Concepts of Neuroanatomical
Dominance and Localization in Language
As previously noted, Broca’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.
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