Concepts of Neuroanatomical Dominance and Localization in Language
As previously noted, Brocaâ€™s discovery of the connection be-tween aphasia and damage to the left hemisphere was the pre-cursor of all contemporary investigations of the structural and functional organization of the brain. Broca pointed out the sig-nificance of the left inferior frontal area, and Wernicke predicted the importance of the left temporoparietal region. In the pasttwo decades, functional mapping studies of language disorders have clearly established dominance of the left hemisphere for language functions through the use of radiological techniques. Studies of the relationship between handedness and dominance for language have suggested that in about 98 to 99% of right-handed individuals the left side is dominant for language. About 1 to 2% of right-handed individuals show right side dominance (Gling et al., 1969). This reversed pattern of dominance (i.e., right side dominance) is therefore considered to be exceptional and has been reported in a few studies (Fischer et al., 1991).
Although the issue of neuroanatomical dominance for language abilities in right-handed adults has been more or less settled for many years, the debate over localization has evolved into new di-rections. One classical approach to localization has been syndro-mic, with attempts made to find structural substrates for each of the subtypes of aphasia. This method involves lesion-syndrome correlations (Cappa and Vignolo, 1983).
An approach to localization that has yielded an extensive literature on neuroanatomical correlates of language focuses on specific linguistic functions and processes. This approach uses data from lesion-deficit correlational studies and functional neu-roimaging techniques have further made it possible to identify brain regions that are activated during the performance of lan-guage tasks even in normal individuals. In the past few years, investigations into the neural correlates of specific functions, such as naming, comprehension and reading, have led to the well-established conclusion that these functions are not unitary. Rather, they are complex language processes that involve various components of language, including phonology, lexicon, syntax and semantics. Thus, the new direction in localization research involves attempts to find the loci for specific language operations within the various component processes and, in turn, to map these components onto brain regions.
Evidence from various sources has clearly established that linguistic functions are multifaceted in terms of both neural mech-anisms and components of processing. As a result, impairment of a function does not involve global loss; instead, there are selective patterns of dissociation even between aspects of a specific linguis-tic function. This finding has given rise to the view that the vari-ous aspects of a function have corresponding neural mechanisms that are localized in different regions of the brain. The different regions are interconnected in ways that are thought of as form-ing neural networks. A breakdown in their connections results in selective patterns of dissociation. Because these networks can in-volve distant regions of the brain, the earlier ideas of localization of a function in terms of dichotomous subdivisions such as anterior versus posterior and sensory versus motor regions are now consid-ered untenable. In addition, the patterns of dissociation suggest differential breakdown of the interconnections between levels of processing. Some of the patterns of dissociation observed in func-tions such as word finding and comprehension are presented to elucidate the involvement of different regions.