tracing response parameter preparation strategies using transition times

Human Movement Science 4 (1985) 39-53

North-Holland

39

TRACING RESPONSE PARAMETER PREPARATION

STRATEGIES USING TRANSITION TIMES *

Ann HARRISON and Andrew BISHOP

Sheffield University, UK

Harrison, A. and A. Bishop, 1985. Tracing response parameter preparation strategies using transition times. Human Movement Science 4, 39-53.

Latencies were measured using a modified four-choice reaction time (CCRT) test procedure; the responses comprised forward and backward displacements of left- and right-hand levers. Two

stimuli were presented, separated by an interval (1%) of 25, 165, 415 or 815 msec. Sl designated

which response should be readied; and on 62% of occasions, S2 indicated that the prepared

response should be executed immediately (~atne trials). During transition trials, S2 specified which

one of the alternative responses should be substituted. Same conditions produced significantly

shorter reaction times. Transition latencies varied depending on the modification required:

changing direction proved easier than changing either hand or hand and direction. This is

compatible with a response coding strategy in which limb is designated before direction. Latencies

and exchange function analyses suggest that preparations were discontinued after response

selection; even at the longest ISI, there was no convincing evidence of preparatory motor

programming.

Introduction

Rosenbaum and Kornblum (1982) used a method of ‘priming’ to investigate the programming demands of voluntary movements. The four actions studied were button presses executed with the ring and index fingers of the left and right hands. A modified 2-CRT test procedure was employed in which one response was demanded on 75%

* The work reported in this paper forms part of a research program supported by the National Fund for Research into Crippling Diseases (grant number 421 700). Dr. Bishop was supported by a

research studentship from the Social Science Research Council. Both are gratefully acknowledged. Mailing address: A. Harrison, Dept. of Community Medicine and Behavioural Sciences,

Faculty of Medicine, Kuwait University, P.O. Box 24923, Kuwait.

0167-9457/85/$3.30 0 1985, Elsevier Science Publishers B.V. (North-Holland)

40 A. Harrison, A. Bishop / Tracing response prepararion

of trials. The aim was to encourage selective preparation (priming) of the predominant response, and the manipulation proved successful: when an action represented the predominant response in a pair, its latency was significantly shorter than when it served as the minority response. Indeed, predominant response reaction times were unaffected by the type of response with which they were paired. Minority response latencies, in contrast, varied significantly depending on the characteris- tics of the partner action. Rosenbaum and Kornblum (1982) conclude that when a minority response was demanded, subjects followed a procedure of adapting the prepared motor program, the time required to do so varying depending on what program dimensions and values were shared by the paired actions. Changing the finger dimension of a prepared response was the most-consuming modification; surprisingly, changing both hand and finger dimensions proved no more difficult than amending just the hand component. The explanation offered for the latter finding, based on introspective data, is that subjects were able to ready both responses if these involved different hands, but were unable to prepare two actions for the same hand. One of the reasons given for adopting a priming rather than a precue methodology was to reduce stimulus identification and response selection effects, but it is not possible to estimate what role these factors actually played in the latency differences recorded.

A disappointing feature of research on response selection and preparation has been the lack of generality (the methodological depen- dence) of findings. In the above study, within-hand transitions proved the most difficult. Various 2-CRT studies have similarly shown that within-hand response pairings produce slower latencies than between- hand pairings (e.g. Kornblum 1965); but the reverse advantage has been demonstrated with serial tapping tests. Both Rabbitt (1965) and Kornblum (1973) found that within-hand transitions were completed more speedily than between-hand transitions; however, stimulus scan- ning variables were not adequately controlled (Rosenbaum and Korn- blum 1982). Miller (1982) used a precue procedure, and found that reaction times were briefer when response options were reduced to two within-hand actions rather than two between-hand responses. Bishop and Harrison (1983) contrasted overt and implicit precuing procedures, and found that while a within-hand advantage emerged with the former, subjects did not differ in their execution of within- and between-hand pairings when precuing was implicit. The explanation

A. Harrison, A. Bishop / Tracing response preparation 41

offered is that subjects are able to implement a variety of response selection and preparation strategies. When the test procedure provides sufficient time, as was judged to be the case with implicit precuing, the performer is able to utilise strategies which while relatively time-consuming enable him to make full use of the precues provided so that he is maximally prepared when the reaction signal appears. When time is limited, however, the greater efficiency of certain strategies is revealed; and so, under overt precuing test conditions, the greater efficiency of designating the hand component of a response before direction emerges.

Rosenbaum and Kornblum (1982) classified actions in terms of hand and finger (index or ring) dimensions. It cannot be assumed, however, that this is necessarily the strategy adopted by all of their subjects; some may have selected to code fingers in terms of their spatial location on a hand (i.e. leftmost vs rightmost), in which case presumed and actual response matches are not the same. In an earlier study (Bishop and Harrison 1983), such potential confusions were avoided by using movements of left- and right-hand levers towards and away from the body. A system of different coloured stimuli at a single locus was chosen in preference to arrow representations in order to avoid orienta- tion discrimination differences; and within-hand actions which had no effector muscle groups in common were selected so that general hand facilitation effects would not be contaminated by muscle-specific effects. A major worry with this study, however, is that the response preparation strategy identified may have been adopted to cope with the particular experimental procedure employed, and may not be one which subjects generally use. Precues were presented successively which would seem to encourage serial selection and preparation of parameters, whereas parallel processing may be the more usual approach. In order to avoid this problem in the present study, a priming paradigm was adopted; the four responses, timing and stimulus coding systems used successfully in the previous study (Bishop and Harrison 1983) were retained for comparability. As in Rosenbaum and Kornblum’s study, subjects were encouraged to ready one of the responses; after a variable interval, a second stimulus indicated whether the prepared response should be executed or which of the other three should be substituted. In contrast to Rosenbaum and Komblum’s procedure, subjects were prevented from switching preparation strategies to accommodate a single, known, current transition; in the present study ipsilateral, contralateral and diagonal transitions were equally likely to

42 A. Harrison, A. Bishop / Tracing response preparaiion

be demanded. Our aim was to discover whether subjects would again opt to designate hand before direction, and complete ipsilateral transition most readily, when there was no procedural encouragement to do so. The interval between the priming and reaction signal stimuli was varied in an attempt to separate response identification and preparatory motor programming components.

Method

Subjects

Twelve subjects (8 females, 4 males; median age 20 years) were recruited from among the staff and students of the Department of Psychology.

Apparatus

The response levers (fig. 1) were mounted in rubber blocks which acted to produce zeroing after displacement; forward and backward displacements were detected by microswitches positioned near the base. The top of a lever had to be moved 8 mm with a force of 750 gm wt to activate a switch. Presentation of stimuli, assessment of responses, and feedback about errors were managed on-line using a specially-written program running on a Nova 840 computer. The computer’s millisecond timer was used throughout. Red, green, blue and white 1 cm squares were used to differentiate which action should be prepared (Sl) and

error +- light cl

OK4 h

stimulus ZOW

response levers

Fig. 1. The apparatus used.


which should be substituted (S2). A yellow triangle appearing at S2 signified that the prepared response should be executed; this fifth stimulus was introduced because subjects could not detect reliably when Sl was replaced by an identical S2 symbol. Stimuli were back- projected to precisely the same location on a frosted plastic screen and illuminated by 1.2 watt incandescent bulbs. A fast rise time (less than 5 msecs for 90% maximum brightness) was used to reduce RT variance. Stimuli were viewed in semi-darkness; the display tube was positioned in the visual midline about 60 cm from the subject. A light-display box provided error feedback. One light came on whenever any of the following errors was detected: (i) wrong hand, (ii) wrong direction, (iii) wrong hand and direction, (iv) double response - taken as indicating poor control or an attempt to correct the initial response, (v) RT less than 70 msec - taken as denoting a false start or guessing, since the reaction was probably too fast to be a response to the signal, (vi) RT greater than 900 msec - indicating that the subject had for some reason failed to respond. Although only one light was used, different categories of errors were recorded separately.

Procedure

The four responses selected required forward and backward displacements of left-hand (Lf and Lb) and right-hand (Rf and Rb) levers. Subjects held each lever knob lightly between thumb and forefinger. Backward displacement was effected by flexing the forefinger with the thumb relaxed; and forward displacement was achieved by adducting the thumb with the forefinger relaxed. Mapping of stimulus colours to responses was varied across subjects using a Latin square procedure. Fig. 2 outlines the trial procedure followed. Sl indicated which response should be prepared. Subjects were informed that 70% of trials (in reality, 62.5%) would be Same trials, i.e. a yellow triangle would appear indicating that the prepared response should be executed. Great emphasis was placed on subjects making full use of the IS1 for prepar- ing the selected response: ‘It is most important that during the interval between stimuli you attempt to prepare the response indicated by the first stimulus. Do not move the lever during the interval, simply ‘get ready’. Of course, on 30% of trials, this preparation will be inap- propriate, but it is still your ‘best bet’.’ Whether S2 indicated a change of action (transition trials) or that the prepared action should be

44 A. Harrison, A. Bishop / Trrrcing response preparation

Fig. 2. Trial sequence.

executed (same trials), the requirement on subjects was the same, namely to respond as quickly and accurately as possible.

The test period was partitioned into four successive stages, each of which involved a different IS1 (25, 165, 415 or 815 msec). Latin squares were used to balance the distribution of ISI’s across stages for the subject groups. Each stage comprised 5 blocks of 32 test trials (five Same trials plus one of each of the three types of transition for the four responses set, demanded in randomised order). When errors were detected, feedback was given, and extra trials were appended to a block (again using randomised presentation) until a full complement of each response category had been logged. Trial length was adjusted to take account of different ISI’s, in order to secure a constant response rate across conditions.

The experiment was completed in a single session lasting about 75 min. The above instructions were read to subjects before they tackled four practice blocks, each of which introduced a different ISI. A 2-min rest followed; the instructions were reiterated, and then subjects worked through the four stipulated test stages. Short rests were permitted between stages if needed.

Results

The first block of trials for each test stage was counted as practice, and only the last four blocks were analysed. For each response, at each of the ISI’s, a total of 20 same and 12 transition trials (4 of each type) were, therefore, considered. In order to simplify data presentation, transitions were classified as follows: (i) ipsiluteral - same hand, opposite direction, (ii) contralateral - same direction, different hand, (iii) diagonal - different hand, opposite direction.


Errors

Transition trials produced twice as many errors as same trials (table 1); but in both cases, the majority of errors (58-75%) involved producing a wrong response rather than an invalid one. Wilcoxon tests were used to analyse substitution errors. For same trials, direction errors and hand errors did not differ significantly in their frequency (p > 0.05), nor were short ISI’s (25 and 165 msec) more prone to error (p > 0.05). Analysis of transition trials similarly showed that error rate was not significantly affected by IS1 length (p > 0.05) or by the type of transition demanded (p > 0.05). The majority of substitutions recorded during transition trials involved production of the Sl response. Analyses failed to confirm the suspicion that contralateral conditions were more prone to this type of substitution (p > 0.05); neither was Sl substitution rate found to be affected by IS1 length (p > 0.05).

Reaction times

The median reaction time produced by a subject for each condition was computed; and based on these data, group means were derived (table 2, fig. 3). Separate ANOVA tests were used to compare ipsilateral, contralateral and diagonal transition latencies with performance under same conditions. In every case, same trials produced significantly faster responses (vs ipsilateral F(1,ll) = 78.45, p < 0.0001; vs contralateral F(l,ll) = 120.94, p -C 0.0001; vs diagonal F(1,ll) = 135.28, p c O.OOOl), and lengthening IS1 had the effect of speeding up responding (ipsilateral/same - F(3,33) = 52.71, p < 0.0001; contralateral/same - F(3,33) = 42.59, p < 0.0001; diagonal/same - F(3,33) = 36.11, p < 0.0001). A significant ISI/trunsition vs same interaction was found for ipsilateral (F(3,33) = 3.71, p -C 0.05) and diagonal (P(3,33) = 3.60, p -C 0.05) test conditions, indicating that increasing IS1 improved same performance to a differentially greater extent; but the equivalent comparison for contralateral testing just failed to reach significance (F(3,33) = 2.85, p = 0.052). When performance under the three transition conditions were compared using an ANOVA test, it was found that reaction times differed significantly depending on whether an ipsilateral, contralateral or diagonal modification was called for (F(2,22) = 11.01, p < 0.0005). As would be predicted from the above analyses, increasing IS1 significantly reduced reaction times (F(3,33) = 32.74,

Tab

le

1 T

otal

nu

mbe

rs

of e

rror

s re

cord

ed

(sub

ject

s N

= 1

2).

IS.1

(mse

c)

Cat

egor

y of

err

or

Inva

lid

resp

onse

s Su

bstit

utio

ns

RT

R

T

Dou

ble

Tot

al

Wro

ng

Wro

ng

Wro

ng

hand

T

otal

i 70

mse

c >

1100

m

sec

resp

onse

di

rect

ion

hand

an

d di

rect

ion

Sam

e tr

ials

(N

= 3

840)

25

0

15

13

28

14

19

9 42

16

5 0

18

11

29

14

11

6 31

41

5 0

3 7

10

9 8

5 22

81

5 0

1 1

2 10

4

3 17

-

Tot

al

6 75

32

69

v

42

23

112

Sum

mar

y:

4.7%

tri

als

(N

= 18

1) i

nvol

ved

erro

rs;

61.9

%

of e

rror

s w

ere

subs

titut

ions

.

Ipsi

late

ral

tran

siti

on

tria

ls

(N =

768

) 25

0

9 16

5 0

1 41

5 1

4 81

5 0

3

Tot

al

T

ii

5 14

6

6 1

13

2 3

5 4

0 9

5 10

7

3 1

11

2 5

5 4

3 12

14

3?

25”

fl

5 m

Sum

mar

y:

10.0

%

tria

ls

(N

= 77

) in

volv

ed

erro

rs;

58.4

%

of e

rror

s w

ere

subs

titut

ions

.

--_,

._

, _

,, __

_

Con

tral

ater

al

tran

siti

on

tria

ls

(N =

768

) 25

0

3 4

7 2

8 0

10

165

0 5

2 I

3 14

0

11

415

0 2

0 2

5 15

1

21

815

0 3

1 4

8 5

0 13

Tot

al

0 13

7

XI

18

42 a

i

61

Sum

mar

y:

10.6

%

tria

ls

(N

= 81

) in

volv

ed

erro

rs;

75.3

%

of e

rror

s w

ere

subs

titut

ions

.

Dia

gona

l tr

ansi

tion

tr

ials

(N

=

768

) 25

0

13

165

0 3

415

0 1

815

0 1

Tot

al

0 ix

3 16

1

4 8

13

1 4

1 0

5 6

0 1

4 6

7 17

0 1

3 2

9 14

z 22

!J

12

29

=

50

Sum

mar

y:

9.4%

tri

als

(N

= 72

) in

volv

ed

erro

rs;

69.4

%

of e

rror

s w

ere

subs

titut

ions

.

a Sl

su

bstit

utio

n.

48

RT [msecsi

MO-

600 -

A. Harrison, A. Bishop / Tracing response preparation

x

* x

0 *

0 x * x CONTRALATERAL

0 * OIAGONAL

WSILATERAL

0

0

0

OSH 4004 , , , , 25 165 415 815

ISI [msecsl

Fig. 3. Group reaction times for same and transition test conditions.

p < 0.0001); but importantly, the ISI/transition type interaction was non significant (F < 1). A Tukey post-hoc analysis of trial means (table 3) indicated that ipsilateral transitions were executed more quickly than

Table 2 Group reaction time data (means of medians).

ISI (msec)

Latency (msec)

Same Transition trials

trials Ipsilateral Contralateral Diagonal Mean

25- 571 709 763 739 737

165 519 671 730 705 702

415 447 631 678 652 654

815 401 608 658 639 635

Mean 485 655 707 684 682


Table 3

Post hoc analyses of transition trial means (Tukey test)

Matrix of differences in means (msec)

Iusilateral Contralateral Diagonal

Ipsilateral

Contralateral

Diagonal

_

52 b _ _

29 a 23

= p < 0.05 (K = 3, df = 22, d( p < 0.05) = 28.05);

bp<0.01(K=3,df=22,d(p<0.01)=36.26).

Table 4

Paired t-test comparisons of performance under different transition conditions, at each IS1

(subjects N = 12).

IS1 ‘t’ value

(msec) Ipsilateral Contralateral Ipsilateral “S

Contralateral

“S

Diagonal vs

Diagonal

25 2.87 b 1.75 1.77 165 3.99 = 1.76 1.79 415 2.59 a 1.91 1.33 815 3.15 c 2.45 2.20 =

a p < 0.05; bp -= 0.02; cp -C 0.01.

800

1 MEAN TRANSITION i5 TRIAL LATENCY

700 1iS

b I I

[msecsl

600 i,

a;5 4;5

400 500 600

MEAN SAME TRIAL LATENCY

[ msecsl

Fig. 4. An exchange function plot of transition and same reaction time covariance (after Audley

1973).

50 A. Harmon, A. Bishop / Tracing response preparatmn

contralateral or diagonal ones, but that diagonal transitions were not significantly faster than contralateral (p > 0.05). A detailed comparison of transition types was carried out at each of the ISI’s using paired t-tests (table 4). At all ISI’s, ipsilaterul transitions were faster than contralateral; but only at the longest interval did ipsilateral performance significantly exceed diagonal, and diagonal exceed contralateral. Fig. 4 provides an ‘exchange function’ (Audley 1973) plot of the covariance of transition and Same reaction times as IS1 increased. The data were best fitted by a linear function with a slope of +0.6, indicating that same and transition latencies both improved as IS1 increased, but that this improvement was greatest for same trials.

Discussion

The present study clearly demonstrated that subjects were able to execute ipsilateral transitions more quickly than contralateral ones. Both involve updating just one response ‘dimension’ (Rosenbaum and Kornblum 1982), but changing hand proved significantly less time-consuming. Error rates for the two conditions were equivalent, and so the ipsilateral advantage does not result from a greater speed : accuracy trade-off.

It is evident that subjects complied with the instruction to process Sl information because they were able to interpret the triangle stimulus in same trials, and because Sl substitutions predominated. Beyond identi- fying the Sl response, what preparations were carried out during the IS1 period? The observation that the ipsilateral : contralateral advantage was present at the shortest IS1 and did not increase as ISI lengthened suggests that no preparatory motor programming was undertaken, even though time was available at the longer ISI’s. This interpretation is supported by the ‘exchange function’ obtained. Audley (1973) con- ducted a series of studies in which items from a response set were demanded with unequal frequencies. Generally, as the reaction time for the most common response declined, those for competing responses increased, producing negative exchange functions. The explanation offered is that only one prepared response can be accommodated in the ‘response buffer’, and so preprogramming one response retards the execution of the other options. The fact that a positive, rather than a negative, exchange function was produced in the present experiment is


compatible with preparations being curtailed prior to motor programming. The reaction times recorded also support this interpretation. Based on earlier studies (Bishop and Harrison 1983) using equivalent responses and stimuli, the expected 4-CRT for the present task would be about 520 msec, the 2-CRT about 400 msec, and the simple RT below 300 msec. In theory, the longest ISI’s provided plenty of time for preprogramming the Sl response; but the reaction times recorded for same trials are well in excess of a simple RT, and so provide no indication that the Sl response had been preprogrammed. Admittedly the task facing subjects during transition trials was complicated in that they had to decide how to modify the Sl response; but the reaction times recorded were consistently above the 4-CRT level, and certainly never approached the 2-CRT expected if some usable motor programming had been completed during the IS1 period.

The procedure used in the present study did not constrain or encourage subjects to designate response parameters sequentially (as opposed to in parallel) or to stipulate the hand parameter first; but based on the data obtained, this is the strategy subjects adopted. The results confirm the findings of an earlier precuing study (Bishop and Harrison 1983) in which responses were executed more quickly when hand was revealed before direction than when direction was revealed before hand. Effective preparation was possible with both orders, but the former proved more efficient in the context studied. Rosenbaum (1983) claims that a variable-order specification system, or ‘distinctive feature’ model of response parameter preparation (Rosenbaum 1980), will lead to faster and more flexible motor performance. This does not, however, exclude the possibility that certain preparation orders will be more efficient, perhaps because of physiological considerations. For example, the ‘hand then direction’ preparation preference observed may be linked to the fact that activity overflow errors are easier to safeguard against when priming is limited to just one hemisphere (Welford 1968).

If it is accepted that preparatory motor programming was not undertaken during the IS1 period, some other explanation is needed for why latencies declined as IS1 lengthened. Psychological refractory period (PRP) effects are likely to retard performance at the briefest ISI’s (Welford 1952), but should not affect intervals as long as 415 msec; and so release from PRP cannot be invoked to explain the latency reduction observed between the 415 and 815 msec ISI’s. At the longer ISI’s,

52 A. Harrison, A. Bishop / Tracing response preparation

subjects had time to prepare for the advent of S2 after finishing defining Sl; while at shorter ISI’s, they were probably having to divide their attention between decoding Sl and monitoring for S2. The fact that both transition and same latencies improved as IS1 lengthened is consistent with an increase in available attentional capacity.

It is important to consider why subjects might choose not to utilise the IS1 period for preprogramming the Sl response and therefore lose the opportunity to minimise Same trial reaction times. It could simply be that the frequency of same trials was too low to encourage preparation; but also, if Audley (1973) is correct, preprogramming may not be wholly advantageous, for when preprogramming is engaged in the latencies of the alternative responses are retarded, and overall task performance may well be worse. Rosenbaum and Kornblum (1982) used a slightly higher ratio of same trials (75%) and claim that responses were preferentially prepared; but as already stated, a purely response selection explanation for their data cannot be ruled out. Indeed, it may be that the dimensions which the selected actions had in common are ones which are relevant to response identification but not to motor programming (Kerr 1978; Goodman and Kelso 1980). In order to investigate programming savings it may be necessary to select actions which have common acceleration, force and timing dimensions rather than common global direction or hand parameters. Further research is indicated to investigate this possibility.

‘There is mounting evidence that when people are called upon to select and execute responses, the strategy they choose is affected by a variety of task demands, such as the likelihood of having to change the response, what alternative actions may be demanded, whether the response must be perfect from the moment of execution or can be updated while in progress, the relative importance placed on speed and accuracy, and the type of stimulus presentation and mapping used. There is always the danger, therefore, that the strategy adopted in a given study may have very limited generality. With this caveat, one strength of the current study is that subjects were free to process response dimensions in any order, either sequentially or in parallel, and yet a consistent preference for a hand then direction strategy emerged. Further research is needed to establish how commonly and under what conditions this strategy is selected, investigating different actions, different response combinations and task demands. A range of movement and reaction time measures, exchange function plots and possibly EMG

A. Harrison, A. Bishop / Tracing response preparaiion 53

monitoring will be needed to trace motor programming and response selection contributions.

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tracing response parameter preparation strategies using transition times

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