[Biological determinants of cerebral dominance]

Abstract

This review summarizes the present evidence for a biological basis of functional brain asymmetry. Morphological asymmetries, despite long-standing knowledge, have only recently aroused general interest. The most striking asymmetries involve regions of the cerebral cortex located around the posterior end of the Sylvian fissures: mainly studied was the planum temporale, but others include the parietal operculum, inferior parietal lobule, and inferior frontal gyrus. In most instances, cortical areas proved to bear asymmetries favouring the left hemisphere. Architectonic studies also revealed asymmetrical features; especially area "Tpt", roughly similar to Wernicke's posterior language area, has been found up to 7 times larger on the left than on the right hemisphere. Similar asymmetries were discovered in fetal brains, as early as the 30th gestational week, as well as in the cerebral cortex of some apes. These data suggest that morphological asymmetries need not be the consequence of functional effects but rather a predetermined feature, probably widely spread throughout animal kingdom. An open question remains as to the functional significance of these asymmetries. In this regard, studies have dealt with possible correlations between morphological asymmetries (as assessed in vivo by cerebral neuro-imaging methods) and features of functional asymmetry, especially handedness and hemispheric dominance for language. Despite incomplete results, available data suggest a significant correlation, at least in dextrals. The exact nature of these relationships remains speculative. Knowledge about the contribution of genetic factors in determining cerebral dominance followed observations of familial clustering of functional asymmetries, especially sinistrality, as well as studies in mono- and dizygotic twins. However, a purely genetic model seems to be unable to account consistently for the data. Hypotheses emphasizing birth stress may be only exceptionally verified. Actually, current evidence points to a specific period in the fetal life, probably around the 6th gestational month, during which neurons in post-migrational stage set up their synaptic contacts. The final gyral pattern of the brain probably builds up during this period, as a consequence of a mechanism of competition for synapses among different cortical areas which regulates the amount of neuronal growth and death. Such a mechanism probably also accounts for the development of cortical asymmetries.(ABSTRACT TRUNCATED AT 400 WORDS)