Organization Of The Perisylvian Cortex For Language Processing

Two general classes of theories of the relationship of portions of the perisylvian association cortex to components of the language processing system have been developed—one based on "holist" or distributed views of neural function and one based on localiza-tionist principles. Although theories within each of these two major groupings vary, there are a number of features common to theories within each class.

The basic tenet of holist/distributed theories of the functional neuroanatomy for language is that linguistic representations are distributed widely and that language processing conmponents rely on broad areas of perisylvian association cortex. Karl Lashley identified two functional features of holist/distributed models that determine the effects of lesions on performance: equipotentiality (every portion of a particular brain region can carry out a specific function in every individual) and mass action (the larger the neuronal pool that carries out a particular function, the more efficiently that function is accomplished). The features of equipotentiality and mass action jointly entail that lesions of similar sizes anywhere in a specified brain region have equivalent effects on function, and that the magnitude of any functional deficit is directly proportional to the size of a lesion in this specified area. Recently, models of lesions in parallel distributed processing simulations of language and other cognitive functions have provided a mathematical basis for these properties of these systems.

All the traditional theories that postulate localization of components of the language processing system maintain the view that, discounting lateralization, the localization of components of the language processing system is invariant across the normal adult population. Thus, all the traditional localizationist theories have as a corollary that lesions in particular areas of the perisylvian association cortex interrupt the same language processing components in all individuals. Many localizationist theories also maintain that the specific localization of language processing components results from a computational advantage inherent in juxtaposing particular language processing components to each other or to cortex supporting arousal, sensory, and motor processes. Modern localizationist models relax these assertions, incorporating the ideas of individual variability and some degree of specialization wthin large-scale neural nets.

Because of the plethora of specific theories within each of these two general camps, it is impossible to critically review the empirical basis of all theories that have present-day adherents. I focus on the most widely cited theories as examples of each class.

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