Introduction

Over the past century, studies of behavioral deficits produced by unilateral cerebral lesions and cerebral disconnections, as well as anatomical, behavioral, and more recent neurophysiological studies, all indicate that certain homologous areas of the cerebral hemispheres of humans are not simply duplicates of each other (as are a number of other homologous organs such as the kidneys) but differ both structurally and functionally.

The first scientific descriptions of hemispheric specialization focused on the asymmetrical cerebral representation of language. Reports by Broca (1865) and Wernicke (1874), based on different types of aphasic patients, indicated that the posterior part of the inferior frontal gyrus (Broca's area) and the posterior part of the superior temporal gyrus (Wernicke's area) of the left cerebral hemisphere play a major role in language production and comprehension, respectively. More recent evidence presented by Geschwind and Levitsky (1968), Teszner et al. (1972), Wada, Clark, and Hamm (1975), and Witelson and Pallie (1973) has shown that, indeed, Wernicke's area differs in its anatomical structure from the homologous area of the right hemisphere. Using injections of sodium amytal into the carotid artery, H. W. Gordon and Bogen (1974), Wada and Rasmussen (1960), and others have further substantiated the importance of the left hemisphere for language in most individuals.