In Part II, we discussed a number of clinically derived theories of how language is represented and processed in the brain. In Part III, we presented recent studies of the linguistic and psycholinguistic aspects of aphasic disturbances. In the final part of this book, we shall consider a number of recent studies of the neural basis for language. Chapters 20 and 21 review studies based upon new techniques – recording electrical activity in the brain, and stimulating cortical and subcortical areas during neurosurgical operations. In Chapter 23, we shall consider theoretical approaches to modeling neural activity which can be related to language and language breakdown. In this chapter and the next, we shall take up two old themes – lateralization, and localization of language functions – in the light of recent studies.

One of the major neurobiological discoveries of the nineteenth century was that language functions were primarily carried out in one hemisphere of the brain. This feature, known as lateralization of language functions, was first brought to widespread scientific attention by Broca in 1865 (see Chapter 3). Broca recognized that the fact that eight consecutive aphasic patients had lesions in the left hemisphere was unlikely to have occurred by chance, and he therefore hypothesized that the left hemisphere was dominant for language. Broca also recognized that the left hemisphere was responsible for right-handedness, and he postulated that left-hemisphere dominance for language and for manual preference were linked. The connection between the two, according to Broca, was due to the fact that the convolutions in the left hemisphere developed earlier than in the right, a finding which he attributed to Gratiolet.