A variety of neurologic observations can be interpreted on an hypothesis of two distinct modes of thought, appositional and propositional, which are largely lateralized to right and left hemispheres respectively (1, 2, 3). This A-P hypothesis has sociologic implications whose discussion is the main purpose of this paper. We also include some data illustrating a confluence of neurologic and sociologic observations.

The A-P hypothesis is based upon recognition of a right hemisphere superiority for certain performances. In addition to evidence previously reviewed (1, 2), particularly strong support for a special right hemisphere capacity has come from further tests of patients with cerebral commissurotomy (4, 5, 6, 7). Closer characterization of "appositional" and "propositional" thinking are still in progress; on the basis of cerebral commissurotomy data, the hemispheric specializations have been described as "gestalt-synthetic'' for the right hemisphere and "logical-analytic" for the left (8).

Two distinct modes of thought or cognitive styles have been postulated by anthropologists, physicists, and chemists as well as many psychologists (see Bogen (2) for references, to which others have since been added (9-20)). The A-P hypothesis asserts that the most useful dichotomy is to be made in terms of hemispheric specialization." Assuming two kinds of cognition implies that one will be more effective than the other in dealing with certain problems. It follows from the A-P hypothesis that, in an individual with customary lateralization, each hemisphere will contribute a varying proportion depending upon the situation or problem presented to the individual. Evidence for differential hemispheric participation in different circumstances, when the commissures are intact, has recently been obtained by EEG methods (21-29), by discriminative reaction times (29a), and by the observation of initial conjugate lateral eye movements (30-31b), first described by Day (30a).

This leads us to the concept of individual "hemisphericity", i.e. a tendency for a person to rely more on one hemisphere than the other. If such "hemisphericity" exists, it may well reflect influences of early cultural exposure. In a culture where most individuals are exposed to intensive education of the left hemisphere potential for reading, writing, grammar, etc. we could reasonably expect a tendency for the propositional mode to dominate, even when dealing with problems for which it is less appropriate. Conversely, persons raised in a non-literate culture emphasizing different training, in spatial skills for example, should exhibit a reverse tendency.

For a particular population, although the individuals will vary widely, we expect some central tendency in the relative reliance placed upon one or the other mode, a central tendency differing from that of another population. Any culture might then be characterized in terms of its ratio of appositionality to propositionality, that is, an A/P ratio (32).

Finding a culture without educational stimuli for either one or the other hemisphere is probably impossible. All men learn, in Hughlings Jackson's phrase, to propositionize; all men learn likewise to appositionize, whether they are formally trained or not. However, in a complex society comprised of subgroups with different origins and educational exposures, the A/P ratio could differ among the subgroups. One might suppose that in the U. S., for example, Blacks or Hopi Indians would differ from middle-class Whites in having less success in problems making greater demands upon the left hemisphere than the right. More specifically, these two minority groups can be expected to do relatively less well on verbal tests (32a, 32b), whereas there are some reasons to expect that they would be better on a test emphasizing visuospatial recognition. These reasons include: function in one mode may be inhibitory of the other mode (9-11); a genetic predilection for one or the other basic process has been postulated (12) ; and we have the impression that subdominant groups in a technological society are provided less access to propositionising and consequently must rely more often upon the alternative appositional strategy.

Whatever the reason--nature or nurture or interaction--this expectation has been confirmed in a recent demographic survey (33, 34) in which both Blacks and Hopis scored lower on the Similarities subtest of the WAIS, and higher on the Street figure-completion test.

The Similarities test requires the subject to give a verbal characterization of the similarity between two verbally designated objects. The subject is asked in what way an orange and a banana are alike, or a poem and a statue, or praise and punishment, etc.

The Street test (35) involves the presentation of silhouettes which have been partially obliterated to make their recognition difficult. Examples of the Street test are shown in Figure 1. The answers to the Street test are verbal (the name of the object pictured) so that whatever the cerebral locus of solution, the answer must issue from the left hemisphere. It is readily apparent, however, that verbal-logical skills are crucial to the Similarities test, but of relatively little help in the Street test.*

It is a reasonable supposition that there will be cultural preferences for one mode over the other, once a decision has been made to distill the available array of neurologic facts into the hypothesis of two modes of thought, hemispherically lateralized. Substantial empirical validation of this extrapolation is not yet available; but we can provide some supportive data in illustration of our theoretical suggestions. Before presenting these we note that the evidence must appear in two steps: First, there must be data to show that an ethnically defined population does better on one type of test than another type of test, whereas a different population does better on the second type of test than the first. The next step is to show that each type of test is assignable to one or the other hemisphere.

In this preliminary paper, we represent each mode of thought by a single test. This simplistic approach is reminiscent of the discredited assumption of a single "intelligence" by which a brain can be measured. However, the hypothesis of two complementary modes of thought, appositional and propositional, carries with it the implication that each mode of thought (or type of "intelligence") can, at least in theory, be measured by a single test. Indeed, this could be considered an operational trial of the A-P hypothesis; that is, to the extent that such a procedure (a test for each mode) provides new data or sheds new light on old data, there would then be pragmatic justification for the hypothesis. From the factor analytic point of view (9, 36, 37), our theory means that we assume three principal factors: a, p, and b, the last subsuming those influences which are symmetrical for the two hemispheres. What we require, therefore, are two tests with comparable q loading hut with a minimal load of p and a respectively.

To represent propositionality, we require a test which is highly verbal in both presentation and response, and which is scored so as to reward a tendency to abstraction. These characteristics are possessed by the Similarities test, including its scoring method which rates more highly, for example, the answer "both are fruit," than the answer "both have peeling" (for orange-banana) and rates more highly "works of art" than the answer "made by people" (for poemstatue). In view of the well-established dominance of the left hemisphere for language, it seems reasonable to associate the Similarities test with the left hemisphere. But we also require empirical data.

The correlation of brain lesion site with the results of psychological testing involves many uncertainties and ambiguities (38). However, most of the available evidence (39-42) indicates that the Similarities test is preferentially impaired by left rather than right hemisphere lesions. This includes the observation (43-46) that right hemispherectomy for tumor in the adult produces marked impairment on many other tests but not Similarities, leading Smith (46) to the conclusion that, " ... the capacities involved in verbal abstract reasoning were not noticeably impaired" by right hemispherectomy."

In addition to the ablative data, lateral specialization can be demonstrated by testing the two hemispheres separately following their disconnection. In this circumstance the right hemisphere may understand some simple language (47-53).But its capacity for verbal expression is minimal and quite inadequate for responding to the Similarities test. Any achievement on this test following cerebral commissurotomy can therefore be confidently assigned to the left hemisphere. To represent appositionality, we require a test as non-verbal as possible in both presentation and response, and which puts a premium on the right hemisphere's ability to infer a spatial whole from several parts (4).These character istics are possessed by the Street test whose only verbal instruction is, "what is that?". This is not only simple but has in actual practice required no further explanation or elaboration. As structured today, the Street test requires a verbal answer, but this is only a single word.*

Table I: Means and standard deviations for various samples in the population survey on the Similarities and Street tests. The data are similar to those previously reported (33, 34) differing mainly in lower N because, for this paper, all subjects have been eliminated who gave any evidence of left-handedness.
Also shown (last column) are St/Sim ratios for various groups. These were computed, not by using the means ill the preceding columns, but by computing the St/Sim ratio for each of 1220 individuals and then computing the means and standard deviations shown here. These St/Sim values are used in this paper as surrogates for the hypothesized A/P ratios. It is apparent that the numerical values shown in the tables and in Figure 2 result from an arbitrary choice of scoring systems. With the introduction of appropriate constants, any one of these could be set to 1.0 as a reference standard. This would seem to involve a judgment of cultural values for which we see as yet no objective basis. The problem is related to the equally (as yet) unsettled and pressing questions of what constitutes for it given individual a desirable balance [A/P ~ 1.0 or (A -- P/A + P) ~ zero], and whether homogeneity within a society is indeed desirable.

The effect on Street performance of lateralized brain lesions has been published, so far as we are aware, only by DeRenxi and Spinnler (54). They found that right hemisphere damage impaired performance more than damage to the left hemisphere (p < .02, x2 = 5.27).

As for the brain-bisected patient, the present form of the Street test prevents its presentation to the two hemispheres separately. However, psycho metric performance in the absence of the corpus callosum is characterized by impairment of those capacities proper to the right hemisphere (55, 56).t Therefore, if the Street test were given to commissurotomy patients, we would expect a very low score, particularly in the patients operated at an older age when cerebral lateralization was more advanced.

It is important that the Similarities test be given simultaneously with the Street, so that both will be equally affected by any overall or generalized intellectual impairment. Having these two scores, we can then use the ratio (St/Sim) to represent the left hemisphere's ability to utilize its proper mode (propositionality) as compared with its utilization of any appositionality still residual within the left hemisphere.

The Street and Similarities tests were given (by.T.E.B.) to ten right-handed cerebral commisurotomy patients (of Professor P. J. Vogel), most of whose histories have been previously summarized (1, 6). The tests were administered in the same form used in the sampling (33, 34) of the ethnic populations (under the direction of W.D.T. and J.F.M.) and they were graded by the same individual R.n.) The tests were also administered to two patients having partial section (with preservation of the splenium). Exhaustive testing has indicated that in these two patients right hemisphere processing is available for left hemisphere speech (57). It was expected, therefore, that these two patients would not show a depression of Street versus Similarities. That is, they could reasonably be expected to have essentially normal St/Sim ratios.

The results are shown in table 2 and in figure 2. They are much as predicted, and thus provide support for the supposition that attainment on the Street test ordinarily draws upon the ability of the right hemisphere.

For the patients with splenium intact (D.NI. & N.F.) the Street scores and hence the St/Sim ratios are relatively high, reflecting access to the right hemisphere. In the three young male patients with complete section (A.A., C.C. and L.B.) the ratios average lower than normal but are still higher than in those males operated as adults; this presumably indicates a lesser degree of lateralization sit the time of operation, or alternatively, a greater capacity for compensation through left hemisphere reacquisition of appositionality. The older male patients all have markedly depressed A/P ratios, as compared with the normals in the demographic survey (Figure 2.)

The difference between the scores of the female and the male adult patients is significant (p < .01. See Table 3). This can be explained in the same way as the data for the younger males, that appositionality was less lateralized before operation. It is a common impression that the verbalization of women, in our present society at least, is less inclined to abstraction and more accessible to intuition. It could be supposed that this is attributable to a more easy transcommissural access, or it could alternatively be considered the result of a lesser degree of lateralization (with a greater residual appositionality in the left hemisphere).The present data offer support for the latter explanation, based on less lateralization (54b,c). However, on the present data alone, we are possessed of an explanation for a non-existent fact, since a sex difference in A/P ratio received very little support from the population survey (Tables I & 3).

Commissurotomy patients' scores on Street (St) and Similarities (Sim). These were combined to obtain the St/Sim ratio in the final column. Sim was graded 1, 2, 3 as ill the population survey. The column. labeled Sim1 shows the same tests graded ill the usual Wechsler manner 0, 1, 2. Although we have included L.B. whose high Street score has diluted our results, it should be noted that systematic examinations of the commissurotomy patients have consistently suggested that he possesses some form of interhemispheric communication not present in the other patients (1, 4, 0).

Fig. 2. Comparison of A/P ratios for ethnic groups (means and standard deviations from the last column of Table 1) and commissurotomy patients (individual scores from Table 2).

TABLE III

	1 RH	2 RW	3a UBF	3b UBM	4a UWF	4b UWM	5 PS	6 CC	7 CCY	8 CCA	9 CCFA	10 CCMA
1. Rural Hopi (49)		<.001	0.02	0.004	<.001	<.001	<.001	<.001	<.001	<.001	<.001	<.001
2. Rural White (73)	3.68		0.02	NS	0.03	0.04	<.06	<.001	<.001	<.001	<.001	<.001
3a. Urban Black Fem (270)	2.32	2.29		NS	<.001	<.001	<.001	<.001	<.001	<.001	<.001	<.001
3b. Urban Black Male (224)	2.89	1.47	0.99		<.001	<.001	<.001	<.001	<.001	<.001	<.001	<.001
4a. Urban White Fem (227)	6.31	2.22	7.00	5.27		NS	NS	<.001	<.002	<.001	<.001	<.001
4b. Urban White Male (327)	6.15	2.05	6.67	4.99	0.03		NS	<.001	<.001	<.001	<.001	<.001
5. Partial Section (2)	5.03	1.88	4.09	3.36	0.47	0.60		<.05	NS	<.05	<.05	<.01
6. Complete Section Total (10)	9.57	6.82	9.99	9.05	6.51	6.63	2.54		NS	NS	NS	NS
7. Complete Section Young (3)	6.51	4.02	5.83	5.27	3.17	3.21	1.66	0.30		NS	NS	<.05
8. Complete Section Adult (7)	8.94	6.43	8.81	8.10	5.90	6.00	2.62	0.79	1.23		NS	NS
9. Complete Adult Fem (4)	9.40	6.39	10.67	9.30	6.34	6.48	2.93	0.28	0.13	1.40		<.01
10. Complete Adult Male (3)	13.71	11.32	11.19	14.83	12.18	12.77	6.28	1.56	3.14	1.54	4.86

Statistical comparisons of the A/P ratio of each group against each of the others. Below the diagonal are shown student's t values. Above the diagonal are shown the values for p, the two-tailed probability that the intergroup differences could have occurred by chance. (Computations by Roberta Runnestrand. Skewness measures as well as the original data are available on postpaid request).

The present results can be criticized on numerous grounds; but in almost every case, the sources of error would be such as to minimize rather than exaggerate our results. For example, the pre-operative seizures of the commissurotomy patients often interfered with their education (as did the surgery). This is particularly true of the younger patients. This influence would tend to depress scores on the Similarities and thereby raise the A/P ratio especially in the younger patients.

The Street test as originally standardized (35) includes a number of antiquated items (such as a biplane of the 1930s, for example). In spite of this, the younger commissurotomy patients scored higher than those who were operated on as adults. The Street test also contained some obviously culture-bound items (such as the sailboat shown in Figure 1). This would tend to lower the Street score for the Hopis, who scored highest in spite of this influence. Such peculiarities in the specific test items have attenuated rather than exaggerated the differences which we have elicited.

A defect in both tests as used by us is the small number of items (twelve in each test), such that failure on one or two items can significantly affect the final score. This is a serious difficulty and probably the principal methodological weakness in our research to date. Doubling (at least) the number of items is clearly required. It would be especially desirable to have a Street-type test permitting lateralized input and having a non-verbal readout so that the hemispheres of the commissurotornized patients can be tested separately. These modifications are in progress.

Another source of error is that since these patients were tested previously on several occasions with the Similarities but not the Street, their scores on Similarities might reflect some learning. This would exaggerate the differences we have found by decreasing the St/Sim ratio. However, a number of experienced psychometricians have informed us in personal communication that practice effects are minimal and rarely a serious source of confusion on the Similarities subtest.

The presence of extra-callosal brain damage in time operated group must necessarily affect the test results. Since both tests require a verbal reply (without multiple choice), both tests are measuring left hemisphere activity. In the case of the Street test, it is supposed that the left hemisphere is able to draw upon right hemisphere capacity in the unoperated respondents; but the left hemisphere cannot do this in the patients who have had cerebral disconnection. In the latter group, therefore, the state of the right hemisphere can have little influence on the results. And in these patients, tiny left hemisphere damage can be expected to preferentially depress Similarities. To the extent, therefore, that extra-callosal damage is present in the operated subjects, it would raise the A/P ratio and detract from our final conclusions rather than falsely supporting them.

The data are subject to a variety of important criticisms; and conclusions must, therefore, be considered tentative. However, the data indicate that hemispheric disconnection has preferentially lowered the capacity for Street performance. This strengthens the supposition that a normal person's Street performance draws upon the right hemisphere. This in turn is consistent with the notion that higher performance on Street by certain ethnic groups (such as the Hopi) may be attributed to relatively greater development of right hemisphere potential.

The ethnic differences possibly can be explained on a genetic rather than an environmental basis. Against this is the finding of a higher St/Sim ratio in a white sub-culture of rural farmers (Figure 2). In any case, the nature-nurture issue is peripheral to our main point. Either explanation is compatible with our suggestion that cultural differences can be interpreted in part as a result of asymmetry in hemispheric utilization.

Patients having complete cerebral commissurotomy do poorly on the Street figure-completion test, whereas they do relatively better on the Similarities subtest of the WAIS. This is interpreted as evidence that the Street test ordinarily requires right hemisphere processing. The data provide some support for the hypothesis that cultural sub-groups differing in their Street/Similarities ratios do so because of differences in the availability of appositional and propositional (right and left hemisphere) capacities.

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1. Ross-Loos Medical Group, Los Angeles 90017.

2. Department of Sociology, University of California, Los Angeles.

3. Department of Sociology, Fitter College, Claremont.

*For the purposes of this paper one can (as many do) admit hemispheric specialization without accepting that the two hemispheres can function independently. The combination of these two ideas (hemispheric specialization and hemispheric independence) can be called "cerebral complementarity" and requires separate consideration (35a).