Relevant Sources Of Evidence

In the remaining sections, actual clinical and experimental evidence bearing on brain organization of language in bilinguals is reviewed and evaluated. The evidence is examined with respect to the notion of whether bilinguals show differential intrahemispheric localization and/or differential interhemispheric organization of language, either in one of their languages relative to the other or in either language relative to monolingual users or other bilingual users.

There exist many potentially informative sources of evidence. For example, there are four studies of cortical electrical stimulation in a total of 11 polyglot patients, five studies representing a total of 9 bilingual epileptic patients who were administered sodium amytal, two studies ofelectroconvulsive shock therapy administered to a single bilingual psychiatric patient, and one study of the effects of anesthesia on language recovery in a bilingual patient. While clearly relevant to the present discussion, inferences drawn from these largely clinical studies to language organization in brain-intact bilinguals may be problematic given the very likely possibility that early damage to the brain in many of these patients may have led to functional reorganization of language.

We focus instead on four sources of evidence on which there is much more information. Two of these involve clinical populations and two involve normative samples. The sources to be reviewed include (i) clinical reports of language recovery patterns in polyglot aphasia, (ii) studies of the incidence of crossed aphasia in bilinguals versus monolinguals, (iii) studies of cerebral lateralization of language in brain-intact bilinguals, and (iv) studies of functional neuroimaging in brain-intact bilinguals. For each source, criteria that ideal studies of each type would need to meet to provide a better test of the question are outlined. The extent to which each of these four sources provides an adequate test of the question of differential intra- and/ or interhemispheric involvement of language in bilin-guals is then addressed.

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