The acquired aphasias as covered here refer to language disorders that arise as a consequence of focal brain damage in the adult. The clinical observations of such disorders have yielded two broad generalizations: (1) Left hemisphere lesions, but not right hemisphere lesions, disrupt language capacity, and (2) within the left hemisphere, lesions in different locations undermine language differently. These generalizations are not in serious dispute; what to make of them—their significance for uncovering the neurological foundations of language—is. It is assumed, of course, that aphasic syndromes are real entities: natural categories imposed by focal brain damage and not solely the unstable reflection of the simultaneous variation of such characteristics as age of patient and emotional reaction to disease. And it is also assumed that the selective manner in which language breaks down bears some relation to its normal organization in the brain. Even so, it is still far from clear how precisely to describe these neurologically produced lines of cleavage. Indeed, the history of aphasia research can best be regarded as a series of attempts to specify these lines, to provide, in short, a neurologically defensible account of the components of the language mechanism.
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