non-verbal perceptual abilities in relation to left-handedness and cerebral lateralization

Nouroprycholopia. 1977, Vol. 15, pp. 779 to 791. Pergamon Press. Printed in England

NON-VERBAL PERCEPTUAL ABILITIES IN RELATION TO LEFT-HANDEDNESS AND CEREBRAL

LATERALIZATION”

CHRISTOPHER GILBERT

Central Bergen Community Mental Health Center, Saddle Brook, New Jersey, U.S.A.

(Received 28 February 1977)

Abstract-Performance on a facial recognition test and on the WAIS subtests of Block De- sign and Object Assembly were measured in relation to asymmetrical dexterity, hand usage, and familial left-handedness (FLH) with 64 Ss. Degree of perceptual asymmetry was measured by lateral reaction times to faces and letters. Results did not support a simple link between left-handedness and non-verbal perceptual deficit. Facial recognition ability seemed diminished by FLH, and FLH seemed closely associated with decreased cerebral lateralization. Right- hemisphere localization of face-processing seemed independent of any handedness variable.

SKILLS involving spatial orientation and appreciation of shapes and melodies are consider- ably impaired by right-hemisphere damage [l, 21, whereas the opposite is true for verbal skills. The fact that left-handedness is often associated with variations in this pattern of hemisphere specialization, at least for speech function, has led to questions of possible verbal or perceptual deficits peculiar to left-handers. The most widely-considered theory recently is that of LEVY [3]. She proposed that the brains of left-handers are less well “segre- gated” into mutually complementary spheres of activity, and that language processing, perhaps because of differing neuron-linkage characteristics, is incompatible with non- verbal “holistic” perception. When both specializations exist in the same cortical area, one skill may be impaired. Levy offered preliminary evidence from I.Q. testing that the holistic processing suffers, rather than language ability.

MILLER [4] presented confirming evidence for Levy’s idea with a larger sample, using verbal and performance I.Q. tests; NEBES [5] found a deficit in a Gestalt-like activity in left-handers. Others [6-81 presented negative evidence, finding no verbal or performance deficits in left-handed persons.

Unfortunately, some of the preceding studies and many others in the handedness literature treat left-handedness as a unitary variable; upon closer examination it seems not to be. For one thing, the degree of handedness, measured either by manual dexterity or by hand- usage questionnaires, certainly seems to influence performance in perceptual measures of cerebral lateralization. This has been found in studies of dichotic listening [9-121 and for some other measures, including facial recognition 1131. The presence of left-handedness among family members also seems to diminish the degree of hemispheric specialization [14-161 as judged by perceptual asymmetries. From a survey of the occurrence of aphasia following unilateral brain damage, HBCAEN and SAUGUET [17] found that particularly in subjects having a left-handed relative, language impairment was associated equally often with left and right hemisphere damage.

*This article is based on a doctoral dissertation submitted to Michigan State University.

179

7x0 CHRISTOPHER Gmmi

Most published data so far, then, suggests that the variable of less complete lateralization (greater symmetry of cortical functioning) is associated with at least two handedness factors: less consistent manual specialization and presence of familial left-handedness. Following from this is the possibility that persons with these traits would be slightly impaired in non-verbal perceptual skills.

Nowverbal perceptual skills

Judging from studies on the consequences of brain damage, facial recognition is strongly dependent on intact right-hemisphere functioning [ 1 g-201. Damage particularly to the right temporal lobes commonly impairs the person’s ability to recognize faces, while such impairment is rare from left-hemisphere damage alone. Therefore facial recognition was chosen for the present study to measure right-hemisphere performance on a function most unique to it.

A related question of interest concerns the cerebral localization of face-processing in relation to handedness. Facial-recognition ability may be a firmly-right-hemisphere-based skill, regardless of speech localization, or it may shift in complementary fashion to speech. Stated more generally: is the “minor” (dominant for non-verbal skills) hemisphere ahwq’s the opposite of the dominant-for-speech hemisphere ? In the majority of human brains the two functions do seem to occur in opposite hemispheres, but since attention has usually focused on more easily tested verbal abilities, there is much less evidence that in the case of right-hemisphere speech, minor-hemisphere perceptual functions actually switch to the left hemisphere.

Related to facial recognition in nature are two sub-tests of the WAIS: Block Design and Object Assembly. These were included as supplementary non-verbal performance measures because they were found by LANSDELL [21] and VEGA et al. [22] to be the most influenced by right-hemisphere damage of all WAIS subtests. NEWCOMBE and RATLIFF [23], following this lead, tested a number of subjects classified by handedness and found no evidence of of lower performance in left-handers as Levy’s results would predict.

Another way to measure hypothesized asymmetries in perception is to record reaction times to stimuli for which one hemisphere is specialized. Many researchers have shown in various ways that a stimulus appropriate to a hemisphere’s specialization will be perceived more quickly when presented in that hemisphere’s contralateral, or primary receiving, sensory field [24-261. This is true for both verbal-symbolic stimuli and for non-verbal stimuli (faces, dots and abstract shapes). Another well-researched approach has been to demonstrate hemisphere-related lateral differences in discrimination ability [27-291. These findings have been consistent with the reaction-time data.

The most frequently-offered explanation for these perceptual asymmetries is that a stimulus requiring, for example, verbal discrimination must be processed in the verbal, usually left, hemisphere. Stimuli in the left visual field must be transferred from right to left hemispheres; this not only takes extra time, but some information is probably lost

through signal degradation [33]. Quick-flash presentation is used both to ensure stimulus placement in the proper retinal area and to minimize the available information, thus maximizing any deficit. For both vision and audition, it has been found that contralateral input of hemisphere-specific stimuli preserves the most information and allows for the most rapid discrimination.

NON-VERBAL PERCEPTUAL ABILITIES IN RELATION TO LEFT-HANDEDNESS AND CEREBRAL LATERALIZATION 781

The letter discrimination RT task chosen for the present study was based on a letter- judgment paradigm originated by POSNER and MITCHELL [30] in which pairs of letters are presented by tachistoscope to one visual field only. Subjects decide whether two letters are the same or different. Pairs such as “Aa” are physically dissimilar but symbolically identical; this discrimination requires knowledge of symbolic equivalency and calls on language skills rather than simple shape-matching. At least three studies [6, 31,321 have demonstrated a right-field (left-hemisphere) superiority for the symbolic discrimination. A face-matching task, described later, was designed for non-verbal RT testing.

In summary, the variables measured in the present study were: 1. Handedness: defined by (a) the difference in manual dexterity between the hands;

(b) hand usage-the consistency of stated hand preference for many different skilled activities; (c) the presence of left-handedness in the subject’s immediate family.

2. Performance in right-hemisphere-related perceptual skills: Block Design, Object Assembly, and facial recognition ability (free-view conditions).

3. Lateral differences in visual half-field reaction times to: (a) identification of faces in left and right visual fields; (b) a letter discrimination based on symbolic vs form identity.

Thus the study was designed to measure several aspects of handedness, several aspects of right-hemisphere perceptual ability, and degree of cerebral lateralization based on asymmetric reaction times to hemisphere-specific stimuli.

METHOD Subjects

An equal number of subjects were needed to fill four handedness categories: strongly left-handed, weakly left-handed, strongly right-handed, and weakly right-handed. Subjects were recruited at Michigan State University from among undergraduates aged 18-25, either by giving class credit for participation or--in some cases-advertising among students for the specific handedness types.

Handedness In screening subjects, the initial classification was made according to the subject’s writing hand. Next,

each subject’s degree of manual preference was assessed by both a manual-dexterity test and a standard hand-usage questionnaire. The questionnaire was designed by CROVITZ and ZENER [34] and lists 14 common activities such as using scissors, holding a hammer, and throwing. Responses were made on a 5-point scale from “right hand always” to “left hand always”. Scores ranged from 14 (complete right-hand usage) to 70 (complete left-hand usage). In the score distribution for left-handers, the approximate median of 54 served as the dividing point for separating left-handers into “weak” and “strong”; a corresponding median of 30 divided the right-handed group.

The dexterity test was modelled after the principles of the Crawford Small Parts Dexterity Test. Subjects were directed to quickly place twelve small metal washers on twelve straight pins which protruded from a Styrofoam ball, using tweezers, and then remove them one by one. Total elapsed time constituted the first score for the hand. In all cases the dominant (writing) hand was tested first. Usually two trials, alternating, for each hand sufficed; if results were unclear, a third and fourth trial were run. If the nondominant hand completed the task significantly faster or if there was no clear difference between the hands (less than 5 set) then the person was classified as weakly left- or right-handed.

It is not clear whether a greater variability of reported hand usage in the questionnaire or a smaller difference in tested dexterity should be the better definer of “weak” handedness. Therefore, if a subject was classified “weak” on either the questionnaire or the dexterity test, that became his final classification. All other persons were designated strongly left- or right-handed. This screening eventually produced 64 subjects, 16 in each of the four handedness groups.

Reaction time stimuli For the face discrimination task, stimuli consisted of four small black-and-white portraits, two male and

two female. Each picture was 2.5 x 3.0 cm in size, mounted on a white card so that the inner edge was 15 mm either right or left of the central fixation point (1.1’) and the outer edge was 40 mm (3’) from fixation.

For the letter discrimination task, letter-pairs 5 mm high were drawn with India ink on white cards. The closest edge of the closest letter to the central fixation point was 16 mm to the left or right (1.22” of visual angle) and the outer edge of the outer letter was 28 mm (2.14”) from the center. All possible combinations

782 CHRISTOPHER GILBERT

between upper and lower cases of letters A and E, and C and R. were used, making a total of 64 separate stimuli, since each pair was presented in both visual fields.

Reaction time testing apparatus Stimulus materials were presented on a Scientific Prototype model 800 2-field tachistoscope with binocular

viewing. Exposure duration was kept constant at 150 msec. Illumination was 0.20 log ft-L at both the pre-exposure field (containing the fixation point) and the stimulus field. Eye movements were not moni- tored since there is adequate evidence that subjects in similar experiments do fixate properly [31, 321. A switch controlled by the experimenter simultaneously exposed the stimulus field and activated a Hunter Clockounter millisecond timer. The subject’s response switch consisted of a lever which stopped the timer when moved either forward or backward. For half of the subjects, a forward movement of the lever signalled a discrimination of “same” for the letter pairs; for the other half the direction was reversed. Subjects were instructed to use their whole arm for moving the switch, and to keep their wrist and fingers stiff. This was done to minimize finger movements, which might have introduced an unwanted contralateral hemisphere advantage.

Reaction-time procedure Subjects were familiarized with the letter-discrimination task with approximately fifty practice trials in

the tachistoscope. The 64 stimulus cards were then presented in random order twice, once through for each hand. Half of the subjects began with the right hand and half with the left. The experimenter gave a verbal “ready” signal approximately one second before stimulus onset.

For the face discrimination, subjects studied a card containing duplicates of four stimulus faces to learn a simple relationship: two of the faces called for a forward response of the lever, and lwo called for a backward response. Subjects underwent fifty practice trials, more if their learning was obviously slow. The cards were then shuffled and presented in random order both with respect to stimulus and to visual field. Sixty- four observations were collected from each subject, 32 for each visual field, with the responding hand switched halfway through.

For both the letter and face reaction times, errors were treated by informing the subject of the error, recording where the error occurred, and re-inserting the card at random within the remaining cards. Times for errors were not recorded.

Block Design and Object Assembly These subtests of the Wechsler Adult Intelligence Scale were administered as described in the WAIS

manual. Scores in both cases depended on the elapsed time for correct construction of a design or a complete picture, and were assigned according to the WAN norms.

Facial recognition The stimuli for this test were the same as those for an earlier study [13] and presentation was identical

except for more closely controlled lighting conditions. Faces from old university yearbooks were combined to make four arrays of forty faces each, two all male and two all female. All clothing was blocked out, and prominent earrings, unusual hairdos, and obvious identifying characteristics were causes for rejection of a picture. From each array eight faces were individually photographed and enlarged. These sets were re- photographed onto slide film, as were the 40-face arrays, for group presentation. Subjects were tested in groups of five or six, mixed with respect to handedness category. After instructions, the first set of eight test faces was projected onto a screen for 25 set at a distance of approx 6 m from the subjects. Immediately afterward, the corresponding 40-face array was projected. On answer sheets, subjects identified the faces they recognized from the first group, with no time limit. This procedure was repeated for the other three sets, always in the same order. The final score was the total of correct identifications for the four sets.

RESULTS

Faces. A 2 x 4 analysis of variance with repeated measures on visual fields was done on the means of correct responses for each visual field and subject group. Table 1 shows the mean reaction times for each group and visual field.

Only the main effect of visual field was significant (F,,,, = 9.42, P < 0.01). The lack of interaction between visual field and handedness indicates that all four groups had a comparable LVF bias. Approximately 70% of subjects in each group showed this pattern, with insignificant variation from group to group.

For a given subject, the absolute difference (regardless of direction) between the mean LVF and RVF reaction times is a potential measure of functional asymmetry. This measure


Table 1. Group means of reaction times to faces in left and right visual fields (in msec)

Handedness group LVF S.E.M.* RVF S.E.M.

SRH 708 SLH 702 g;

715 (18) 716 (18)

WLH 710 725 WRH 688 I::; 702

*Standard error of measurement.

indicates only the magnitude of hemisphere difference and disregards the left-right localization of the processing function. All four group means for this variable fell between 23.3 and 28.1 msec.

Letters. Data from the letter RT task, unfortunately, did not constitute an adequate measurement of verbal hemifield differences. The response variance was quite large in relation to the number of analyzable RTs per subject (16 per visual field). Neither the results nor the conditions of measurement were comparable to other studies using this paradigm; analysis of variance revealed no difference due to handedness group or to visual field. Mean times clustered around 730 msec. The only meaningful data derived from letter RTs was the absolute-difference scores, described in the section on familial left-handedness.

Reversed face bias Characteristics of persons with “reversed” biases for faces were examined. Around 30%

of the subjects had RVF biases for face RTs; 12 were left-handed, nine were right-handed; there was no clear relationship to other handedness variables. Within this group, however, the correlation between degree of rightward bias for faces and facial recognition performance was - 0.48 (P < OeOl), suggesting that the more the left hemisphere dominates for face processing, the more general facial-recognition ability is lowered. For all 64 subjects, the correlation between facial recognition (FR) ability and degree of absolute face bias was - 0.10, and between FR and degree of leftward bias - 0.02.

A reverse bias in reaction time could be due to at least three situations: the normally dominant hemisphere performs much more slowly, the opposite hemisphere performs faster than usual, or overall reaction time is lowered. Figure 1 shows the pattern of face RTs

0 RVF faster

0 LVF foster

BOO -

780 -

Faces Letters

680- I I I I

LVF RVF LVF RVF

FIG. 1. Means of discriminative reaction times to faces and letters, with subjects direction of individual bias.

divided by


separated on the basis of subject’s direction of bias. Oddly, the corresponding distribution of RTs to the letter discrimination looks quite similar: in both cases there is a large, significant difference (P .< 0.01, t-test) in the left visual field, but not in the right.

Multivariate arzalysis. A multivariate analysis was carried out for all measures combined. Table 2 shows the group means for the variables not already presented. The overall multivariate F-ratio was 1.10, which was not significant.

Table 2. Group means for perceptual measures other than reaction times

Handedness group P

SRH SLH WLH WRH less than:

Facial recognition 20.5 It 0.68 21.1 i 0.74 20.2 I 0.64 18.7 $ 0.87 0.14

Block design 40.8 & 1.5 38.0 + 1.8 41.5 i 1.3 36.4 * 2.2 0.12

Object assembly 35.5 + 1.4 35.1 + 1.4 33.9 * 1.5 32.7 _c 1.8 0.59

The initial broad classification of persons into handedness groups based on the dexterity and hand usage questionnaire showed no promise for disclosing differences in right- hemisphere performance. The next logical step was to examine separately each component of handedness: dexterity asymmetry, hand-usage, writing hand, and presence of familial left-handedness. Sex of subject was also examined as a possible variable.

Dexterity Subjects were classified into two groups: those whose writing hands were superior on the

dexterity test (N = 42) and those whose hands were equal or whose writing hand performed more poorly (N = 22). This comparison produced no differences significant at the 5% level. The multivariate F-ratio was 1.20: P > 0.30.

Handedness questionnaire

Subjects were divided next into two groups based on the weak-strong distinction. One group contained those whose questionnaire scores were either between 24 and 30 or 54 and 70 (N = 43); these score ranges indicated a strong, consistent lateral preference for manual activities, in contrast to those in the second group, whose scores covered the mid-range of 30 to 54 (N = 21). No differences were significant at the 5 “/;: level. The multivariate F-ratio was 1.5, P > 0.18. A division of scores based on writing hand alone likewise yielded no significant differences on any of the measures.

When the criteria for “weak” handedness was broadened to include symmetrical dexterity, mid-range questionnaire score, or both, weakly-lateralized persons (N = 32) were lower than strongly-lateralized persons (N = 32‘) in facial-recognition performance (t 1 1.85, P < 0.05).

Familial left-handedness Regardless of their own handedness, subjects were divided on the basis of whether at

least one parent or full sibling was left-handed. Thirty-seven were thus designated “FLH” and the other 27 were “non-FLH”. This analysis revealed a strong difference in facial recognition ability between the groups (Table 3). The FLH group had significantly lower scores

(FI,u = 17.8, P > 0.0001).


Table 3. Group means for subjects divided by presence of left-handedness in the family

Face* PI NI F.R. B.D. O.A. RT RT RT

Non-FLH (N = 27) 21.8+0.46 4O.lztl.3 35.4kl.O 1~ 13.0&6.7 -12.316.5 -6.9hl1.9 FLH (N = 37) 18.9+0.47 38.511.2 33.5hl.l 1 11*1&4.9 -t-0.62&6,8 15.817.9 F-ratio 17.8 0.74 1.6 0.05 1.8 0.86 P less than : 0~0001 0.39 0.22 0.81 0.19 0.36

*RT values denote overall lateral bias, in msec; positive numbers show LVF bias, negative numbers show RVF bias.

The multivariate F-ratio was 2.71 (P < 0.01). No other differences were significant. Within the FLH group, the facial-recognition scores of the 20 subjects with a left-handed

parent (or parent and sibling) were significantly lower than the scores of those with only a left-handed sibling (N = 17; t = 1.73, P < 0.05 ; see Fig. 2). One would expect that the handedness of the subject would influence his FR performance, but this was not the case; scores of left-handers overall did not differ from those of right-handers. Within the sub- group of subjects with a left-handed parent, left-handers did no worse than right- handers.

FIG. 2. Facial recognition performance in relation to presence of familial left-handedness.

Sex of subject Females showed superior performance on the facial recognition test (FI,62 = 7.14,

P-C O-01). No other differences related to lateralization or to non-verbal performance were apparent. Thirty-one percent of the females, as compared to 29% of the males, had a reversed bias for the face RTs. Females did not differ from males in average absolute bias on either letter or face RT tasks. (Of the 64 subjects, 35 were female and 29 were male.)

Summary of correlations A complete correlation matrix was obtained. The variable of facial recognition showed

t,he strongest overall relationship with the others : besides the previously-mentioned relations with sex of subject (r = O-32), “weak” handedness (r = -0.24) and FLH (r = -0*47), FR correlated 0.26 with performance on the Object Assembly test. Correlation with Block Design scores was O-14. (For these correlations, 0.24 is necessary for significance at a minimum O-05 level.) In general, RTs to faces showed the lowest correlations with the handedness variables. None was higher than 0.10.

The - 0.47 correlation between FLH and facial recognition scores does not in itself


implicate lateralization as a factor. To examine this more directly, the average absolute visual-field differences in letter RTs were compared in relation to familial left-handedness. A 2 x 2 analysis of variance performed on the letter conditions confirmed that the mean hemisphere difference for letter RTs in non-FLH subjects (53 msec) was greater than the mean for FLH subjects (36 msec; Fl.R2 = 5.8, P < 0.03). In other words, both left and right-handed subjects with left-handed relatives had a significantly smaller letter-processing asymmetry than those without left-handed relatives.

To clarify the relationships among aspects of handedness in these subjects, Table 4 contains only those correlations.

Table 4. Correlations among variables of handedness

Writing hand 0 = right 1 = left

Familial left-handedness 0 = no 1 = yes

Equal dexterity 0 = unequal 1 = equal

Handedness questionnaire 0 = strong 1 = weak

Writing hand Familial 1.h.

- -0.09

-0.09 -

-0.13 0.02

0.30 0.06

Equal dex.

-0.13

-0.02

0.26

“Weak” quest.

0.30

0.06

0.26

DISCUSSION

Reaction time to faces The significantly faster LVF reaction time to faces, averaging 13 msec, is consistent with

findings of RIZZOLATTI et al. (15 msec) [24], GEFFEN et al. (25 msec) [31], and MOSCOVITCI-~

and CATLIN (5 msec) [36]. All four handedness groups had the same proportion of leftward bias, of approximately the same magnitude. If this bias is truly due to right-hemisphere specialization for face processing, then it seems that localization of this function is un- related to handedness and its correlates. This conclusion is supported by the consistently low correlations between face reaction time bias and all other handedness variables. This consistent LVF bias may, of course, not be due to right-hemisphere specialization, but no other explanation is immediately obvious. If some unevenness of lighting existed in the tachistoscope, or if a post-exposure memory scanning effect existed, these factors should have affected the letter RTs in the same way. Theorists in this area have often assumed that when speech dominates in the right hemisphere, the left hemisphere naturally takes over the spatial, non-verbal functions. This assumption is not borne out by the present study, and there is other evidence along this line:

LEVY, TREVARTHEN and SPERRY [37], in their chimeric-face study with split-brain patients, located one commissurotomized patient with the right hemisphere dominant for language. This person’s perception of faces and his ability to name objects both showed a left-field bias, indicating right-hemisphere specialization for non-verbal as well as for verbal processing. Also, DEE [l l] demonstrated a relationship between strength of manual preference and degree of lateralization on a verbal dichotic listening task. However, neither handedness nor strength of handedness affected this subject’s performance on a non-verbal dichotic listening task involving melodies. Melody recognition has been fairly well established as a right-


hemisphere function [2, 381, yet in Dee’s sample all handedness groups, regardless of their biases on the verbal dichotic test, had a right-hemisphere superiority for melody discrimination. Localization of verbal function was related to variables of lateralization, but localization of non-verbal functions was not. Finally, TZAVARAS, H~AEN, and LE BRAS [39] found in a sample of brain-damaged subjects that occurrence of right-sided posterior cerebral lesions correlated with a drop in facial recognition ability. This correlation seemed independent of subject and familial left-handedness.

If right hemisphere functions eventually prove to be less “movable”, this would give new weight to the idea that non-verbal perception is impaired when speech function is not confined to the left hemisphere. If the two functions simply changed places, no interference would logically be expected, but if facial recognition and other visual-spatial abilities are firmly embedded in the right hemisphere, then the occurrence of speech in that same hemisphere could interfere with a more fundamental neural organization. Yet several studies, e.g. LEVY and REID [40], have shown an apparent reciprocal shift in verbal and non-verbal perceptual asymmetries, using tasks other than facial matching. So the present results with faces may not be generalizable to other right-hemisphere skills.

Perceptual dejicits As to the original question regarding non-verbal perceptual deficits in left-handers, this

study found none which allow any blanket statement about left-handers in general. The four handedness groups did not differ in scores for Block Design, Object Assembly, FR ability, or the RT measures. Comparing both weakly-lateralized to both strongly-lateralized groups did reveal a significant difference in facial recognition performance favoring the more strongly lateralized subjects, with corresponding but non-significant trends for Block Design and Object Assembly. This finding approximately replicates an earlier study [ 131 in which weakly left-handed persons had lower FR scores than both strongly left-handed and unselected (not tested for lateralization) right-handed subjects.

Results for left-handers as a group are not in strong agreement with Levy’s 1969 report showing a drop in non-verbal performance for left-handed subjects. However, Levy used the entire WAIS Performance Scale for assessment of non-verbal ability, and did not report which subtests contributed most to the left-handed deficit. Part of this deficit could have been due to the Digit Symbol subtest, for reasons having nothing to do with hemisphere dominance. BONIER and HANLEY [41] found that performance on this test is impaired when the writing hand obscures the symbols which have just been written. This is what happens with most left-handers because of their handwriting position. In that study, rearranging the response sheet layout eliminated the difference between left-handers and right-handers.

Familial left-handedness At first glance, handedness did not seem to affect non-verbal perceptual performance. A

strong right-hemisphere performance deficit appeared only when subjects were divided on the basis of familial left-handedness: those subjects with FLH were about 15 ‘A worse at recognizing faces. This effect has considerable support. H~~CAEN and SAUGUET’S survey of components of aphasia following brain damage [17] showed a clear association between familial left-handedness and cerebral ambilaterality: in FLH left-handers, disturbances of oral language and of reading occurred equally often following right and left-hemisphere damage. Left-handers without FLH showed almost no disturbances of language following right-hemisphere damage. The study did not include right-handers, unfortunately.


Several other studies are relevant to this issue. HINES and SATZ [16], using a letter- detection test, noted that right-handers with FLH had much less visual-field asymmetry. ZURIF and BRYDEN [14] found a diminished asymmetry in left-handed and right-handed FLH subjects on both dichotic listening and tachistoscope letter-detection tasks. MCKEEVER et al. [15, 421 presented evidence that FLH decreases visual lateral asymmetry in a letter masking task, and HANNAY and MALONE [43] used a tachistoscopic verbal-memory task to test laterality effects in right-handed females with and without FLH. They interpreted their results as indicating that FLH decreased asymmetry for recognition of verbal material. VARNEY and BENTON [12] showed that FLH had a complex influence of its own on tactile perception which was independent of subject handedness. In their study, non-FLH right- handers showed the greatest left-hand (right-hemisphere) superiority. Presence of a left- handed sibling lowered the degree of asymmetry between the hands, and presence of a left-handed parent lowered it even more, quite similar to the pattern from the present data (Fig. 2).

The present study offers more direct support for the hypothesis that FLH is associated with decreased lateralization. Analysis of RTs in the letter-discrimination task showed that FLH subjects had significantly smaller differences between the two visual fields. This finding adds to the likelihood that lateralization, rather than some other factor, is the variable responsible for the drop in FR ability within the FLH group.

Taken together, these results imply a genetic component to both lateralization and facial recognition ability. The puzzle is why subject handedness seems less important than the presence of left-handedness in the family. One clue may be in a conclusion reached by COLLINS [44] : his review of the existing research on the inheritance of handedness led to the suggestion that while degree of lateralization may be genetically determined, the direction of asymmetry may not be. That is, a genotype may confer a greater potential for lateral variability in some people without specifying direction.

LEVY [45] reviewed evidence for genetic determination of handedness and several other structural and functional asymmetries, and emphasized the point that hand usage is quite subject to cultural pressures, and also to perinatal brain damage. These two sources of variation may interact with genetic determination or may be independent of it.

Reversed bias The most noteworthy characteristic of subjects with rightward biases for face RTs was

the -0.48 correlation between degree of rightward bias and facial recognition score. No such correlation existed for the subjects in general, or with any component of the letter reaction times. Some related correlations are: degree of rightward bias x letter RT absolute difference; - 0.29; face RT absolute difference x FR ability: 0.07: letter RT absolute difference x FR ability; 0.17; FR ability x face RT absolute difference: 0.10.

CONCLUSIONS

Two factors were associated more or less independently with decreased FR ability reversed RT bias and familial left-handedness. The results point tentatively toward a conclusion that decreased lateralization for verbal processing somehow interferes with face processing. Reduced verbal lateralization seems to exert its effects, if any, on FR ability through interference with the normal leftward bias for face RTs. Perhaps these subjects are looking at faces the wrong way, using a cognitive mode and hemisphere less suited to face analysis.

NON-VERBAL PERCEPTUAL ABILITIES IN RELATION TO LEFT-HANDEDNESS AND CERtRRAL LATERALIZATION 789

The distribution of face RTs shown in Fig. 1 suggests that RT speed is bilaterally diminished when the bias is reversed. The fact that this pattern is the same for the letter RTs cannot be explained the same way, for in this case, a reversed bias seems to enhance overall RT performance. An alternate interpretation is that the direction of the RT bias for both letters and faces is determined by the right hemisphere only.

Correlations among the various components of handedness were remarkably low (Table 4). Even the correlation between equal dexterity and “weak” categorization from the handedness questionnaire was only O-26. Self-report may be a poor indicator of strength of lateralization. The weak vs strong lateralization dichotomy, based on a combination of dexterity and questionnaire responses, is some improvement over a simple handedness classification. A more central measure of laterality such as EEG or thorough perceptual asymmetry assessment would bypass variation due to learned manual control.

The lack of correlation between FLH and other components of handedness is puzzling: - O-09, 0.02, and - 0.06 were the correlations with writing hand, dexterity, and hand-usage questionnaire, respectively. Hand usage and other lateralization indices may be peripheral correlates of some central factor which determines the variability of both speech and manual-control localization. These are probably closely related, since language dominance seems to apply less to speech comprehension than to production, which is, like skilled manual activity, ultimately a motor act, (KIMURA [46] found a relationship between the speech hemisphere and the gestures of the contralateral hand during speech.)

At any rate, these results support a growing conviction among laterality researchers that to maximize the chance of obtaining a “standard” language-on-the-left human brain/ subject, handedness questionnaires and dexterity tests are not enough. Familial left-handedness represents an important moderating variable. The possibility of a genetic factor which determines speech and motor lateralization, perceptual asymmetries, and right-hemisphere performance deficits should soon produce new research based on the genealogy of handedness.

REFERENCES

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On a mesur6 la performance wr on test de reconnaisnanre visuelle et

sur deux subtests de MIS, cubes it assemblage d’objets, en relation avec L’asymC-

trie de la dextdrit6, la prevalence manuelle et le stock de gauchcrir familinle

(GF) chez 64 sujecs. On mesurait le degr6 d’asymnitrie perceptive par les temps de

rsaction 1acGrale aux visages et aux lettres. l.es rlsultats ne sent pas en faveur

d’une liaison simple entre la prgvalence manuelle gauche e( le dgficit perceptif

non verbal. La capacit$ de reconnaissance des visages semble diminuae par C,F et

GF paraie Btroitement assaciee avec la diminution de la lateralisation cGr&bralr.

I.a 1atGralisarion h6mispheriquc droite du traitement de In reconnaissance des vi-

sages parait indipendante de route variable de pr&alence manuelle.

Deutschsprachige Zusammenfassung:

Die Leistungen bei einem Cesichtserkennungstest und bei

Wechsler-Subtests (z. B. Mosaik-Test u. a.) wurden in Be-

ziehung zu asymmetrischer Geschicklichkeit, Gebrauch der

HBnde und fami.liPrer Linkshgndigkeit (FLH) mit 64 Priifauf-

gaben gemessen. Das AusmaR der optisch-perzeptuellen Asym-

metrie wurde iiber die halbseitige Expositionszeit fiir Ge-

sichter und Buchstaben berechnet. Die Ergebnisse haben eine

einfache Verbindung zwischen Linkshgndigkeit und nonverba-

len perzeptuellen MBngeln nicht gestiitzt. Die Leistungen

beim Physiognomieerkennen schienen geringer bei LinkshSn-

digkeit und Linkshgndigkeit schien eng verkntipft mit einer

geringeren cerebralen Lateralisation. Eine rechtshemisphS-

rische Lokalisation des Physiognomieerkennens schien unab-

hSingig von irgendeiner Hgndigkeitsvariablen zu sein.

non-verbal perceptual abilities in relation to left-handedness and cerebral lateralization

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