.45, p > 0.05. Indeed the trend as expressed through the mean values was toward greater variability for the Right
hand.
HAND BY VISUAL CONDITION
RIGHTHAND LEFrHAND
fl ! I I d
ILLUMINATED NON-UUM
VISUAL CONDlTION
FIGURE 11.16 X Variable Error(rnm) Hand by Visual Condition.
TABLE 11.15 X Variable Error (rnm) as a Function of Visual Field and Pattern.
P a m Left Right mean
There was a statistically significant main effect for the Factor of Pattern, F(3,21) =
4.01, p < 0.05. A post hoc analysis involving a pairwise comparison of mean differences,
using the Tukey (HSD) procedure (Table 11.15b) revealed the presence of statistically
signiscant pairwise differences between the Variable Enor in the X.direction for movements made in response to Pattern 3 and for responses made to all other Pattern presentations. TABLE 11.1% X Variable Enm (mm) Differences among Pattern Means.
**p < 0.01. I
In addition, there was revealed a Visual Field by Pattern interaction, F(3,21) =
7.45, p < 0.005. As illustrated by Figure 1 1.17,the increased variability associated with
Pattern 3 was more pronounced for movements made in response to target positions located in the left visual field.
VISUAL FIELD BY PATTERN
LElTFIELD RIGHT FIELD
FIGURE 1 1.17 X Variable Error (mm) Visual Field By Pattern.
As for X Variable Error, the cell structure for error values in the Z direction was collapsed over the factors of Midpoint and Target Eccentricity, again providing twelve values within each unique cell. The Variable Error scores were subjected to a Four Way Repeated Measure ANOVA, again the design being 2 x 2 x 2 x 4 factorial with Hand (left, right), Visual Condition (illuminated, non-illuminated), Visual Field (left, right) and Pattern (1,2,3, & 4) as factors.
TABLE 11.16 Z Variable Error (rmn) as a Function of Hand. Visual Condition and Visual Field,
Hand
Right Right Left Left
- - - - -
Visual Field Left Right mean Left Right mean mean
Visual Condition
Illuminated 22.1 19.1 20.6 23.3 20.6 21.9 21.3
Non-illum 24.6 24.9 24.7 25.3 23.0 24.1 24.4
mean 23.3 22.0 24.3 21.8 22.8
The inferential analysis indicated the presence of a main effect for visual Condition, F(l, 7) = 6.64, p < 0.05, there was greater variability in the magnitude of error in the Z direction when movements were made in the absence of ambient lighting.
Similarly, there was greater variability for movements made in response to targets located in the left visual field, F(l, 7) = 6.10, p < 0.05.
There was, however, no apparent effect of the hand making the response (mean left hand = 22..7 mm, mean right hand = 23.0 mm), F(l, 7) = 0.36, p > 0.05. No higher interactions between these factorsattained statistical sigmficance. Some indication of the relative contributions of the factors of Hand and Visual Field is ~rovided by Figure 1 1.18
HAND BY VISUAL FIELD
a ! I I d
LEFT RIGHT
VISUAL FIELD
FIGURE 1 1.18 Z Variable Error(rnrn) Hand by Visual Field.
The factor of Pattern was associated with a main effect of statistical significance, F(3,21) =
5.68, p < 0.01, pairwise post hoc analyses employing the Tukey (HSD) method revealed the presence of statistically significant pairwise differences in Z Variable Exror fur movements made following presentation of Pattern 1 and Variable Error associated with all
other patterns. Further, Z Variable Error when movements were in response to the presentation of Pattern 3 was appreciably lower, to a statistically significant degree, than
the Z Variable Error associated with Patterns 2 and 4 (Table 11.17b).
In addition, the interaction of Pattern and Visual Condition, as illustrated by Figure 11.19,
was of statistical sigruficance, F(3,21) = 3.65, p < 0.05. It appears that this may be
accounted for in terms of the greater variability associated with responses made to pattern 1, when ambient illumination was not present.
TABLE 1 1.17 Z Variable Error (mm) as a Function of Visual Condition and Pattern,
Visual Condition
Pattern Illuminated Non-illum mean
mean 21.3 24.4 22.8
TABLE 1 1.l7b Z Variable Enm (mm) Differences amonq Pattern Means..
PATTERN BY VISUAL CONDITION
FIGURE 1 1.19 Z Variable Error (mm) Visual Condition by Pattern.
VARIABIL,lTY IN TJ!E TIME TO PEAK VELOCITY,
In order to establish a legitimate measure of the variability in the time taken to'reach
peak velocity, the cell structure was collapsed ova the factors of Midpoint and Target Eccentricity, latencies fur individual trials thus provided twelve values within each unique
cell, from which a measure of the variability could be obtained. The Variability indices thus
calculated were subjected to a Four Way Repeated Measure ANOVA, the design being a 2 x 2 x 2 x 4 factorial with Hand (left, right), Visual Condition (illuminated, non-
illuminated), Visual Field (left, right) and Pattern (1,2,3, & 4) as factors.
The analysis failed to reveal the presence of a main effect fur either the factors of
Hand (mean right hand = 32.8 ms, mean left hand = 38.1 ms), F(l, 7) = 3.97, p > 0.05 or
for Visual Field, F(l, 7) = 0.1 1, p > 0.05. The factor of Visual Condition was, however,
associated with a main effect of statistical sigdicance, F(1,7) = 6.19, p c 0.05, (mean illuminated = 33.5 ms, mean non-illuminated = 37.4 ms). The relationship between the
mean values of variability obtained for all Hand-Visual Conditions is illustrated in Figure
11.20
HAND BY VISUAL CONDITION
30 ! I I I
illrrminated nan-ill1m.I
VISUAL CONDmON
FIGURE 11.20 Variabilitv in the Time to Peak Velocity as a Function of Hand and Visual
Condition,
Table 11.18 comprises the mean values of the variability for all combinations of the factors
of Visual Field and Hand. An interaction of these factors was observed, F(l, 7) = 7.87, p
< 0.05, representing a tendency toward increased variability for right handed movements 145
made into the left visual field and for left handed movements made into the right visual
field. This trend is explicitly illustrated in Figure 1 1.21
TABLE 11.18 Variabilitv in the Time to Peak Velocitv as a Function of Hand and Visual
Field.
Hand
Visual Field Right Left
Left
Right 30.3 40.1
FIGURE Field.
HAND BY VISUAL FIELD
42 1
righthaad left hwd
-- I I
left right
VISUAL FIELD
11.21 Variability in the T i e to Peak Velocitv as a Function of Hand and Visual
Mean Peak Velocity measures were obtained b m each cell of the 128 unique. combinations of the six factors, each mean value being derived from three trials. As previously described, for other measures, a repeated measures ANOVA was performed, employing a 2 x 2 x 2 x 4 x 2 x 2 factorial design, with hand (right, left), visual condition (illuminated, non-illuminated), visual field (left, right), pattern (1,2,3, & 4), midpoint
(above, below) and eccentricity (outer, inner) as factors. The analysis revealed main effects of statistical significance for the factors of Hand,
F(1,7) = 24.55, p < 0.005, and for Field, F(1y 7) = 7.87, p < 0.05 and an interaction of these factors, F(1,7) = 75.44, p c 0.0001. Illustrated most clearly by Figure 11.22, and by the mean values contained in Table 11.20, the highest peak velocities were obtained for the respective hands when the response movements were being made into the ipsilateral visual field.
TAB= 11.19 Mean Peak Velocitv ( 4 s ) as a Function of Hand. Visual Condition and
- Visual Field,
Hand
Right Right Left Left
VisualField Left Right mean Left Right mean
Visual Condition
Illuminated 2.41 2.56 2.48 2.36 2.23 2.30
Non-illum 2.42 2.61 2.52 2.38 2.29 2.33
mean 2.41 2.59 2.50 2.37 2.26 2.32
TAB= 1 1.20 Mean Peak Velocitv h / s ) as a Function of Hand and Visual Field,
Hand
Visual Field Right Left mean
Left 2.4 1
Right 2.59
mean
HAND BY VISUAL FlELD
RIGHTHAND *- EITHAND
22 ! I I d
LEFT RIGm
VISUAL FIELD
FIGURE 11.22 Mean Peak Velocity (m/s) as a Function of Hand and Visual Field,
The factors of Visual Condition and Visual Field interacted in a fashion which
reached statistical significance at the 0.05 leveL As indicated by Figure 11.23 and by Table
1121, for movements made into the right visual field, mean peak velocities were greater
when ambient illumination was not present, F(1,7) = 7.13, p < 0.05.
TABLE 11.21 Mean Peak Velocitv ( 4 s ) as a Function of Visual Condition and Visual Field.
V i i Field
Visual Condition Left Right mean
Illuminated
VISUAL FIELD BY VISUAL CONDITION 2.46 1
LVF. R.V.F.
2 3 8 ! I I J
ILLUMINATED NON-UUM
VISUAL CONDITLON
FIGURE 1 1.23 Mean Peak Velocitv ( 4 s ) as a Function of Visual Condition and Visual
Field.
The effect of the Pattern factor was such that a statistically si@cant main effect
was obtained, F(3,21) = 9.29, p < 0.0005. A Tukey (H.S.D.) post hoc pairwise
analysis.of differences in the means produced three comparisons which were of statistical
s iwcance (Table 11.22b). Mean Peak Velocities achieved during reaches in response to
the presentation of Pattern 1 were appreciably higher than those achieved in response to all
other Patterns. There was observed an additional interaction of this factor with Target
Eccentricity, F(3,21) = 4.36, p < 0.05. From inspection of Figure 11.24 it would appear
that this interaction reflects an elevated peak velocity for movements made to outer targets
in response to the presentation of pattem 4.
TABLE 11.22 Mean Peak Velocitv ( d s ) as a Function of Pattern and Target Eccentricitv.
Pattern
Eccentricity 1 2 3 4
Inner 2.52 2.4 1
Outer 2.50 2.4 1 2.32 2.43
TABLE 11.22b Mean Peak Velocitv (ds). Differences amow Pattern Means.
PATTERN BY ECCENTRICl'l'Y
1 Q INNER
0 1 2 3 4 5
PATTERN
FIGURE 11.24 Mean Peak Velocitv (rnls) as a Function of Pattern and Target
Eccentricitv.
The Pattern factor also interacted with the Relation to the Midpoint, F(3,21) = 9.69, p < 0.0005. Indications obtained from the distribution of relevant means appear to
indicate that the mean peak velocities for movements made to above the midpoint were less
critically affected by variations apparently due to the presentation pattern. A main effect of
statistical ~ i ~ c a n c e was also observed for the factor of Midpoint, F(l, 7) = 61.f34, p < 0.0001. Clearly, higher mean peak velocities were obtained for movements made toward
targets located above the midpoint.
TABLE 11.23 Mean Peak Velocitv (mls) as a Function of Pattern and Relation to
-.
Pattern
Midpoint
Lower 2.42 2.28 2.14 2.3 1
PATTERNBYMIDPOINT
FIGURE 11.25 Mean Peak Velocitv (m/s) as a Function of Pattern and Relation to
Miduoint.
Two further, higher order, interactions were also present, illustrated by Table 11.24
and Table 11.25, and by Figure 11.26 and Figure 11.27 respectively. Hand interacted with
Visual Field and Target Eccentricity, F(l, 7) = 28.46, p < 0.005, in a fashion which
indicates that, for the right hand, movements made into the right visual field were accompanied by mean peak velocities which increased with target eccentricities, whilst for
movements made toward the left visual field, peak velocity decreased with target
eccentricity. For the left hand, the pattern was reversed, peak velocity increased with
eccentricity for movements made toward the left visual field and decreased with increasing
eccentricity for movements made to targets in the right visual field. Or more succinctly, for ipsilateral movements, peak velocity increased with eccentricity, whilst for contralateral movements, peak velocity decreased with eccentricity. This illustrates once again the pervasive nature of what are apparently spatial compatibility effects.
TABLE 11.24 Mean Peak Velocity (m/s) as a Function of Hand. Visual Field and Tarrret
Eccentricity.
Visual Field Right Left
Left Inner
Left Outer
Right Inner 2.54
Right Outer 2.64
HAND BY VISUAL FIELD BY ECCENTRICITY
u - I
INNER
FIGURE 1126 Mean Peak Velocitv (mls) as a Function of Hand. Visual Field and Tarpet
E-
The second three way interaction was that of Hand by Visual Condition by Pattern,
F(3,21) = 3.34, p < 0.05. As illustrated by Figure 1 1.27, this interaction was somewhat
complex. There is some reason to suggest that the more general, though weak, trend for the
mean peak velocity of right and left hand movements to be greater when ambient
illumination was not present, was to some extent reversed for the right hand only when
responses were made to pattem 3. One can only assume that the interaction had some
consistency which is not reflected by initial examination of the means.
TABLE 11.25 Mean Peak Velocity (mls) as a Function of Hand. Visual Condition and
Pattern.
Hand
Right ,' Right Left Left
Visual Condition Illllminated Non-illum Illuminated Non-illum
Pattern
4 2.48 2.5 1 2.27 2.33
mean 2.48 2.52 2.30 2.33
HAND BY VISUAL CONDlTION BY PAITERN
FIGURE 11.27 Mean Peak Velocitv (1x11s) as a Function of Hand. Visual Condition and
Pattern.
1 1.4 DISCUSSION One of the primary considerations of this study was the examination of the potential
differences which may exist between the preparation for, and the regulation of, movements
made by the preferred and non-preferred hands. In all cases, in experiments 1 and 2, subject's preferred hand was their right. Reaction Time may be considered as being some index of the time required for movement preparation , though at this stage it is important
that it should be clearly distinguished from accounts of Reaction Time as the time for
movement "programming". In this experiment, as in Experiment 1, there was no statistically significant main effect for reaction time associated with the hand making the response, although in both cases, the mean latency to initiate responses was lower for the
left hand in six of eight subjects. The magnitude of this mean difference was approximately 10 milliseconds in each experiment, yet was obviously expressed with an inconsistency
such that the attribution of any functional significance to this difference must be approached with considerable caution. It is tempting to speculate that for movements which were
themselves matially complex, as was seemingly the case for the movements made in these
experiments, the right hemisphere would be involved in the preparation for action to a
greater extent than for less spatially complex movements, which perhaps did not involve
this progression into extrapersonal space. Certainly, in simple or choice reaction time studies, in which the spatial topology of the response movement is considerably diminished, a right hand advantage is more generally observed (e.g., Anzola, Bertoloni, Buchtel & Rizzolatti, 1977), while Kimura and her associates have argued, on the basis of
clinical evidence (e.g., Kirnura, 1974; 1977; 1979; Lomas & Kimura, 1976; Lomas, 1980)
that the left hemisphere is superior for "movement programming", an advantage which is
ostensibly reflected in the pattern of impairments for tasks which emphasise
preprogramming". The trend towards a left hand advantage in the present experiment may
reflect the presumed shift toward greater right hemisphere involvement which is thought to
accompany an increasing relative spatial complexity of the response movement.
Much greater between hand differences were exhibited, in this experiment, in terms
of measures which are assumed to reflect other facets of movement regulation. Responses
initiated by the right hand were completed more rapidly than those made by the left hand.
This effect may usefully be compared with trend for Mean Peak Velocity which is the
inverse of that for Movement Time, it is not unexpected that the larger peak velocities
exhibited by the right hand were associated with shorter movement times, though this was
not assessed on an individual trial basis. If the Mean Peak Velocity may indeed be regarded
as some indicator of the applied impulse, it would appear that greater force was generated
by the preferred hand, a result which would not be unanticipated, or at least that an equivalent force was applied for a longer period.
In view of the claim made by Annett, Annett, Hudson and Turner (1979) that the
non-preferred hand is more variable in its output of force, the relative variability in the time
required to reach peak velocity may be seen as a preliminary means of testing this
assumption. Although there w& a relatively consistent tendency for movements made by
the left hand to be associated with greater variability in the time to peak velocity, this apparent effect failed to meet the criterion adopted for defining statistical sigdicance.
Obviously a measure of Jerk, had this been available, would have been a more direct means
of assessing the variability in applied force. Variable Error has been considered a measure of the accuracy with which a pre-
existing motor program is executed (e.g., Guiard, Diaz & Beaubaton, 1983). Although one may find strong cause to reject this interpretation, there may still be some utility in simply taking Variable Error as a pointer as to the fluctuation of the terminal locations of
movements, to "equivalent" targets over a course of trials. In this respect there was no
appreciable difference between the hands for variability in either the X or Z directions. One might again speculate that, for spatially complex movements, the manifestations of a presumed greater right hemisphere involvement in &ding with an "abstraction of space"
are not confined to events temporally preceding the initiation of movement. Therefore, for movements of this nature one would perhaps not expect a left hand advantage for a measure of the accuracy, or at least variation in accuracy of execution.
Radial Error may be regarded as the most comprehensive indicator of the accuracy with which subjects completed their responses, providing, as it does, information on the
absolute distance between the position of the phantom target and the terminal location of the movement. In more concrete terms, it is a demonstration of how successful the subject was in accomplishing his action goal, albeit a goal which was &fined by the experimenter.
Overall, individuals were remarkably accurate, bearing in mind the fact that the target
position was never actually displayed. Movements were completed a mean distance of
approximately 40 mm from the target. There was however no advantage for the preferred
right hand. Although there are problems associated with the interpretation of any non-effect
in this context, this trend is particularly noteworthy as a preferzed hand advantage, in turns
of overall accuracy, has been widely anticipated in literature which to date has been
insensitive to the influence of any more than a limited set of specific task demands. As will
be outlined in the General Discussion, experimental identification with the null hypothesis
is not beyond the scope of current research in "Motor Control". It does not appear that the
measure of Radial Error was insensitive as such, there was detected a main effect 158
associated with visual conditions which will be discussed in greater detail below. Rather, it simply appeared that responses made the preferred and non-preferred hands were ,
equivalent in teminal accuracy. In view of the global charactefistics of this pointing task, a progression to locate a point in space, it is again possible to hazard- the explanation that the
usual increased left hemisphere activity which is contiguous with the regulation of simple movements is for complex movements, on a temporal basis, accompanied by, and on a
quasiphysical basis, transacts with an increased right hemisphere involvement. This
transaction to be expressed in measures sensitive to the regulation of the ongoing
movement. Rather than considering Constant Error as indicative of the accuracy of "central
programming" (Guiard et al., 1983), one should perhaps maintain rather less grandiose expectations and regard this measure in its primary form, as merely the directional bias of a series of movements. In this experiment there was essentially no evidence of a difference
between the responding hands in terms of Constant Error in the X direction. Similarly, for
measures of Z Constant Error, although it appeared that right hand movements tended to
slightly undershoot target positions and left hand movements to more appreciably overshoot (in this case terminate above targets), this effect had little consistency.
Clearly a central consideration in this investigation was the effect of, what was presumed to be, an imposed manipulation of spatial complexity. As discussed with respect to Experiment 1, it was felt that the desired manipulation might only be achieved by giving
subjects a prior conscious awareness of the precise nature of the pattern sequences. The
pre-experimental test sessions with example displays indicated that apprehension of the
sequences could be achieved most rapidly. Thus there may be some assurance that
individuals were able to appreciate the correspondence between a pattern and the
appropriate target location. Post practice session and post experimental debrieiings at least indicated that subjects experienced some confidence as to their ability to extrapolate to target
locations. Reaction Time measures failed to indicate the presence of any appreciable
differences between patterns, though ktt3re~tinf& a trend across both responding hands
was for responses to the presentation of Pattern 2 to be initiated more rapidly than, for
example those to what was supposedly the more "simple" pattern 1. The magnitudes of
these differences in the means were small and statistically not sigdicant. There was no
evidence of interaction between the factor of Pattern and either the hand making the
response movement or the visual field in which the sequence was presented.
The absence of an interaction with Visual Field is particularly important. When the
task of interest requires a relatively uncomplicated manual response, reaction time measures 159
may be regarded as reasonably good indicators of the processing charactastics of the
cerebral hemisphere which first receives the stimulus information, in this case the ,
hemisphere contralateral to the visual field in which the pattern was displayed. In these
circumstances one might anticipate that a right hemisphere advantage, in terms of the
rapidity with which spatial relationships are "appreciated", would be manifested as a left
visual field superiority, and extending this reasoning expect an interaction with Pattern.
However, as Bashore (1981) points out, the characteristics of reaction time indices may
vary considerably as a function of the type of movement required. In these current
experiments, it is likely that the highly complex nature of the movements was such that the
idiosyncratic stimulus processing characteristics of the receiving hemispheres were overshadowed to a large extent. Certainly, the reaction time latencies obtained in both
Experiment 1 and 2 (404 and 355 ms, respectively) are appreciably higher than those obtained in the simple reaction time studies reviewed by Bashore (1981), which are almost
universally below 300 ms. As a choice element is introduced, in the current experiments through the use of a number of target positions and indeed a variety of patterns, the likely
contribution of various factors becomes even more complex, as Bashore (1981) comments: The result is that variables known not to influence simple reactions do alter the time required to make choice decisions. The cerebral mechanisms responsible for producing these effects are unknown (p. 374).
Another possibility exists, that the nature of the pattern was such that, containing elements
which were both spatial in nature and encapsulating a progression which was sequential, there may have been inherent characteristics which favoured the processing "functionalities" of each hemisphere. Whilst continuing to maintain that it is not plausible that one hemisphere is uniquely specialized for the sequential processing of information
(see Chapter 3), it does appear to be the case that the displays used in these experiments may comprise aspects which involve the partial involvement of both hemispheres. Certainly
it did appear that most of the effects associated with the Pattern manipulation were
expressed symmetrically.
In section 11.1 it was suggested that a subject of interest was an enquiry as to
whether any effects associated with the characteristics of the visual information speclfylng
target position recurred throughout the course of the movement. This recursion, if present
was, it was hypothesized, to be expressed particularly in terms of kinematic indices. This
potential recursion was in contrast to the view, expressed by Fisk and Goodale (1985), that
the influence of spatial localization was confined to "neural systems" prior to those
concerned with motor output. In this enterprise it is worth reemphasizing that the pattern
presentation was terminated some time prior to the initiation of movement, there was on
average a delay of over 200 ms from sequence offset to response initiation. 160
A systematic variation in movement time was associated with the presentation
pattern, the required to complete responses to targets specified by Pattern 3 was appreciably greater than when indicated by any othcr pattem. FranLs (note 3) has speculated that some
degree of "tracking" may be occurring. This seems unlikely as the stimulus is never actually present during the movement itself. Also, if some form of imagery is involved, as a substrate for tracking, one would anticipate that a positive relationship would exist
between movement time and the magnitude of the distance between successive presentation
points (c.f., Cooper & Shepard, 1973). If this were the case, the movement times
associated with Pattem 4 would presumably be longer than those for Pattern 3, clearly this
was not the case.
Higher initial impulses, such as those which, it is inferred, accompanied right hand
movements have been associated with greater variability and consequently reduced terminal accuracy (Schmidt, Zelaznik, Hawkins, Franks & Quinn, 1979), yet in these instance, the left hand was slightly, if not sigmficantly, mare variable in terms of the time to reach peak velocity, and no more variable in measures of tenninal accuracy as expressed by X and Z
Variable Error. It may be the movement, in its time course to "peak velocity", although not in itself unmodified or ballistic, may bring the right hand closer to the target position, in
spite of the greater initial impulse. Indeed this possibility has been discussed by Todor and
Cisneros (1985). One might further suggest that the variation in accuracy as a function of
initial force production observed by Schmidt et al. (1979) in response to increased accuracy
demands, is qualitatively different from the relationship between initial impulse and
variability which appears to differentiate the hands. Following this reasoning, movements
made in response to the presentation of Pattern 3, perhaps arising indirectly from some
"ambiguity" associated with the nature of the pattern, may result in motion which in the
initial phases is further from the target location. This may be regarded as purely speculative at present and a topic for further investigation.
As Prablanc, Eschallier, Komallis & Jeannerod, 1979 have demonstrated, the relationship described by Fitts Law is maintained in the absence of visual feedback. Carlton (1979) has suggested that kinematic analysis of movement profiles supports a "discrete
feedback interpretation" of this relationship. Clearly for the movements presently being
discussed, discrete modifications were not necessarily subject to the direct influence of
visual information. Prablanc, Pelisson & Goodale (1986) have commented that 'open loop' movements (open loop with respect to visual feedback about the relative positions of the target and the moving limb) are far from being uncorrected or ballistic. Indeed, the prime effect of target duration on pointing accuracy suggests that visual information about target location is somehow used to control the movement during its execution.
To this it might be added that "open loop" movements are far from being
"uncorrected", or at least unmoditi.ed, in the absence of concurrent visual information
relating to the moving limb and the target, aithough some information initially derived
from the optic array is used to regulate the movement during execution. One must conclude
that some affardance as to the position of a "target" in extrapersonal space is provided by a change in the optic array ova the &tially static eye, even when the position of the target
itself is never explicitly defined. The mechanisms through which this may occur are discussed in greater detail below.
There was no effect of Pattern upon the absolute accuracy of the response as
expressed by Radial Error, though there was an effect of Pattern associated with X Constant Emr. Interestingly, movements made in response to the presentation of Pattern 1 exhibited less variability of terminal error in the X direction, yet appreciably more variation
for errors in the Z direction. In the latter case, the Variable Error of responses made to Pattern 1 was sigmficantly larger than for all other patterns and might conceivably represent
a sensitivity to occasions on which the direction of the sequence, the temporal progression,
was misapprehended. Subject's comments during practice trials indicated that there was
some initial ambiguity associated with Pattern 1 in this respect. However, it is p e g that
this potential equivocality was not also expressed in terms of X Variable Error, as
misapprehension of the pattern, and a resultant response at the "wrong end" of the pattern
represents error in both directions. The magnitu& of the peak velocity achieved during the course of the movement
was directly influenced by the nature of the pattern which had defined the target, Responses
to Pattern 1 were accompanied by peak velocities higher than those for all other patterns.
The lowest peak velocities were observed for responses to Pattern 3. As for considerations of the hand making the response, there appears to be no
straightfmard relationship which exists between, the distribution of initial impulses as
reflected by Peak Velocity and the variability of terminal location as reflected by both
Variable Emr measures. The trend in terms of peak velocities was not unambiguously
related to Variable Emr, although Z Variable Error (figure 11.19) more closely resembled the trend in Peak Velocities (figure 11.24). This somewhat confusing picture serves to emphasis the caution which should be exercised in attaching impo~ance to the measures of
Constant and Variable Enw= The axes utilized were defined with respect to the display board and are thus somewhat arbitrarily imposed upon the subject. There is no direct
reference to whole body or joint axes, other than in terms of alignment with the fixation
point. The body segments may be potentially described with respect to a Cartesian
Coordinate system, yet they are certainly not organized on this basis (c.f., Turvey &
Carello, 1986). Perhaps the most striking characteristics of the data collected were the indications
that visual infomation related to the motion of the responding hand contributed
signifcantly to the terminal accuracy of the movement, as expressed by measures of Radial
Error. It should be recalled that visual infomation of the position of the target was never
explicitly made available, and the visual stimuli comprised implicit indications of the target
location were never available during the course of the movement. As a consequence, at no
stage was there present, visual information regarding the relative positions of the target and the responding hand.
Vision of the responding hand was obviously useful for the regulation of movement toward a target which was never explicitly defined in equivalent terms. It was never possible, for example, for individuals to direct an eye saccade towards the actual position of the target in space. Indeed it is a topic of enquiry as to the role assumed by the terminal position of the first saccade following sequence presentation. Fisk and Goodale (1985)
have discussed that, in circumstances in which targets are presented only briefly and are extinguished prior to movement initiation, the point fixated by the eyes may serve as the
target for the subsequent limb movement. These authors observed a positive correlation
between the final positions of the eye and the limb movement, again suggesting that the
area of space fixated by the eyes may have constituted the best available indicator of target location. It was in the present circumstances impossible to establish whether eye
movements were directed to the position of the final point of light projected, to some other
element of the sequence or to some related point in extrapersonal space which had not been illuminated. There was no variation in terminal accuracy associated with the pattern which
had been displayed. This perhaps may be taken as circumstantial evidence that the final dot
presented was not taken as the best indicatur of the target, the final displayed point for
Pattern 1 was closer to the target than, for example, the final displayed point of Pattern 4,
though there were no accompanying differences in accuracy. The extK'aI't2~al signal
associated with eye movements may have been equivalent across all patterns. Certainly if
an eye movement was made to an area of space rather than to a specific point of the target
display this may well have been the case. If extraretinal signals were used as the
predominant basis for the approximate localization of target position, the non-specifi.city of
this signal across all Pattern presentations would be well carrelated with the equivalence in
tenninal position. Although it has been observed that extra-retinal signals represent a rather
impoverished source of information when other cues are available (Prablanc et al., 1986),
in the absence of accurate foveal infoxmation as to the position of the target, these imprecise 163
extra-retinal signals must represent the best basis on which the subject may proceed. One
might enquire whether a saccade to an ill defined region of space, providing some foxm of extra-retinal signal provides more potent information than the flow of information over the
static eye, a situation which occurs during stimulus prsmtation. Obviously further
research, in which eye movements following presentation were not permitted would provide some resolution of issue.
The directional biases of the pointing movements may themselves be directly
influenced by the nature of the, initially contiguous, eye movements,and as such may
provide some indication of the relative contributions of extra-retinal and optic-flow related
signals. As touched upon in Chapter 7, one of the most consistent characteristics of the oculu-motor system is for initial saccades to undershoot the target. Conventionally, when
target information is C O I I M U O ~ ~ ~ ~ available, a second "corrective" saccade of smaller magnitude brings the target into foveal vision. This has a practical utility, for eye movements, as for limb movements, it is advantageous to initially undershoot than
overshoot, movements involving reversals being at least more time consuming. In this
experiment, the subject was never afforded the opportunity to make a second corrective saccade, initial saccades were not only likely to have been somewhat inaccurate, they were also likely to have been undershoots. If extra-retinal information pertaining to the initial
saccade was used as the primary estimate of target position, it is likely that the resultant
movements would also exhibit some degree of undershootingHowever as there was some evidence of the presence of interactions for Constant Error measures, particularly involving
the factor of Midpoint, it may be possible that biomechanical factm or at least factors
associated with the execution of the limb movements account, in part, for the tendency to
undershoot. It may have been that the biomechanical system constraints were such that
undershoots were more likely, though whilst there was a Hand by Visual Field by
Midpoint interaction for Radial Error (the possible mechanisms underlying which are discussed below) there was no consistent directional bias which would lead one to
emphasise the exigency of such constraints. Importantly, Fisk and Goodale (1985) observed that the correlation of the terminal positions of eye and limb movements varied
significantly as a function of target exposure duration. Then was a stronger relationship
between the respective tenninal accuracies when a brief, 100 ms exposure condition was
used, in which the only information related to target position was apparently that provided
by the extra-retinal information. In the longer duration condition, subjects were able to
locate the target in foveal vision and obtain indications as to the relative positions of the
target and the responding had duxing the latter phase of the movement, therefore the
position of the eye on completion of the first saccade was likely to have contributed 164
proportionately less to the regulation of the movement. Although in the present experiment
it would appear impossible to disentangle the possible effects of biomechanical factors from
those relating to extraretinal signals in terms of influencing terminal accuracy, and therefore
becomes difficult to assess the relative contributions of retinal (in the form of changes in the
optic flow field over the static eye) information and extra-retinal information, to the initial
establishment of general target location, it does appear reasonable to assume that extra-
retinal information accompanying the first saccadic eye movement was particularly
important. To consider again the issues raised in Chapter 7, it may be a normal characteristic of
normal reachindpointing movements to exhibit "ongoing corrections" subserved by motion
related, non-visual information and visual target information which require very little time
(Prablanc, Pelisson & Goodale, 1986). One should note of course, the similarities and
differences between the present experiment, and the Pelisson, Prablanc, Goodale and
Jeannerod (1986) study in which "pseudocontinuous" modifications of the movement
trajectory were made in the absence of visual information of the relative positions of the
target and the limb. In the latter study, modifications of the movement trajectory were
necessary to accommodate perturbations of the target. There was no visual information
available as to the position of the moving limb. The authors argue, on the basis of the Prablanc et al. (1986) results, that:
a dynamic control system existed in which extra-visual information about the position of the moving limb is compared with visual information about the position of the target @. 303).
In this experiment, however, it was the case that there were improvements in
terminal accuracy, potentially arising from non-discrete, or at least non time consuming,
modifications of the movement trajectory, when information about the position of the
moving limb was available. Information relating to the position of the target could, during
the course of the movement, only have been extra-visual, and was probably initially extra-
retinal. Paillard (1982) has emphasized that movement cues, quite possibly those obtained
in the periphery as vision of ones body segments, are closely associated with motion
relative to the visual axis previously established by foveal grasp of the target. The results of
the present study would appear entirely consistent with this synopsis, in that, both the visual axis and the target position were probably established by foveal gasp, and in this
case foveal vision could be considered as relatively unimportant in the regulation of
movements for which a terminal comparison of target and hand positions, ostensibly
subscrved by foveal vision, could never occur.
The results of this experiment also provided confirmation of the highly pervasive
nature of what might be termed spatial compatibility effects. It is the nature of these effects I . 165
to which attention will now be devoted. In line with the results obtained from Experiment
1, there were no indications of Visual Field by Hand interactions for measures of Reaction Time. Whilst, as outline above, there is reason to believe that the patterns of movement
initiation latencies for complex pointing or reaching movements will differ appreciably from those for simple or choice reaction time paradigms, it is puzzling that there was no
convergence with the results obtained by Fisk and Goodale (1985) who used similar
reaching tasks and elicited spatial compatibility effects for reaction time measures.
Nevertheless what are apparently consistent effects of this nature appeared to transcend the
most simple behavioural description of the pointing movements. For the measure of
Movement Time, there was an appreciable Hand by Visual Field interaction. For both
hands, contralateral movements consumed more time than the ipsilateral equivalents. Although there was no expression of a similar effect upon the terminal accuracy of
the movements, there was for Radial Enor a higher order interaction of Hand by Visual
Field by Midpoint. This trend will be discussed in greater detail below, suffice to say at
present, contralateral movements for both hands were associated with greater error when
made to above the midpoint, whereas for ipsilateral responses there was some divergence
of the hands. Left hand responses to the left visual field were more accurate when to upper
targets, whereas, right hand responses were more accurate to lower targets presented in the right visual field.
There was no evidence of a Hand by Visual Field interaction in terms of the tendency to undershoot targets as expressed by X Constant Error, nor any equivalent interaction for Z Constant Error, nor for either measure of terminal variability. In terms of the variability in the time to achieve peak velocity, there was a statistically significant
interaction of Hand and Visual Field, contralateral movements were associated with greater
variability than the ipsilateral equivalents. If this measure, as has been suggested, may be regarded as some index of the initial impulse, there may be some reason for considering
that there was greater variability in the initial production of force for contralateral movements. One should also be aware that this measure may also be sensitive to the time
course over which the impulse was described. It may be that electromyographical monitoring of muscular activity during these movements may serve as a suitable basis for
comparison of the initiation of contralateral and ipsilateral movements. The Hand by Visual
Field interaction in terms of the Mean Peak Velocity achieved during the movement
suggested that contralateral movements were associated with lower peak velocities and
potentially lower initial impulses. These indices may collectively suggest that the reasons
for the increased movement times for contralateral movements were not due to
biomechanical factors exerting an influence while the movement was "unfolding". 1 6 6
Although the distance moved by the tip of the stylus may have been the same for ipsilateral
and contralateral movements, the displacements of the centre of mass of the responding
limb may not have been equivalent. The inertial characteristics of contralateral movements
are unlikely to be the same as inertial characteristics for ipsilateral movements. Although
biomechanical factors cannot be completely excluded, there are additional reasons to believe
that these influences do not form the primary basis of spatial compatibility effects.
Fisk and Goodale (1985) devoted particular attention to examination of whether the
kinematics of limb movements, and in particular the differences between ipsilateral and
contralateral reaches could be accounted for by differential biomechanical constraints or
patterns of muscular activation. The monitoring of eye movements revealed that, as with
limb movements, saccades were initiated more slowly when they were accompanying
contralateral reaches. Perhaps more importantly, the scaling of movement times observed
for limb movements was reproduced for eye movements, even though it was the case that
for contralateral and ipsilateral limb movements into for example the right visual field, the
eye movements required to fixate the target were identical in either case. Although the eye
movements were completed more rapidly than limb movements, the eyes having smaller
inertia to overcome, they were initiated equivalently. In concrete terms, the reaction times
were highly correlated Fisk and Goodale (1985) argue that the temporal synchrony of the
two motor systems must be accounted for as a "common integration of motor
programming .... at higher levels in the central nervous system".
These authors also suggest that it is unlikely that the pattern of response latencies
and velocities profiles was influenced by purely mechanical constraints, as for contralateral
reaches, peak velocity was achieved some time before the hand crossed the body midline,
movements at this point were topologically still ipsilateral. They argue that "braking" due to
antagonist activity in contralateral reaches was a contributing factor only in the final stages
of the movement. In this sense the claim that differences in patterns of muscular activation
operating in each case are not the basis of the observed differences, one must distinguish
between the muscular activity associated with the initial impulse, presumably agonist firing,
and the subsequent activity which contributes to what appears to be the regulated nature of
the trajectory.
In view of the clear differences between ipsilateral and contralateral reaches, in
terms of the peak velocity achieved and the variation in the time required to reach that peak
velocity, and by implication the initial impulse, and associatively the initial pattern of
muscular activity, one must conclude that spatial compatibility effects may be at least
partially reflected in the characteristics of initial muscular activation. There is other
circumstantial evidence which appears to justify an exclusion of biomechanical factors in 167
favour of some explanation in terms of the neural activity associated with the preparation
and execution of movement. Electrophysiological data collected by Georgeopoulos, Kalaska and Massey (1981) has been taken to suggest that the activity of single neurons in the motor cortex is highly dated to the direction of an associated hand movement.
Movements in eight possible directions were considered, and it was noted that motor
neurons displayed "directional specificity", the highest level of activity in a particular motor
neuron was associated with movement in one direction. Signrficantly, the level of discharge
was generally greatest for ipsilateral movements made foxward into space and outward
from the body axis. Finally one must also consider that in studies examining spatial
compatibility effects and employing a choice reaction time paradigm (e.g., Anzola et al.,
1977), the response movements are such that they are essentially uninfluenced by biomechanical constraints. Whilst it is the case that reaction time in these instances must be
considered in part a reaction time/movement time composite, spatial compatibility effects
are clearly expressed.
It may be concluded that it is a strongly exhibited characteristic of the "motor
system" to exhibit differences, both qualitative and quantitative, between movements which
cross the visuallbody axis and are completed in contralateral space, and those which
proceed entirely ipsilaterally. However, the underlying mechanisms must still be regarded
as unclear, although it does appear that the phenomenon must be considered "higher order"
rather than as arising as a consequence of biomechanical constraints. Ladavas (1987) has demonstrated that spatial compatibility effects may exist in the
absence of any overt correspondence between the spatial properties of the stimulus and the
required response. Generally the right hand exhibited a greater compatibility with "upper"
targets and the non-dominant left hand responded more quickly to lower visual stimuli. Although the experiments described by Ladavas were conducted within the context of a
choice reaction time paradigm, it was of some interest to examine whether these presumed
relationships would be maintained, or of any other consistent correspondences would apply
in situations in which the response act was a more complex movement involving a
progression through space. Once again it is the nature of interactions which are of primary
interest. Main effects associated with factors such as the relationship of the target to the midpoint are themselves of little intrinsic interest. If these effects are present and are expressed in a way similar to the more conventional spatial compatibility effects, these
correspondences are likely to be consistently revealed by a number of dependent measures,
themselves assumed to be sensitive to varying but related aspects of the preparation for and
execution of movement.
Reaction Time measures failed to reveal what, on the basis of the Ladavas work,
might have been an anticipated Hand by Relation to Midpoint interaction. More generally
Reaction Time measures were not associated with main effects or with interactions of
statistical significance, other than a theoretically uninterpretable four way interaction. It is conceivable that the use of m a a n values failed to compensate for problems related to the high variability associated with reaction time measures. Each of 128 cell values for a given subject was composed of the median of three values, this may not have been sufficient to
compensate for the anticipated skewness of within cell distributions.
Indications from measures of Median Movement Time were similarly unrevealing.
Although, as previously discussed, there was an anticipated main effect associated with the
relation of the target to the midpoint, there was no apparent interaction with the responding
hand It was of some interest to note that, in terms of X Constant Error, movements made to targets located above the midpoint were associated with a lesser degree of undershooting when these targets were also in the right visual field, the reverse holding true for lower
targets. However, there was again no interaction with the responding hand This bias was
also exhibited most clearly for movements made to outer targets, Visual Field, Eccentricity and Relation to Midpoint interacted, indicating that the reduced degree of undershooting to
outer targets was greater when these were also upper targets. None of these trends appear
to bear directly on the spatial compatibility effects noted by Ladavas (1987). Similarly, Z
Constant Error scores and Radial Error provided no indications of particular interest in this
respect. Given that the measures of Variable Error and the Variability in the Time to Peak
Velocity were collapsed over the factors of Midpoint and Target Eccentricity, they will not
be considered further. Likewise the Mean Peak Velocity provided no illumination of this
supposed phenomena.
It is necessary to exercise some caution. In the absence of any a priori outline of
this presumed effect, and with a coexisting ambiguity as to the mechanisms underlying this
phenomena, there could be no specification of the means and measures through which it
would be expressed. As such there is some reason to place limited trust in the results of
"data snooping" of this kind, one is usurping the role of chance to some degree. In
summary, there does appear to be no evidence to support the presence of spatial
compatibility effects, for complex movements, which are not based on quite overt
relationships between the target position and the visual/body axis.
SUMMARY AND CONCLUSIONS
12.1 SUMMARY OF RESULTS Both experiments provided confihmation that the relationship between the spatial
location of the movement goal and the visual/body axis is associated with effects exerted
upon a variety of movement of parameters. These spatial compatibility effects did not
appear to reflect purely biomechanical constraints but may have arisen as a consequence of
the particular characteristics of "higher order" organization.
There was little evidence of asymmetries in response movements, other than in
terms of parameters possibly related to the nature of the impulses applied during the
initiation of movement, specifically the Peak Velocity of the movement and the movement
duration. The spatially complex nature of the response movement itself was conceived as
being one possible reason why preferred hand "advantages" were not exhibited. In
addition, anticipated asymmetries arising from processes related to the localization of
targets in extrapersonal space were not exhibited. The functional unity of perception and
action was stressed throughout, yet it was also suggested that suitable measures may
provide indications of the physical/physiological correlates of this behaviour. It may have
been the case that, indeed, the establishment of a target in space cannot be dissociated from
the preparation for action directed toward that spatial location.
Vision of the responding hand, in conditions in which ambient illumination was
present, was associated with an increased terminal accuracy of the movement, although
individuals were never afforded the opportunity to make a visual comparison of the relative
positions of the target and the responding hand. This increase in terminal accuracy was not
accompanied by increases in movement time. The possible means through which retinal
and extraretinal information may contribute to this regulation could not be delineated in this
study.
12.2 CONCLUDING REMARKS
A substantial portion of the literature review was devoted to consideration of the
relative contributions of a variety of factors to what are observed as manual asymmetries. It
was tentatively concluded that a complete account of manual asymmetries would encapsulate a particular sensitivity to specific task demands and consider that, in particular,
as the nature of the movements studies equate more closely with the spatially oriented goal
directed behaviour characteristic of our daily lives, a proportionate increase in right
hemisphere involvement should be anticipated. The results of Experiments 1 and 2, whilst
providing no strong evidence in support of this claim, similarly did not encourage a
rejection of this approach.
In both experiments, advantages for the left hand, in terms of reaction time, although not statistically significant, may indicate an increased involvement of the right hemisphere, in terms of preparation for action which is spatially complex. There were also no benefits observed for the prefened right hand in terms of the final accuracy of the movement, in cases both in which visual information regarding the position of the responding hand was and was not available. Whilst there is always likely to be some
hazard in associating the theory with the null hypothesis, in that some encouragement is
provided for ill controlled experimentation, it may now be the case that, in the field of
motor control, theoretical positions may be sufficiently well defined and measurement techniques be of such precision, that a priori identification with the null hypothesis may be
possible. Meehl(1967) in stating, what was to become the paradox to bear his name, outlined
a substantial problem associated with the logic of psychological research: In the physical sciences, the usual result of an improvement in experimental design, instrumentation, or numerical mass of data, is to increase the difficulty of the 'observational hurdle' which the physical theory of interest must successfully surmount; whereas, in psychology and some of the allied behaviour sciences, the usual effects of such improvement in experimental precision is to provide an easier hurdle for the theory to surmount (p. 103).
Mendela (1972) has suggested a possible solution to this paradox, is to adopt the
conventional strategy of identifying with the alternative (non null) hypothesis until
experimental precision has increased to some criterion level, at which point a switch can be
made to a strategy where the theory is identified with the null hypothesis. One problem
with this approach, as Mendela (1972) acknowledges is selection of the time at which this
switch should occur. In addition, it is not necessary that there be adoption of a "point" null
hypothesis, which is itself nearly always false (Grant, 1962; Bakan, 1966), the alternative
is to select a bandwidth to be considered as null. Therefore, differences have to exceed this,
experimental as opposed to statistical, bandwidth. In order to achieve a practical 171
implementation of this strategy, it may be necessary to design experiments such that, for
example, the theory of interest predicts no differences versus some differences (Wilson &
Miller, 1 964).
The utility of this strategy in the examination of manual asymmetries may be clearly
seen. If it indeed proves practically impossible to provide some manipulation of spatial complexity which is independent of other parameters of the movement, and therefore not
feasible to identify levels of complexity permitting examination of interactions, the
alternative may be to examine a series of movement types which are selected on the basis of
topological spatial complexity. Due to other intrinsic aspects of preparation and execution, it may be the case that, at this stage of phylogical development, at least for right handed
adults, a consistent population level left hand "advantage" in terms of, for example,
terminal accuracy, will not be exhibited even for the most spatially complex movements.
Therefore, it may only be possible to identify with the null hypothesis for spatially complex movements (as one cannot anticipate a left hand superiority of any appreciable magnitude to
arise from anything other than chance) and to predict some differences (in favour of the
right hand) for movements which are less spatially complex.
In view of the precision associated with the methodology current in examination of motor behaviour, such as that provided by sophisticated motion analysis systems, it may be possible to adopt as an experimental strategy, identification with a band null hypothesis. To
relate this directly to the present study, in a subsequent "replication", one might again suggest that no right hand advantage would be anticipated. In this case the theory is
identified directly with the null hypothesis and may be tested as such.
Clearly Experiment 2 provided indications that the environmental information
specifying the action goal exerted effects which recurred throughout the time course of the
resultant movement. Obviously this perception does not precede action. There are ways in
which attempts may be made to account for this behaviour in terms of a computational
model, through the formal specification of complex relationships, of a recursive nature,
assumed to link the "perceptual and motor systems". However, by adopting a commitment to "ecological realism" (Kugler, Kelso & Turvey, 1982) and a consideration of the animal-
environmental synergy, one precludes the need for consideration of formalized intermediate
relationships. The more general problem perhaps arises in conceiving of the stimulus as
something upon which the organism acts. As Merleau-Ponty suggests, when an individual
acquires a skill he: does not weld together individual movements and individual stimuli but acquires the power to respond with a certain type of solution to situations of a certain general form. The situations may differ widely from place to place,
1 7 2
and the response movements may be entrusted sometimes to one operative organ, sometimes to another, both situations and responses in the various cases having in common not so much a partial identity of elements as a shared sigmficance. (1962, p. 142).
The significance in this case being that which affords the accomplishment of a previously
established task goal, in addition to the task goal itself. It may well be the case that
particular patterns of enviror&ental information provides less in the way of affordance, in
terms of that action than others, identifiably in this instance Pattern 3. The
comments of Dreyfus (1979) are particularly noteworthy in this regard: A machine can, at best, make a specific set of hypotheses and then find out if they have been confirmed or refuted by the data. The body can constantly modify its expectations in terms of a more flexible criterion: as embodied, we need not check for specific characteristics or a specific range of characteristics, but simply whether, on the basis of our expectations, we are coping with the object. (p. 250).
One can see how the affordaxes provided by visual/environrnental information pertaining
as to the position of the responding hand in extrapersonal space may be accommodated
within this perspective. It is not the case that there should exist an ideal or specified
trajectory, in Dreyfus's terms, "a range of characteristics", rather the individual may
regulate his action on the basis of expectations, not so much of the object, but of the
outcome of that action.
Whilst in theoretical terms, progress may be brought about by a shift in, or
reevaluation of the epistemological assumptions which accompany approaches in this area,
in more practical terms, in examining the physica.l/physiologica1 correlates of behaviour
described at the phenomenological level, the progress may be brought about by the
implementation of the use of a collection of dependent measures more likely sensitive to
characteristics of the physical/physiological substrate to be described
In the continued development of the present investigation, this may take the form of
application of jerk analysis as a means of directly assessing the relative variability in force
production for the two hands, some monitoring of muscular activity and analysis of the
biomechanical constraints inherent in contralateral and ipsilateral limb movements, as a
means of establishing the relative contributions of these factors to what may be regarded as
potent spatial compatibility effects, as well as continued integration of circumstantial
evidence, such as single cell recordings of cortical activity accompanying similar
movements. In addition,it seems imperative that there be an implementation of analysis of
oculomotor behaviour as an attempt to estimate the importance of optic flow and extra-
retinal information in establishing the positions of task goals in extrapersonal space and in
assessing the relative contributions of central and peripheral vision in the regulation and
modification of related limb movement. It should be reemphasized that this course of
investigation may proceed entirely without need for recourse to intermediate "pseudo- explanatorytt constructs.
APPENDIX A
SCHEMATIC REPRESENTATION OF PATI'ERN DISPLAY SEQUENCES.
FIGURE 1
TARGET
LINEAR
CUBIC
TARGET
TARGET v QUADRATIC
TARGET
APPENDIX B
SCHEMATIC REPRESENTATION OF TARGET POSITIONS RELATIVE TO THE DISPLAY PANEL.
APPENDIX C
PRELIMINARY ANALYSIS OF CROSSINGS OF THE ZERO ACCELERATION
PROFILE
Although custom designed WATSMART software was used with the intention of applying
a 30 Hz, low pass filter, there is some reason to be sceptical about the success of this
procedure. Examination of the data indicated that a high frequency 'noise' component may
have remained present in the signal. Although the instantaneous vector acceleration was
subject to succesive stages of smoothing, and the subsequent imposition of a windowing
filter, this was probably not entirely appropriate for the removal of the phasic noise
component. In this respect the data must be regarded as somewhat suspect. In view of what
would be associated problems in the interpretation of the measure of Zero Crossings, this
analysis may be considered as no more than suggestive, and as such has been included in
this appendix, rather than in the main body of the results. The analysis did appear to
indicate the presence of differences across levels of the factor of Pattern, however, in view
of the nature of the signal obtained, the evaluation of these trends should be approached
with caution. It is likely that the values presented represent overestimates of the number of
crossings.
Mean values for the number of crossings of the acceleration profile were obtained
from three trials within each of 128 cells constituting all possible combinations of the six
factors of Hand, Visual Condition, Visual Field, Pattern, Relation to Midpoint and Target
Eccentricity. A repeated measures ANOVA was performed on the mean scores, using a 2 x
2 x 2 x 4 x 2 x 2 factorial design.
The preliminary analysis indicated the presence of a main effect associated with the
factor of Hand, F(l, 7) = 6.60, p < 0.05, there were a greater number of zero crossings
when subjects performed movements with their left hand.
TABLE 1A Mean Number of Zero Crossings as a Function of Hand and Pattern.
Hand
Pattern Right Left mean
mean 6.3 7.5
There was also evidence of a main effect for the factor of Pattern, F(3,21) = 10.95,
p < 0.0005. It appeared that the mean number of crossings occurring during movements
made in response to pattern 3 was greater than during other presentation sequences, indeed,
a post hoc analysis, using the Tukey (HSD) procedure (Table l.lB), indicated that this
trend did achieve statistical ~ i ~ c a n c e at the 0.01 level, in cases of comparison with all
other levels of this factor.
TABLE 1.1A Mean Number of Zero crossing:^. Differences among: Pattern Means.
HAND BY PATTERN
11 1
FIGURE 1A Mean Number of Zero Crossings by Hand and Pattern.
Most importantly, there was no main effect associated with the factor of Relation to
Midpoint, F(1, 7) = 0.02, p > 0.05, nor an statistically significant interaction of Hand and Visual Field, F(l, 7) = 3.41, p > 0.05. The presence of ambient illumination during the
course of the movement had no consistent effect on the number of zero crossings, F(1,7)
= 0.17, p > 0.05. Indeed, somewhat counter to intuition, the mean number of crossings was greater for the Non-illuminated condition (mean illuminated = 6.7, mean non-
illuminated = 7.1). No main effects corresponding to the remaining primary factors approached statistical significance at the 0.05 level.
Higher order interactions were however present. Hand interacted with the factor of
Relation to Midpoint, F(l, 7) = 6.43, p < 0.05. The relevant mean values are presented in
Table 2A, and are represented in Figure 2A
TABLE 2A Number of Zero Crossings as a Function of Hand and Relation to Midpoint..
Hand
Midpoint Right Left mean
Lower
upper
mean 6.3 7.5
HAND BY MIDPOINT
* RIGHTHAND + LEFTHAND
FIGURE 2A
lower Upper
MIDPOINT
Mean Number of Zero Crossings by Hand and Relation to Midpoint.
Clearly, for movements made with the right hand, there were an increasing number
of zero crossings of the acceleration profile for targets located above the midpoint, whilst
for left-handed movements the tendency was reversed.
A three way interaction, of the factors of Visual Condition, Visual Field and
Relation to Midpoint, was also of statistical significance, F(l, 7) = 6.50, p < 0.05.
As suggested by examination of Figure 3A and by inspection of the mean values in
Table 3A, for movements made toward targets located in the right visual field, the number 180
of zero crossings increased for upper targets relative to lower targets when ambient lighting
was not present, and decreased from lower to upper targets when illuminated In contrast,
for movements made into the left visual field, when illuminated the number of zero
crossings increased for upper targets relative to lower targets, yet for conditions in which
ambient lighting was not present the tendency was reversed.
TABLE 3A Number of Zero Crossings as a Function of Visual Condition. Visual Field
and Relation to Mid~oint.
Visual Condition
Illuminated Illuminated Non-illurn Non-illurn
Visual Field Left Right Left Right
Midpoint
Lower 6.0 7.1 7.0 7.3
VISUAL CONDITION BY VISUAL FIELD BY MIDPOINT
LOWER UPPER
MIDPOINT
FIGURE 3A Mean Number of Zero Crossings by Visual Condition, Visual Field and
Relation to Midpoint.
APPENDIX D
SUMMARY TABLES FOR ANALYSES OF VARIANCE, EXPERIMENTS 1 AND 2.
SO
UR
CE
ME
AN
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RR
OR
ha
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RR
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fi
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h c
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OR
f c
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hfc
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E
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OR
hm
E
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OR
fm
E
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OR
hfm
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OR
cm
E
RR
OR
hc
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ER
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fcm
E
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OR
hf
cm
ER
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ec
c
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RO
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E
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OR
fe
E
RR
OR
SU
M
OF
SQ
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S
83
72
98
87
.69
53
1
35
56
93
1.4
29
69
11
28
7.5
31
25
1
43
11
1.2
18
75
21
78
.00
00
0
68
93
.43
75
0
30
3.1
95
3 1
3
70
1.3
67
19
36
82
9.8
82
8 1
8
54
48
.24
21
9
14
3.0
78
12
1
06
67
.92
18
8
21
26
.54
68
8
22
03
4.7
65
62
13
89
.57
03
1
92
13
.61
71
9
2.0
00
00
2
71
1.2
50
00
16
8.8
20
31
2
48
6.8
04
69
29
.07
03
1
18
12
.99
21
9
34
.03
12
5
53
89
.90
62
5
22
34
.67
18
7
27
74
1.5
78
12
25
3.1
95
31
1
20
27
.17
96
9
74
2.7
57
8 1
1
81
40
.67
96
9
26
36
.82
81
2
87
60
.73
43
7
17
.25
78
1
40
16
.17
96
9
30
6.2
81
25
3
54
2.4
06
25
1 2
8.0
00
00
1
04
25
.12
50
0
DE
GR
EE
S
OF
M
EA
N
FR
EE
DO
M
SQ
UA
RE
T
AIL
P
RO
6 .
0.0
00
0
0.4
81
6
0.1
80
6
0.4
73
6
0.0
52
7
0.9
62
6
0.5
76
7
0.3
88
9
0.9
44
7
0.5
12
8
0.7
47
4
0.8
39
5
0.6
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Experiment 2: Median RT B
o m - P Or- o m - m 0 0 O N P O O h 0 0 0 0 0 - w - m m o m - w o r c m m - N - a o w m o o m N N m m o m ~ m W N o m . . . . . . . . . . . . o m a - N m O N O N - m - w P W m o 0 0 m - h m - w m o m a m - 0 o
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Experiment 2: Median MT B
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28
0.9
65
22
8
8.5
08
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1.0
49
98
12
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6
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71
08
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64
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2.6
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26
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5.8
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79
1
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7.8
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25
6
8.4
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4
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11
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7
6.9
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64
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56
7
9.7
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91
13
5.1
63
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8
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29
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Experiment 2: Time to Peak Velocity D
n m r-lo cnO 3 - m a 0 - o m n m O N l n m
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m - m - m - m - m - N N N N N
m - N - m a a o a m h m ~ m cnm c n q m - a~ m m iolo m ~ - m O - m a N - O F m q O N O N 0 0 0 - O l n . . . . . . . . . . 0 0 0 0 0 0 0 0 0 0
REFERENCE NOTES
Note 1: Peters, M. (1987), Personal Communication.
Note 2: Comments arising f i ~ m a conversation with D. Elliott and D. Goodman 1987.
Note 3: Comments arising from a discussion involving the author, I.M. Franks, R.B.
Wilberg, and D. Goodman, 26th February, 1988.
Note 4: Ideas arising from a conversation with R. Lonergan, April, 1988.
Note 5: Bawa, P. (1988), Personal Communication.
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