Individual Variation In Laterality

Thus far, I have emphasized contemporary human laterality patterns that could be considered prototypical, as if we were all more or less identical. Although there is sufficient homogeneity to justify consideration of the prototype, there is also sufficiently reliable heterogeneity to warrant consideration of individual variation. Of particular interest has been the possible relationship of functional hemispheric asymmetry to a number of other between-subject factors, including handedness, sex, intellectual ability, and psychopa-thology.

Handedness is of particular interest because it is a behavioral manifestation of brain laterality for certain manual activities and it is also related to other, more cognitive aspects of laterality. Hand dominance is determined by a variety of genetic and environmental factors, both before and after birth. The direction and magnitude of hand dominance may even be determined by different factors, with the magnitude being more heritable and with the direction being more subject to environmental influence. Environmental influences include pre- and postnatal trauma, prenatal levels of testosterone and other hormones (higher prenatal levels of testosterone are associated with greater incidence of left-handedness), asymmetric positioning of the fetus in utero, as well as the biases of the postnatal world. As noted previously handed-ness is at least moderately related to other forms of laterality. On average, laterality for a group of left-handed individuals is in the same direction as that for a group of right-handed individuals, but the magnitude of the asymmetry is smaller. This group difference does not necessarily occur because individual left-handed people are uniformly less lateralized than individual right-handed people since more left-handers than right-handers show a laterality effect in a direction opposite that which is considered prototypical. Rather, there is a greater proportion of left-handed individuals who show a laterality effect in a direction opposite that which is considered prototypical. It is noteworthy that even though the relationship between handedness and other forms of laterality is only of weak to moderate strength, it has been demonstrated by all the various techniques and tools that are used to study laterality. This suggests that if the relationship of laterality to other variables such as biological sex were as strong as the relationship of laterality to handed-ness, it would be relatively straightforward for these techniques to uncover.

There are many effects of fetal hormone levels on brain development in other species, and in humans there are clear relationships between biological sex and cognitive ability. For example, women tend to out-score men on tests of verbal fluency, and men tend to outscore women on tests of spatial ability. Thus, it is plausible that there are sex-related differences in at least some aspects of brain laterality. This possibility receives modest support from the fact that the incidence of left-handedness is slightly higher in men than in women, consistent with the hypothesis that higher levels of fetal testosterone promote development of the right hemisphere relative to the left hemisphere. When handedness is controlled, however, evidence for additional sex-related changes in other forms of laterality is equivocal. The most encouraging indications of such relationships have come from the observation of deficits in patients with unilateral brain damage, with some studies finding evidence of greater functional hemispheric asymmetry in men than in women. However, not all studies have shown such sex differences and there may be alternative explanations for some of them. For example, it has been suggested that there is a lower rate of aphasia after left hemisphere damage in women than in men because of sex-related differences in the organization of language within the left hemisphere rather than because of differential lateralization per se. Behavioral laterality studies using neurologically intact men and women provide, at best, weak support for the hypothesis of greater functional laterality in men because the results have been quite variable. Also, even when sex-related differences are found, there are sometimes alternative explanations in terms of the tendency for men and women to prefer different cognitive strategies that may bias performance toward different hemispheres. Perhaps most clear is that if the laterality differences between men and women were as large as the laterality differences between right-handed and left-handed individuals, they would be well-known by now.

There are indications that individual variations in brain laterality may be related to individual differences in cognitive ability. For example, relative performance on tests of verbal and visuospatial ability are related to handedness, but the relationship appears complex and is moderated by sex and by overall reasoning ability. There is also evidence that extreme intellectual precocity, especially for mathematical reasoning, is related to advanced development of the right hemisphere relative to the left, perhaps as a result of increased testosterone levels during fetal development. It also appears that some forms of dyslexia (impaired acquisition of reading) are related to subtle abnormalities within the language areas of the left hemisphere, perhaps leading to an overreliance on the less efficient mechanisms of the right hemisphere. Considerable additional work is needed, however, to substantiate these relationships and to discover the precise mechanisms that might account for them.

The existence of hemispheric asymmetry for emotion has led to consideration of the possible relationship of laterality to psychopathology. Among the more promising hypotheses is the idea that schizophrenia is related to dysfunction of an anterior region within the left hemisphere, an area that is believed to be important for language and for controlling parietal areas involved in attention. It has also been hypothesized that the corpus callosum, and therefore inter-hemispheric interaction, is dysfunctional in schizophrenics, but these hypotheses remain controversial and may apply to only a subset of schizophrenics. In a complementary way, depression has been linked to disturbances of the right hemisphere, but more work is needed to confirm this link and to understand its functional significance.

Popularized accounts of laterality have suggested that people can be classified as "right-brained" or "left-brained" depending on whether they prefer to use strategies and modes of cognition associated with one hemisphere or the other. Thus, left-brained people are said to be rational and analytic, whereas right-brained people are said to be intuitive, artistic, and creative. A few well-rounded individuals might even be lucky enough to be identified as "whole-brain thinkers." Various paper-and-pencil schemes have been proposed to achieve this sort of classification or measure of "hemisphericity," and exercises have been proposed to help more of us utilize whichever side of the brain we tend to neglect. To be sure, there is individual variation on the various dimensions of laterality, but there is no evidence that any neurologically intact individuals are functionally half-brained in the manner referred to as hemisphericity. Individuals do differ reliably in cognitive style, personality, creativity, and so forth. At the same time, however, we have seen that hemispheric asymmetries are subtle, with no indication that aspects such as rationality and creativity are the exclusive product of one brain hemisphere. Instead, both hemispheres contribute to virtually everything we do.

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