Like the concept of cerebral dominance and specialization for language, the concept of localization of the language system and its components has undergone considerable development since it was first enunciated by Broca in 1861 with respect to motor speech functions. We have traced some of the development of this concept in Part II of this book. The first major innovation to Broca's formulation that we mentioned was that of Wernicke (1874), whose hypothesis that the function of auditory comprehension and the permanent representation of the sound patterns of words for language were located in the association cortex of the first temporal gyrus became the cornerstone of the connectionist theories of language processing in the brain. We traced the development of these connectionist models through the work of Lichtheim (1885) and other nineteenth-century neurologists, and into the twentieth century with the work of Geschwind (1965). In Chapter 9 we reviewed Luria's (1973) theories, which go yet further with respect to the localization of sub-components of a language-processing system. We also reviewed a number of objections to the connectionist theories in Chapters 6,7, and 8 of Part II.

Recent developments in neuroradiology have greatly increased our ability to study the exact location of lesions which cause particular types of aphasia, and therefore have allowed investigators to test the correlations between symptom complexes and lesion sites predicted by the classical connectionist theory. They also permit us to document new sites in the brain in which lesions can produce aphasias. This new work has led to new theories of localization of language functions. In addition, recent psycholinguistic analyses of deficits in patients following localized lesions have also suggested new theories of language localization.