The physical substrate responsible for the representation and processing of human language is the human brain; more specifically, certain portions of telencephalic cortex and possibly some subcortical structures. Though the cerebral hemisphere which is responsible for language varies in different subgroups of the adult human population, the area responsible for language is thought to be relatively circumscribed within the relevant hemisphere in the normal adult human population. The existence of a restricted neocortical area responsible for language is known as the localization of language functions; the location of this area within one telencephalic hemisphere is known as the lateralization of language functions. Many authorities believe that localization of function extends to subcomponents of a language processing mechanism, and a variety of theories of the further extent of localization of language functions has been proposed. Similarly, many workers believe that aspects of the language system can be differentially lateralized. This chapter will present a review of the concepts of localization and lateralization of language functions, focussing upon the identification of the ‘language areas’ of the brain, their internal organization with respect to language functions, and the genetic and environmental determinants of their location and internal organization.

Both clinical and experimental studies of aphasia indicate that the neo-cortex surrounding the Sylvian fissure is responsible for many language functions. Clinical evidence to this effect consists of large-scale studies of pathological and radiological correlates of aphasias, which show that permanent aphasias arise after lesions in this area (Luria 1947; Russell & Espir 1961).