Mechanisms of Brain Asymmetry: New Evidence and Hypotheses

Abstract

Human functional and anatomical asymmetries have been well documented for over 100 years (Galaburda et al., 1978b; Springer and Deutsch, 1985; Sperry, 1974). Thus, the left hemisphere of most individuals is specialized for language functions and the right hemisphere for spatial and nonlinguistic tasks. There are numerous examples of anatomical asymmetries in areas purported to underly language function. The length of the outer border of the planum temporale, an area on the posterior portion of the upper surface of the superior temporal gyrus (and thought to comprise a major portion of Wernicke’s area) was shown to be greater on the left in the majority of cases examined (Geschwind and Levitsky, 1968). Thus, out of 100 cases, 65% had larger left plana, 24% were symmetrical, and 11% had plana that were larger on the right. Similar findings of planum asymmetry were reported by others (Wada et al., 1975; Witelson and Pallie, 1973). Asymmetries in cytoarchitectonic area Tpt (which is located mostly within the planum temporale) are correlated strongly with planum temporale asymmetry and can be as much as 620% larger in the left hemisphere (Galaburda et al., 1978a). Cytoarchitectonic area 44 (part of Broca’s area) is also asymmetrical in favor of the left (Galaburda, 1980).