Patterns of cerebral organization

BRAIN

AND

LANGUAGE

20, 249-262 (1983)

Patterns of Cerebral Organization M. P.

BRYDEN

University of Waterloo AND

H. HBCAEN

AND MARIA

DEAGOSTINI

Unite de Recherches Neuropsychologiques et Neurolinguisiiques de I’INSERM An analysis of the concurrent incidence of aphasia and spatial disorder in 270 patients with unilateral brain damage suggeststhat the two functions are statistically independent. These data can also be used to estimate the distribution of left, right, and bilateral representation of linguistic and spatial functions in the population. In right-handers, sex affects the pattern of cerebral asymmetries, while the familial history of sinistrality has a stronger effect on the pattern of cerebral asymmetries in left-handers. These findings suggest that complementary specialization exists only as a statistical norm: It is suggested that differences in complementary and noncomplementary specialization may underlie individual differences in cognitive skills.

The notion that the two cerebral hemispheres subserve different functions is now well established in the neuropsychological literature (cf. DeRenzi, 1982; Heilman & Valenstein, 1979; Gazzaniga & LeDoux, 1978; Hecaen & Albert, 1978; Springer & Deutsch, 1981). In general, the left hemisphere is considered to be more involved in verbal and linguistic processes, while the right hemisphere is more specialized for nonverbal processes, especially those involved in visuospatial activities. Such a division of function has come to be termed “complementary spacialization,” to distinguish it from the notion of an all-powerful left hemisphere implied by the term “cerebral dominance.” The data analysis reported here was supported in part by a grant from the Natural Science and Engineering Research Council of Canada to M.P.B. An earlier version of this paper was presented at the February 1982meeting of the International Neuropsychological Society by M.P.B. Send requests for reprints to Dr. M. P. Bryden, Department of Psychology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. 249 0093-934X183$3.00 Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.

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The term complementary specialization may be taken in two ways. First, it may be taken in the statistical sense, that is, as a description of the normal state of affairs. In this sense, there is general agreement that most right-handed males show complementarity, in that the two hemispheres subserve quite different processes. The case is not quite so clear for left-handers (Hecaen & Sauguet, 1971; Herron, 1980), nor for right-handed females (McGlone, 1980), for whom concepts of bilateral representation are often invoked. Alternatively, the term complementary specialization may be taken in a causal sense, as implying that one hemisphere carries out a particular set of functions because the complementary functions are localized to the other hemisphere. By this argument, not only do individuals who manifest left-hemisphere language show right-hemisphere visuospatial processing, but those few who exhibit right-hemisphere language simultaneously manifest left-hemispheric visuospatial processing. While such a strong position is rarely made explicit (but see Bradshaw & Nettleton, 1981; Corballis, 1981), it seems to have become part of the lore of contemporary neuropsychology, especially as viewed by the popular press. Bradshaw and Nettleton (1981) argue that the left hemisphere is specialized for analytic, sequential, and time-dependent functions. By their argument, the right hemisphere acquires a superiority in some tasks because processing space in the left hemisphere that would normally be devoted to the task has been preempted by language processes. Such a system must work at the level of the individual, with the languagedominant hemisphere necessarily determining which hemisphere appears to be specialized for nonverbal activities. A similar argument has been advanced by Corballis and Morgan (1978; Morgan & Corballis, 1978), who suggest that hemispheric specialization has its origins in a left-toright gradient of neurological development. Again, this argument implies that the hemisphere specialized for nonverbal activities is necessarily the opposite to that specialized for language activities. Given such notions, a fuller comprehension of the nature of hemispheric specialization may lead to some important insights into the mechanisms involved in hemispheric specialization and their evolution. An explicit test of the notion of causal complementarity requires that we know something about the lateralization of language and nonlanguage functions in the same individuals. Unfortunately, the vast majority of clinical studies focus on one function or the other, and do not report data on the joint incidence of aphasia and visuospatial function. Granted, some evidence for causal complementarity can be seen in the commissurotomy literature, where the nonverbal hemisphere is often reported to have good visuospatial abilities (cf. Gazzaniga, 1970; Gazzaniga & LeDoux, 1978). On the other hand, studies of lateralization of function in normal subjects have often shown the same hemisphere to be superior

251

PATTERNS OF CEREBRAL ORGANIZATION

for both verbal and nonverbal processing (cf. Bryden, 1973; McGlone & Davidson, 1973). Furthermore, some large-scale studies of unilateral brain damage have shown visuospatial deficits following right-hemisphere damage to be approximately equivalent in left- and right-handers (Hecaen & Sauguet, 1971; Newcombe & Ratcliff, 1973). If there is a causal complementarity, this should not be the case, since more left-handers than right-handers have right-hemispheric speech representation. There is, then, at least some evidence to suggest that causal complementarity may not appropriately describe the organization of the human brain at the level of the individual. The present report is based on an analysis of cases of unilateral brain damage originally reported by Hecaen, DeAgostini, and Monzon-Montes (1981). These include 140 left-handers and 130 right-handers. All patients had had a full neuropsychological examination, and presented a unilateral lesion that was verified surgically, clinically, or paraclinically. Each patient was classified by Hecaen et al. (1981) as aphasic or not aphasic, and as showing a visuospatial disorder or not. Aphasia was diagnosed in terms of disorders in verbal fluency, auditory and visual verbal comprehension, naming disorders, and writing disorders. Spatial disorders were determined by the presence or absence of spatial agnosia, spatial dysgraphia, loss of topographic memory, and unilateral spatial agnosia (Hecaen et al., 1981, p. 267). These data are shown in the original publication as Tables 9 and 10. This report presents data on the joint incidence of aphasia and spatial dysfunction (Table 1). One of the left-handers included in Table 9 of Hecaen et al. (1981) has been omitted since no data were available on spatial ability. The incidence of spatial disorder is higher in Table 1 than in the original study since the original categorization of spatial disorder did not include cases of constructional apraxia. In Table 1, patients TABLE ASWCIATION

BETWEEN APHASIA

1 AND SPATIAL

DISORDERS

Right-handed

Left-handed Left-Hemi

Right-Hemi

Left-Hemi

Right-Hemi

AS A0 OS 00

AS A0 OS 00

AS A0 OS 00

AS A0 OS 00

Men

FS+ FS-

10 13 10 8

3 1

6 8

7 2

1 3

4 3 9 12

2 3 4 14

1 2

8 9

1 0

0 6 0 13

5 5

Women

FS+ FS-

5 6 9500

1

2

2 1

0 1

2 2

1 2

0 4

3 7

1 1

0 2

8 9

4 14 1 2 5 16

9 3 12

3 17 4 10 7 27

1 2 3

0 19 10 2 9 17 2 28 27

All men All women Total

20 21 14 11 34 32

2 2

4 13 15 1 4 4 5 17 19

6 4

6 17 3 10 9 27

5 4

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exhibiting constructional apraxia have been classified as showing a spatial disorder. In the patients considered here, constructional apraxia was closely associated with the appearance of other spatial disorders (x2 = 60.46, p < .OOl), and unrelated to the incidence of aphasia (x2 = 0.36, n.s.). Therefore, constructional apraxia appears to cluster with other spatial disorders. Furthermore, omitting these cases does not alter the major points to be made in this report. RELATION BETWEEN APHASIA AND SPATIAL ABILITY The first step was to arrange the data as 2 x 2 contingency tables (presence or absence of aphasia by presence or absence of spatial disorder). These 2 x 2 tables were tested for association by x2 (or Fisher’s exact test where the sample sizes were small). There are, in fact, two directions in which a significant association can exist in such data. The notion of causal complementarity implies a negative association; that is, patients should show aphasia or spatial disorder, but not both. In this case, the larger entries should be along the major diagonal, with entries in the A0 and OS cells. A positive association is also logically possible, with patients showing either both aphasia and spatial disorder or neither. In this case, the larger number of cases should fall along the negative diagonal, in the AS and 00 cells. Of the individual cells, only one is statistically significant, that for lefthanded men with no familial history of sinistrality (FS-) and with left hemisphere lesions (x2 = 4.91, p < .05). This is a positive association, with 18 cases along the negative diagonal and only 9 on the positive diagonal, indicating that, for this subgroup, aphasia and spatial disorder seem to appear concurrently. This one cell makes the combined data for left-handed FS- subjects with left-hemisphere lesions (x2 = 6.55, p < .Ol) and for all left lesions in left-handers (x2 = 4.94, p < .05) statistically significant; in both cases, the associations are positive rather than negative ones. No other subgroups or combinations of subgroups in Table 1 show statistical significance. Furthermore, a subdivision of the subjects into those with anterior lesions and those with posterior lesions (Hecaen et al., 1981, p. 266) indicates no significant associations between aphasia and spatial disorder except for a positive association in the left-handers with posterior left-hemispheric damage (x2 = 6.60, p < .Ol). This is again due to the male FS- subgroup. As a further check on the association between aphasia and spatial disorder, the data were cast into a number of three-way x2 tables to examine the interaction between aphasia, spatial disorder, and sex, and the interaction between aphasia, spatial disorder, and family history of sinistrality. (The cell entries are much too small to attempt a four-way chi-square analysis.) These analyses were carried out separately for each handedness and hemisphere of lesion group.

253

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Such a three-way contingency table yields a x2 with 4 df (Winer, 1971). This can then be partitioned into four separate components, each with I df. The components can be identified as two-way interactions between aphasia and spatial disorder, aphasia and sex (or FS or lesion site), spatial disorder and sex (or FS or lesion site) and a three-way interaction between aphasia, spatial disorder, and sex (or FS or lesion site). The significant components of these analyses are shown in Table 2. As with the initial analyses, the only sign of an association between aphasia and spatial disorder is in the left hemisphere for left-handers, and is a positive association. None of the three-way interactions reached significance, indicating that sex and familial history do not modulate or obscure an association. The remaining significant effects confirm those reported previously (Hecaen et al., 1981). Sex affects the incidence of aphasia in the left hemisphere of left-handers, and the incidence of spatial disorder in the right hemisphere of right-handers. Familial sinistrality interacts with both aphasia and spatial disorder in the left hemisphere of right-handers. Finally, aphasia is more frequent with posterior lesions in the left hemisphere of right handers, and spatial disability is more frequent with posterior lesions in the left hemisphere of left-handers and in the right hemisphere of right-handers. The important point, however, is that these data provide no support for the argument that the two cerebral hemispheres necessarily have complementary functions. In no group is there any sign of a negative association between aphasia and spatial disorder. The one significant association is a positive one, attributable to a single subgroup. On the basis of these data, it would seem logical to assume that language and TABLE 2 SIGNIFICANT CHI-SQUARE VALUES FROM ANALYSIS OF LESION DATA

Hemisphere Right-handers

Left-handers Left

Right

Left

Right

Aphasia X spatial disorder

4.94

-

-

-

Sex X aphasia X spatial

4.06 -

-

-

3.86 3.92 -

-

FS X aphasia X spatial Location X aphasia X spatial p(.O5) = 3.84. p(.Ol) = 6.64.

4.14

6.28 -

5.41 9.67

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spatial functions are independently assigned to the two hemispheres, rather than being causally linked. Thus, the idea that the right hemisphere acquires its functions only in response to the specialization of the left hemisphere (Bradshaw & Nettleton, 1981; Corballis & Morgan, 1978) cannot be correct at the level of the individual, although the lateralization of language may well have preceded that of visuospatial processes in the population. If one wishes to argue that the relative size of the temporal planum in the two hemispheres contributes to determining which hemisphere will be language dominant (cf. Galaburda, LeMay, Kemper, & Geschwind, 1978), then it is also plausible that the relative magnitudes of some other brain region contributes to determining the dominant hemisphere for visuospatial processing (cf. Rubens, 1976). The present data would suggest that interhemispheric differences in language and visuospatial areas are independently determined. They therefore provide no support for the notion of a left-right gradient in neurological development (Corballis & Morgan, 1978). At the same time, the present results should come as no great surprise. We have long known that spatial tasks often do not show the overwhelming right-hemisphere dependence in the same way that language functions are so clearly dependent on left-hemisphere mechanisms in the majority of individuals (Ratcliff & Newcombe, 1973). Furthermore, while left- and right-handers differ in language lateralization (Satz, 1980), such differences are not so clearly evident in tests of visuospatial performance (DeRenzi, 1982; Masure & Benton, 1983). The fact that the distributions differ and that the influence of handedness differs indicates that complementarity cannot be strictly causal. It should be noted that complementarity, in the sense of the two hemispheres being specialized for opposing functions, will remain the rule rather than the exception, for purely statistical reasons, at least in right-handers. If language functions are most likely to be dependent on the left hemisphere, and visuospatial functions on the right hemisphere, than the majority of individuals will show language and visuospatial functions in opposite hemispheres even though the two are causally independent of one another. In other words, while there may be no evidence for causal complementarity, statistical complementarity still exists. LATERALIZATION

OF LANGUAGE FUNCTIONS

The data in Table 1 can also be used to estimate the likelihood of language processes being lateralized to one hemisphere or the other. For example, in right-handers, aphasia is observed in 36170(51.4%) of those with left-hemisphere lesions, and in 5/60 (8.3%) of those with right hemisphere lesions. Assuming that the sampling of cases is random, and that the brain lesions are also random, we then know that the incidence of

PATTERNS

OF CEREBRAL

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255

left hemisphere language to right hemisphere language is in the ratio of 5 1.4 to 8.3. This calculation leads to the conclusion that language is lefthemispheric in 86.1% of right-handers, and right-hemispheric in the remaining 13.9%. This is, of course, a rather high estimate for the incidence of right-hemispheric language representation (cf. Rasmussen & Milner, 1977), and more will be said about this point later. If we engage in similar calculations for left-handers, we find that 66/ 87 or 75.9% of the left lesions are aphasic, and 17/53 or 32.1% of the right lesions are aphasic. By the calculation proposed above, 70.3% of left-handers have left hemisphere language. But, the incidence of aphasia following left lesions is higher than this, so something must be wrong. Note that the two incidence values (75.9 and 32.1%) sum to more than 100%. Thus, there must be some left-handers with bilateral speech representation, in which a lesion of either hemisphere would produce an aphasia. In fact, the incidence of bilateral representation must equal at least 8%, the amount by which the two incidence values exceeds 100%. If this is the case, 67.9% of left-handers would have left-hemisphere language, 24.1% right-hemisphere language, and 8% would be bilateral. Such a situation would require that all lesions to either hemisphere produce aphasia, if that hemisphere is in any way involved in language. Since this was not true for the right-handers, there is no great reason to believe that it should be true for the left-handers. Decreasing the probability that a unilateral lesion will produce aphasia in a language-relevant hemisphere from 1.0 will serve to increase the estimate of bilateral language representation while decreasing the estimates of unilateral representation. It should be noted that this probability cannot drop below .759, the larger of the two incidence values. If we arbitrarily accept a value of .85, the incidence of bilateral representation rises to 19.1% and the estimates of unilateral left and right hemispheric representation become 62.7 and 18.2%, respectively. These figures are very close to those estimates by other procedures (Rasmussen & Milner, 1977; Segalowitz & Bryden, 1983). The logic of these calculations can be expressed more formally, as follows. L, R, and B

Let 1, r Let i Then

= the proportion of individuals with left, right, and bilateral representation, respectively. = the observed incidence of the deficit in question following left and right lesions, respectively. = the proportion of cases in the population in which damage to the relevant hemisphere produces an effect. L + R + B = 1.000 1 = i(L + B) r = i(R + B)

256

BRYDEN,

And, by substitution

HkCAEN,

AND DE AGOSTINI

r = i(l - L) r + 1 = i(l + B).

By these equations, i is limited to values between 1.000 and the larger of 1 and r. Since there are four unknowns (i, L, B, and R) and only three equations, the equations do not have a unique solution. However, they can be used to specify the limits of L, R, and B, and can provide some guidelines to more precise solutions. For example, the observation that r + 1 exceeds 1.000 for left-handers forces the conclusion that there must be individuals with bilateral language representation. Table 3 shows the results of applying these equations to men and women separately, and to those with and without familial sinistrality. Computations have not been reported on the individual subgroups, since the number of cases is so small that the estimates of incidence, 1 and r, have very large standard errors, and computations of L, B, and R become almost meaningless. Several very arbitrary decisions have been made in constructing Table 3. In right-handers, where 1 + r is always less that 1.00, i has been taken as 1 + r. If we were to take i < 1 + r, the figures would change by increasing the incidence of bilateral representation. However, Rasmussen and Mimer (1977) report no cases of bilateral language representation in right-handers, and there seems to be no reason to do so here. In the

Handedness Right

Left

TABLE 3 ESTIMATES OF HEMISPHERIC ASYMMETRY FOR LANGUAGE L B r Group

-

R

i

Men Women

0.535 0.481

0.033 0.133

94.2 78.3

-

5.8 21.7

0.568 0.614

FS+ FS-

0.500 0.522

0.077 0.088

86.7 85.6

-

13.3 14.4

0.577 0.610

Men

Max Min Opt

0.695

0.317

68.3 54.4 62.7

1.2 45.6 19.1

30.5 18.2

1.000 0.695 0.850

Women

Max Min Opt

0.895

0.333

66.7 62.8 64.9

22.6 37.2 29.3

10.7 5.8

1.000 0.895 0.950

FS+

Max Min Opt

0.739

0.476

52.4 35.6 47.1

21.5 64.4 35.0

26.1 17.9

1.000 0.739 0.900

FS-

Max Min Opt

0.780

0.219

78.1 71.9 75.6

28.1 11.0

21.9 13.3

0.999 0.780 0.900

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257

left-handed groups, 1 + r > 1.00, and some incidence of bilateral representation is indicated. The “maximum” value is obtained by setting i = 1.00, the “minimum” value by setting i = 1, and the “optimal” value was selected as being roughly halfway between these two. In right-handers, it is sex that seems to be the major variable in determining language lateralization. The figure for males is extremely close to that reported in other large-scale investigations of language lateralization in right-handers, most of which involved predominately male populations (Rasmussen & Milner, 1977; Satz, 1980; Segalowitz & Bryden, 1983). In contrast, the estimates for females suggest that only about 80% of right-handed women have left-hemisphere language representation. The sex differences in language laterahzation that have been reported (McGlone, 1980) therefore may result not from weaker lateralization in women, but from a higher incidence of right hemisphere language specialization in women, although it should be noted that other studies of unilateral brain damage (e.g., Kimura, 1983) have not found the elevated incidence of right hemisphere aphasias in women seen in Table 1. The “optimal” estimates for left-handed males are again strikingly similar to those shown for left-handers by Rasmussen and Mimer (1977) and by Segalowitz and Bryden (1983). Among left-handers, however, women are fairly similar to men, with possibly a higher incidence of bilaterality. If there is a major effect among left-handers, it lies in the familial history of sinistrality. FS + individuals are far more likely to show bilateral representation than are FS- individuals, and are less likely to show clear left-hemispheric specialization. LATERALIZATION OF SPATIAL FUNCTIONS The same logic can be used to estimate the incidence of spatial representation in the left and right hemispheres. For all right-handers, the estimates are 30.7% left-hemispheric and 69.3% right-hemispheric, while the comparable figures for left-handers are 45 and 55%. Even excluding those patients classified as showing spatial disorder on the basis of constructional apraxia, and employing only the cases reported in Hecaen et al. (1981, Table lo), changes the percentages only slightly, to 28.2 and 71.8% in right-handers. and to 42.5 and 57.5% in left-handers. The simple observation that right-hemispheric specialization for visuospatial processes in right-handers is not nearly as prevalent as left-hemispheric language specialization poses serious difficulties for any concept of causal complementarity: There simply must be some individuals who are left-hemispheric for both language and visuospatial processes. Table 4 shows estimates of the lateralization of spatial ability as a function of sex and familial history. The results are strikingly similar to those obtained with the aphasia data. In right-handers, sex seems to make a difference, with the incidence of right-hemisphere visuospatial

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TABLE 4 ESTIMATES

Handedness

Group

Right

Left

OF HEMISPHERIC

ASYMMETRY

FOR SPATIAL

ABILITY

I

r

L

B

R

i

Men Women

0.209 0.259

0.667 0.367

23.8 41.3

-

76.2 58.7

0.876 0.626

FS+ FS-

0.167 0.261

0.500 0.529

25.0 33.0

-

75.0 66.7

0.667 0.790

Men

Max Min Opt

0.407

0.537

43.1 32.9

75.8 23.9

56.9 24.2 43.2

0.944 0.537 0.800

Women

Max Min Opt

0.536

0.583

41.7 27.1

11.9 91.9 39.9

46.4 8.1 33.0

1.000 0.583 0.800

FS+

Max Min Opt

0.413

0.714

28.6 20.7

12.7 57.8 25.2

58.7 42.2 54.1

1.000 0.714 0.900

FS-

Max Min Opt

0.487

0.438

52.6 10.1 51.3

89.9 2.8

47.4 45.9

0.925 0.487 0.900

processing lower in women than in men. In left-handers, the familial history of sinistrality is more relevant, with bilateral representation being more prevalent in FS + than in FS - cases. Further, it is only in lefthanders that the data force the presumption of bilateral representation. Although estimates of the incidence of left-hemispheric language representation are relatively common (e.g., Rasmussen & Milner, 1977; Satz, 1980; Segalowitz & Bryden, 1983)there seem to be no comparable estimates of the lateralization of spatial processes in one hemisphere or the other. While we recognize that spatial ability is not unitary (cf. McGee, 1979), and that these estimates are at best a first approximation, we do feel that they lead to some interesting insights into cerebral organization. PATTERNS OF CEREBRAL ORGANIZATION If we accept the assertion of the first section that language and visuospatial processes are statistically independent, then the estimates of lateralization given in Tables 3 and 4 can be used to estimate the frequency of different patterns of cerebral organization. Table 5 shows these estimates for right-handers, segregated by sex, since sex was seen to influence the pattern of lateralization for both language and visuospatial abilities. This table shows the prevalence of what we have termed statistical complementarity. Simply because the majority of right-handers are left-hemispheric for language and righthemispheric for visuospatial processes, this pattern is manifested by

PATTERNS

OF CEREBRAL

259

ORGANIZATION

TABLE 5 PATTERNS OF CEREBRAL ORGANIZATION IN RIGHT-HANDERS Visuospatial

Ability Women

Men Language (hemisphere) Left Right

Left

Right

22.4 1.4 23.8

71.8 4.4 76.2

94.2 5.8

Left

Right

32.3 9.0 41.3

46.0 12.7 58.7

78.3 21.7

about 72% of men and 46% of women. Furthermore, since reversed complementarity can also exist, about 73% of men and 55% of women show complementary specialization. The remainder show noncomplementary specialization, and are primarily reliant on a single hemisphere for both language and visuospatial functions. Speculatively, one might argue that if there is to be found any relation between cognitive ability and cerebral organization, it is more likely to lie in the distinction between complementary and noncomplementary specializaton than in whether language or visuospatial abilities are predominately left or right hemispheric. For example, men are generally considered to be better at visuospatial tasks than are women (Maccoby & Jacklin, 1974). Since complementarity is also more evident in men than in women, we might speculate that noncomplementarity leads to poor performance on spatial tasks (cf. Levy, 1972). By this view, sex differences in spatial ability are not so much a matter of sex per se as a function of the specific pattern of cerebral organization with which one is endowed. A similar table can be constructed for left-handers, although it becomes more complex because of the incidence of bilateral representation. Furthermore, familial sinistrality seems to be more important than sex in determining the pattern of organization of left-handers, and we have used this variable in constructing Table 6. Because of the incidence of bilateral representation, it is rather more difficult to make generalizations about this table. However, more FS - subjects (42%) than FS + subjects (29%) exhibit full complementarity. At the same time, however, more FS subjects show clear noncomplementarity (45%) than FS + subjects (28%), largely because so many FS + (43%) individuals exhibit a modified complementarity in which one function is lateralized and the other bilaterally represented. By this table, FS - left-handers of either sex should be rather like right-handed women, especially with respect to language functions (cf. Hecaen & Sauguet, 1971; Zurif & Bryden, 1969), while FS+ left-handers are indeed highly unpredictable. Furthermore, one might

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TABLE 6 PATTERNS OF CEREBRAL ORGANIZATION IN LEFT-HANDERS~

Spatial Ability FS+ Language (hemisphere) Left Bil. Right Total

FS-

Bil.

Right

Total

Left

Bil.

Right

Total

9.7 7.2 3.7

11.9 8.8 4.5

25.5 19.0 9.7

47.1 35.0 17.9

38.8 5.6 6.8

2.1 0.3 0.4

34.7 5.1 6.1

75.6 11.0 13.3

20.7

25.2

54.2

51.3

2.8

45.9

Left

r?Using “optimal”

values from Tables 3 and 4.

expect left-handers, especially those who are FS - , to be poor at spatial tasks if complete noncomplementarity leads to poor spatial ability. At present, few experiments provide data appropriate to test the hypotheses of a relation between complementarity and spatial ability. McGlone and Davidson (1973) found that subjects with complementary specialization were slightly, but not significantly, better than those with noncomplementary specialization on a measure of spatial relations. In this study, dichotic listening and lateralized dot enumeration were used to assess hemispheric specialization for verbal and nonverbal processes. Using somewhat different measures of lateral specialization, Bryden (1978) found a small but significant effect in the same direction. While these data do not constitute strong support for the notion that complementary and noncomplementary individuals differ in spatial ability, they do provide enough encouragement to suggest that further research along these lines is warranted. CONCLUSIONS An analysis of 270 patients with unilateral brain damage suggests that aphasia and spatial disorders are statistically independent. Although a great many simplifying assumptions regarding population sampling, size and location of lesions, and type of aphasic or spatial disturbance have been made in carrying out this analysis, the results suggest that complementary specialization is statistical rather than causal in nature. The data from these patients have also been used to estimate the incidence of right-hemisphere, left-hemisphere, and bilateral representation of language and spatial functions. For right-handers, the results suggest that women are more likely than men to show right-hemispheric language and left-hemispheric visuospatial functions. In left-handers, cerebral organization seems to be more affected by the familial history of sinistrality than by sex, with bilateral representation of language and spatial ability

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261

both more common in those with a familial history of left-handedness. The results for right-handed males are in close agreement with those derived from other approaches to the study of cerebral organization (e.g., Rasmussen & Milner, 1977; Segalowitz & Bryden, 1983). These findings suggest that both clinical and experimental investigators should pay more heed to the relations between aphasia and visuospatial or other nonverbal functions. It is inappropriate to assume that one hemisphere is specialized for nonlinguistic functions simply because the other is involved in linguistic processes. Rather, it seems that the lateralization of verbal and nonverbal processes is determined by different factors, and therefore presumably have different origins. REFERENCES Bradshaw, J. L., & Nettleton, N. C. 1981. The nature of hemispheric specialization in man. The Behavioral and Bruin Sciences, 4, 51-91. Bryden, M. P. 1973. Perceptual asymmetry in vision: Relation to handedness, eyedness, and speech lateralization. Cortex, 9, 418-435. Bryden, M. P. 1978. Strategy effects in the assessment of hemispheric asymmetry. In G. Underwood (Ed.), Strategies of information processing. London: Academic Press. Pp. 117-149. Corballis, M. C. 1981. Toward an evolutionary perspective on hemispheric specialization. The Behavioral

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