HEMISPHERIC SPECIALIZATION AND INTERACTION IN HUMANS

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A research program is outlined dealing with hemispheric specialization at the interface of language and cognition. A convergent approach is proposed combining three experimental populations and three broad types of experimental paradigms. The subjects include (i) commissurotomy (split-brain) patients, (ii) hemisphere-damaged patients and, (iii) normal children and adults. The techniques include (i) continuous viewing of complex lateralized visual stimuli, (ii) metabolic, blood flow and magnetic real time indices of cerebral activation, (iii) more traditional paradigms of hemifield tachistoscopy, dichotic listening and dual task interference. Complementary models of hemispheric specialization are developed, motivated by the clinical populations and applied to the normal brain. One set of hemifield decision and facilitation experiments illustrates the "direct access" model; a second set of dichotic listening experiments illustrates the "callosal relay" model; and a third set studies types, development and modulation of interhemispheric interaction. A fourth set of experiments continues the analysis of hemispheric differences in lexical representation and organization, and compares hemispheric specialization for early, preattentive stages of information processing with specialization for later, conscious processes. The fifth set completes the comparison between hemispheric specialization in the three experimental populations for more complex linguistic and nonlinguistic abilities whose model theoretical status remains to be determined.