predicting spatial ability from hemispheric ‘non-verbal’ lateralisation: sex, handedness and...

Acta Psychologica 46 (1980) 1-14 0 North-Holland Publishing Company

PREDICTING SPATIAL ABILITY FROM HEMISPHERIC

‘NON-VERBAL’ LATERALISATION: SEX, HANDEDNESS AND

TASK DIFFERENCES IMPLICATE ENCODING STRATEGY

EFFECTS *

Paul BIRKETT

Department of Psychology, University of Liverpool, U.K.

Accepted September 1979

Left- and right-handed males and females were given a divided visual field delayed form recognition task and three tests of visuospatial ability of varying degrees of complexity [(a) AH4 diagrammatic section, using 2-D items testing non-verbal reasoning; (b) Revised Minnesota Paper Form Board, using 2-D items testing simple spatial ability;(c) Differential Aptitude Test Space Relations, using 3-D items testing spatial manipulation]. Females of both handedness groups were left hemisphere dominant for the recognition task; males displayed no hemisphere difference. Only the most complex visuospatial test discriminated between groups, both left- and right-handed males were superior to their female counterparts. There were no handedness effects for either laterality or spatial ability. Since it was also possible in some cases to predict spatial ability from lateral&y within sex/handedness groups, the findings were discussed in terms of the contribution of both structural and encoding factors to individual and group differences in laterality and cognitive abilities.

Introduction

Hemispheric laterality effects for tasks employing non-verbal stimuli have been more elusive than have effects for tasks where the stimuli are of an overtly linguistic nature. Interpretations of the absence of a clear right hemisphere superiority for visual stimuli of an ostensibly non- verbal kind generally employ an ‘ease of verbal encoding’ argument.

* The work reported here forms part of a Ph.D. thesis submitted to the Council for National Academic Awards whilst the author was Research Assistant in the Science Department at Bolton Institute of Technology, Deane Road, Bolton BL3 5AB, England. The guidance of my thesis supervisors, Dr. R.H. Ranyard and Christine Gatehouse, is gratefully acknowledged. Geraldine and Noreen McCormack helped with data analysis and preparation of the manuscript. Requests for reprints should be sent to the Bolton address.

2 P. Birkett / Lateralisation and encoding

This proposes that certain non-verbal stimuli (e.g. common geometric shapes) are more amenable to such encoding than are other non-verbal stimuli (e.g. complex random forms). The former will therefore tend to display left hemisphere superiorities whereas the latter will often be better perceived and recognised when presented to the right hemisphere (Bryden 1960; Bryden and Rainey 1963; White 1969, 1972).

However factors other than stimulus-related ones can influence whether or not verbal encoding takes place during performance of ostensibly non-verbal tasks. The results of studies employing random forms classified by complexity and association value (Vanderplas and Garvin 1959) serve to illustrate this point. A left visual field (LVF) superiority has been reported for the delayed recognition of 12-point random forms in right-handed males (Fontenot 1973 ; Dee and Fontenot 1973). However a right visual field (RVF) superiority was found for the recognition of identical forms - also in right-handed males (Hannay et al. 1976). Hannay et al. further demonstrated a RVF advantage for 4-point random forms and no VF advantage for 8-point random forms, using different groups of right-handed males for each type of stimulus. The authors pointed out the difficulties of interpreting between group differences in laterality and proposed that they might depend upon the relative predominance within each group of left hemisphere (verbal encoding) and right hemisphere (visual encoding) modes of processing the same stimuli. They also suggested that a subject’s level of spatial ability might be related to the type of processing mode predominantly employed.

This latter issue has been investigated in a study employing delayed recognition of 4-point random forms presented to right-handed females (Hannay 1976). There was no overall laterality effect but females with left hemisphere superiorities were poorer at Block Design than were those with right hemisphere superiorities. The present author has also reported relationships between laterality and performance in tachistoscopic tasks which are difficult to interpret in terms of functional brain asymmetry (Birkett 1977a, b, 1978). It seems more likely that individual differences in measured laterality for a particular task represent variance introduced by subjects employing various modes of processing the stimuli (cf. Bryden 1978). These modes of processing might reflect characteristic ways of thinking about certain stimuli and might them- selves influence overall performance on tasks employing similar types of stimuli. Thus, in Hannay’s study, the tendency for some subjects to ver-

P. Birkett / Lateralisation and encoding 3

bally encode visuospatial stimuli was reflected in a left hemisphere superiority for form recognition and in poorer performance on a spatial task which would conceivably be better performed with the use of visuospatial processing. This kind of argument might help to account for the apparent failure to formulate adequate general theories of functional brain organisation for visuospatial skills, particularly with regard to sex and handedness differences (Marshall 1973). It may also be the case that some visuospatial tasks do have bilateral cerebral representation (Buffery and Gray 1972).

Considering task variables, it seemed important to extend Hannay’s finding to include visuospatial tasks where verbal encoding would not necessarily lead to poorer performance. Using the form recognition task, direction of laterality should not then be related to the level of performance on such tasks. We therefore employed three visuospatial tests in the present study: ( 1) the diagrammatic section of, the AH4 test of general intelligence (Heim 1955), (2) the Revised Minnesota Paper

Form Board (Likert and Quasha 194 l), (4) the Space Relations subtest of the Differential Aptitude Tests (Bennett et al. 1947). These tests will be fully described later. Laterality was measured by delayed recognition of 12-point random forms presented in a tachistoscopic, divided visual field paradigm. This task was chosen because it seems to allow subjects

to use either left or right hemisphere processing and might therefore provide an indication of the kind of approach adopted by subjects to solving visuospatial tasks.

Considering subject variables, there is evidence that left- and right- handers may be lateralised differently for visuospatial functions, the poorer right hemisphere visuospatial functions of left-handers leading to poorer overall spatial ability relative to right-handers (see Hardyck and Petrinovich 1977 for a review of the conflicting literature). A similar argument has been put forward to account for sex differences in spatial ability (Harris 1978). The present line of thinking would lead to the proposal that sex and handedness differences in the use of verbal encoding strategies might account for some of the data purporting to show that left-handers and females are less lateralised for visuospatial functions than are right-handers and males (Levy 1969; Kimura 1969).

With these issues in mind, data from the present experiment were analysed for sex and handedness differences. It should also be noted that previous studies of left-handedness and visuospatial deficit have been criticised on the grounds of extreme subject selectivity and use of


unsuitable pyschometric measures (Gibson 1973; Hardyck et al. 1976; Heim and Watts 1976). The use of an unselected subject sample and three standard tests of spatial ability allows the present study to over- come these criticisms. Finally, the use of three tests which differ in complexity also allows us to confirm whether sex differences in spatial ability, should they appear at all, are general, or specific to higher level abilities (Buffery and Gray 1972; Fairweather 1976).

Therefore, the central questions asked in the present study were (1) are there sex and handedness differences in lateralisation for form recognition? (2) are there sex and handedness differences in spatial abil-

ity with both simple and complex tasks? (3) do any observed relation-

ships between lateralisation and spatial ability differ according to sex, handedness or the complexity of the spatial task?

The experiment

Method

Subjects 12.5 volunteers were recruited from schools, colleges and the general public in the Bolton area of Lancashire. There were 54 males and 71 females aged between 16 and 42 years (X= 22.5). The breakdown of the full sample gave the following data on age: schools, N = 63, X= 16.9, range = 1.5-19, SD = 0.84; colleges, N = 30, X= 20.4, range = 18-25, SD = 1.82; general public, N = 32, X= 29.0, range = 17-50,

SD = 8.90. Ss were tested individually and were paid an hourly fee for participation in the experiment.

Procedure

(1) Handedness. Ss were asked to complete the Edinburgh Handedness Inventory (Oldfield 1971). This is a simple, 12-item questionnaire for which respondents are required to indicate their hand preferences in the performance of various everyday tasks. The inventory gives a score which can range from -100 (completely left-

handed) to +l 00 (completely right-handed).

(2) Delayed form recognition. Stimuli were twenty 12-point random forms of com- parably high complexity and low association value taken from a previous study (Vanderplas and Garvin 1959). The shapes were drawn as solids (1 sq cm) in black ink on forty white tachistoscope cards (14 X 11 cm) and were centred on 3’ of horizontal visual angle from fixation. Each shape was drawn once on the left and once on the right, giving forty stimuli in all. A verbal ready signal was given by the experimenter after instructing the subject to fixate on the central cross in the pre-


exposure field of a standard 2-field tachistoscope (Labgear Ltd.); the stimulus was

then presented for 10 msec. Stimulus offset started an electronic timer (Electronic Developments) which counted 10 set before a response card containing four foils and the target shape was shown to the S outside the tachistoscope. The S was required to point to the shape he or she recognised as the target, target position being random in the response set. The forty stimuli were presented in random suc- cessive order across visual fields.

(3) Spatial ability. The three tests mentioned in the introduction were administered to Ss in random order. The diagrammatic section of the AH4 test of general intelligence contains items exemplifying five principles: analogies, sames, subtractions, series and superimpositions. All items are two-dimensional and a minimum of shape rotation is required to perform the task. The maximum score is 65.

The Revised Minnesota Paper Form Board contains 64 items for each of which Ss must decide which of five, two-dimensional line drawings of shapes can be made from a given set of fragmented parts. Rotation of shapes in two dimensions is required to solve all items in the test.

The Space Relations subtest of the Differential Aptitude Tests requires Ss to decide which, if any, of five drawings of three-dimensional shapes matches an un-

folded, two-dimensional representation. Mental rotation in three-dimensions is required to perform the test, which has a maximum score of 100.

The experiment was run in two sessions. The first part involved administration of the handedness questionnaire and the delayed form recognition task and took approximately 30 minutes. Ss were asked to return one week later to take the spatial ability tests in a session lasting about 90 minutes.

Results

(1) Handedness. Analysis of responses to the handedness questionnaire resulted in 52 Ss being classified as left-handed (2 1 males, 3 1 females) and 73 as right-handed (33 males, 40 females) [ 11. It is unusual to find an excess of females in a left- handed sample and there may be a volunteer effect here, a study on left-handedness generating more (self)interest amongst the female left-handed. The apparent non- randomness of the handedness groups presents no difficulties for the major issues of this paper.

(2) Laterality. The number of correct form recognitions in each VF wasused to determine whether an S be classified as left or right hemisphere dominant for the task. The results of this classification are given in table 1. Using the sign test (Siegel 1956), a significantly greater number of both left- and right-handed females displayed left as opposed to right hemisphere superiorities for the task. For males of

[l] The data on laterality and spatial ability to be reported below were initially analysed by

familial handedness and by strength of hand preference. These variables had no effects of any

substance and Ss were herefore pooled into left- and right-handed groups for the analyses which follow.


Table 1 Number of subjects with left and right hemisphere superiorities on the delayed form recognition task.

Handedness Male Female

Left (IV = 21) Right (IV= 33) Left (N- 31) Right (iV = 40)

Superior hemisphere Right 10 (12.14) 13 (12.64) 8 (11.42) 9 (11.20) Left 8 (12.43) 16 (12.58) 20 (12.36) 29 (13.00) Equal 3 4 3 2

Z score <l a) <l a) 2.08 b, 3.08 c,

a) p > 0.05 b, p = 0.038 c) p = 0.002 Note: Figures in parentheses refer to the mean numbers of correct recognitions in each hemi-

sphere.

both handedness groups there was no significant difference between the number of Ss with left and right hemisphere advantages.

To allow a comparison of the relative degree of lateralisation between groups a measure of laterality unbiased by total accuracy was computed for each S (Marshall et al. 1975). This coefficient expresses the VF difference as a proportion of either the total correct or the total errors, whichever is the smaller. Negative scores indicate right hemisphere advantages and positive scores indicate lefte hemisphere advantages. Mean and median values of this coefficient are shown in table 2 for each sex/ handedness group. A Kruskal-Wallis ANDVA (Siegel 1956) using the ranked coefficients showed that the four groups differed in asymmetry (N = 14.37, df= 3, p <

0.01, two-tailed). A set of four paired comparisons (Dunn 1964) examined differences in degree of asymmetry between left- and right-handers in males and in females, then between males and females in left- and in right-handers. The only comparison to achieve significance showed that right-handed females were more left hemisphere dominant than were right-handed males (2 = 3.23, p = 0.001, two- tailed).

(3) Spatial ability. Table 3 gives the mean and median scores of each group on the three tests of spatial ability [2]. Kruskal-Wallis ANOVAS on the ranked scores showed no difference between groups on the AH4 test (H = 6.3, df= 3, p > O-05),

or on the Revised Minnesota Paper Form Board (RM) (H = 0.19, df= 3, p > 0.05). For the Space Relations subtest of the Differential Aptitude Tests (DAT) perfor-

[2] It should be noted that 22 Ss did not return to take these tests. Rather than omit them from the laterality analyses just presented it was thought preferable to point out that the results on spatial ability and on relationships between laterality and spatial ability are based on different samples from those used in the laterality analysis alone. This in no way affects the validity of the conclusions to be drawn from the data.


Table 2 Mean and median laterality coefficients for the delayed form recognition task.

N Mean SD Median

Male Left-handed Right-handed

Female Left-handed Right-handed

21 -0.007 0.213 0.00 33 -0.005 0.244 0.00

31 0.072 0.170 0.11 40 0.125 0.201 0.17

mance differed significantly between the groups (H = 14.06, df = 3, p < 0.0 1, two- tailed). Paired comparisons showed that males scored higher than females amongst left-handers (2 = 2.25, p = 0.02, two-tailed) and amongst right-handers (2 = 2.24, p = 0.02, two-tailed). There were no handedness effects.

(4) Relationships between laterality and spatial ability. Polynomial regression anal-

yses (Kerlinger and Pedhazur 1973) were carried out with laterality coefficients and spatial test scores as the independent and dependent variables respectively. Table 4 presents the proportions of variance (R’) in each of the three tests of spatial ability that could be accounted for by laterality scores. In all cases the significance of the linear component of laterality was tested first, followed by the significance of the addition of the quadratic component of laterality. The figures in table 4 refer to the variance accounted for by whichever of these components was significant; if neither component was significant the total variance is given. Three relationships were significant. For left-handed males laterality predicted AH4 performance in terms of a U-shaped curve. The data points and best fit line are shown in fig. 1; the curve was

Table 3 Mean and medium scores on each of the three tests of spatial ability (N = 30).

Left-handedness Right-handedness

Spatial test Spatial test AH4 RM DAT AH4 RM DAT

Male N=20 N=23 Mean 50.50 43.05 64.00 50.43 42.80 69.10 SD 7.30 6.73 20.86 8.55 9.06 16.57 Median 50 42 65 53 43 75

Female N=30 N=30 Mean 44.20 42.37 48.07 48.03 42.17 60.70 SD 11.84 8.58 24.24 8.12 9.15 10.67 Median 45 44 49 47 43 60


Table 4 Proportions of variance on each of the three tests of spatial ability accounted for by laterality scores.

Left-handedness Right-handedness

Male Female Male Female (N = 20) (N = 30) (N = 23) (N = 30)

AH4 R2 0.382Q 0.074 0.006 0.103 F 12.43 c, 1.07 a) <l a) 1.55 a)

RM R2 0.255L 0.015 0.040 0.129 F 6.10 b, <l a) <1 a) 2.00 a)

DAT R2 0.061 0.001 0.066 0.169L F <l a) <l a) <l a) 5.72 b,

a) p > 0.05 b, p < 0.025 c) p < 0.005 L = linear Q = quadratic

drawn from the given regression equation generated by the regression analysis. The graph shows that Ss scoring at both extremes of the laterality distribution tended to perform better on AH4 than did Ss with indeterminate lateralisation. Also for left- handed males, increasing left hemisphere laterailisation on form recognition was

related to decreasing scores on RM. Fig. 2 gives the plotted points, regression equa-

tion and resultant regression line. Finally, right-handed females’ DAT performance

65

52

-1.0 -.6 w.2 .2 .6 1.0

laterality coefficient

65 r

;[ Y:,42.8;-14.,82x , ,

_.6 m.2 .2 .6 1.0


Fig. 1. Relationship between lateral&y and

AH4 performance for left-handed males.

Fig. 2. Relationship between laterality and RM performance for left-handed males.

P. Birkett / Lateralisation and encoding

-1.0 _.6 w.2 .2 .6 1.0


Fig. 3. Relationship between laterality and DAT performance for right-handed females.

was negatively related to increasing left hemisphere lateralisation on form recognition. The regression equation given in fig. 3 predicts the observed straight line relationship from the points shown.

Conclusions and discussion

The following conclusions may be drawn: (1) the delayed form recognition task can be subserved by either left or right hemisphere functions but females are more likely than males to employ left hemisphere processing; (2) our unselected groups of left-handers perform as well as right-handers on all three tests of spatial ability; (3) the male advantage in spatial ability is restricted to the task involving manipulation of spatial relationships in three dimensions; (4) there are only a few group- and task-specific relationships between “lateralisation” and level of spatial ability.

The delayed form recognition task is obviously not a reliable indica- tor of either left or right hemisphere functions alone. Left hemisphere dominance was observed in 73 subjects, right hemisphere dominance in 40 subjects and 12 displayed no difference between the hemispheres in accuracy of recognition. This result was not unexpected in view of the contradictory findings with this task mentioned in the introduction. In brain-damaged subjects too, poorer performance on delayed recognition of nonsense figures has been reported after injury to the left hemisphere (Bisiach and Faglioni 1974) and to the right hemisphere (Fag- lioni and Spinnler 1969).


The experimental parameters considered important in determining whether non-verbal stimuli are likely to be verbally encoded include length of stimulus exposure, number of stimuli, type of response and communication accuracy of stimuli (Brewer 1969). Certainly, these parameters are not constant over studies but even when they are closely comparable the hemisphere difference is unstable. The possibility that individual differences in processing strategies help to account for this lack of repeatability cannot be ignored. Thus, we have found that a subject’s sex can influence whether left or right hemisphere processing is employed. The left hemisphere dominance observed in females parallels other studies which suggest that females are more likely than males to make use of (left hemisphere) verbal encoding when solving visuospatial tasks (Hyde et al. 1975; Kimura 1969; Lansdell 1968; McCall 1955; McGlone and Davidson 1973; McGlone and Kertesz 1973; Meyer and Jones 1957). However the range of laterality scores obtained for females suggests that even they need not necessarily use left hemisphere processing (see fig. 3). The extent to which a group of subjects com- prises individuals who tend to use verbal or visual encoding would thus seem to be an important variable affecting the average laterality effect for the group as a whole (Hannay et al. 1976). This may be one reason why current topological theories of brain organisation possess limited explanatory power (Marshall 1973).

It is important to note that the tendency to use verbal encoding strategies may itself be a consequence of brain organisation; bilateral language representation may “force” a subject to use this kind of approach rather than visuospatial processing because right hemisphere visuospatial functions suffer from “invading” linguistic functions (Levy 1969). Since females are likely to possess some degree of bilateral language representation (Harris 1978) this could help to explain our own and other findings on non-verbal asymmetries in females. Sex differences in visuospatial lateralisation would thus be seen as more apparent than real, although language lateralisation does seem to differ reliably

between the sexes. According to the topological hypothesis (Levy 1969) left-handers

should display the same kinds of results as females because they too possess bilateral language representation. One possible reason for the absence of handedness effects in the present study is that bilateral language may be confined to specific subgroups of left-handers, i.e. those with familial sinistrality or with stronger left-handedness (Subirana


1969; Knox and Boone 1970). This limitation could also account for

the grossly equivocal evidence on handedness differences in visuospatial skills. Without listing all the studies, some have found left-handers to be poorer on a variety of visuospatial tasks (Levy 1969; Miller 197 1; Nebes 197 1; Nebes and Briggs 1974) whilst others have found no difference between handedness groups (Fagan-Dubin 1974; Gibson 1973; Gilbert

1977; Hardyck 1977a, b; Hardyck et al. 1976; Heim and Watts 1976; Kutas et al. 1976; McKeever and VanDeventer 1977; Newcombe and Ratcliff 1973). To the latter list must be added the results of the present study. It would therefore seem fair to concur with the conclusion that the supposed superiority of right-handers on visuospatial tasks is a “just not noticable difference” when subjects are unselected and suit- able tasks are used (Hardyck 1977a).

The spatial tests employed in the present study are not all insensitive to group differences in spatial ability, as confirmed by the male superiority on DAT. Sex differences on AH4 and on RM are minimal (Heim 1955; Petrusic et al. 1978). Indeed, tests where the manipulative component is absent and which involve only two dimensions do not give reliable sex differences, whereas males have an advantage if the test requires manipulation of spatial relationships in three dimensions (Buf- fery and Gray 1972; Fairweather 1976; Maccoby and Jacklin 1975). The present study confirms this distinction.

In the context of encoding strategies, these sex differences between types of spatial task might be explained in terms of verbal encoding being an inefficient method of solving complex spatial problems whilst being adequate to ensure competent performance on less demanding tasks. Thus females will, in general, perform poorly on the former but may score as well as males on the latter. That the DAT Space Relations test can be performed with either (right hemisphere) visual-transforma- tional abilities or less efficient (left hemisphere) verbal-descriptional abilities is apparent from a study employing an adaptation of this test in split-brain patients (Levy-Agresti and Sperry 1968). Our own data also indicate that, within right-handed females, those with left hemisphere superiorities on form recognition were worse on DAT Space Relations than were those with right hemisphere superiorities (see fig. 3). This kind of relationship has been observed in other studies with females (Hannay 1976; McGlone and Davidson 1973) and can be taken as indirect support for the notion that verbal mediation in certain visuospatial tasks leads to females’ poorer performance. Apparently,


some females can use right hemisphere visuospatial skills to good effect; whether this is because of the absence of bilateral language, or because of experiental factors (Nerlove et al. 197 l), or both, is a matter for future investigation.

There were two other relationships between laterality and spatial ability, both in left-handed males. Fig. 2 shows that the verbal encoding argument can account for the poorer RM performance of left-handed males who use left rather than right hemisphere processing for form recognition. However the U-shaped curve in fig. 1 suggests that either left or right hemisphere processing will give adequate AH4 performance in this group.

It is difficult to assess the reliability of these findings on relationships between laterality and spatial ability, particularly in view of their relative absence. Also, we have assumed throughout that left hemisphere superiorities on form recognition reflect verbal encoding whilst right hemisphere advantages indicate visuospatial processing. This assumption may not be unjustified but it requires verification before the kind of data presented here can be properly interpreted. However, the finding in right-handed females does seem to be reliable and this suggests that the methods of analysis and explanation employed here may prove use- ful in future research of this kind. Such research would be directed towards unravelling the interaction between neurological and encoding variables in producing group differences in laterality and cognitive ability. To do this we must be able to describe what kind of information processing is required to perform the tasks that subjects are set and what kind of information processing subjects do in fact use to carry them out.

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predicting spatial ability from hemispheric ‘non-verbal’ lateralisation: sex, handedness and...

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