Evaluating the Functional Asymmetry Evidence

1. The Validity Issue: When Can One Infer Hemispheric Asymmetry?

Performance asymmetries are intended to signal differences between the hemispheres in underlying specialization of function. Unfortunately, there is not always a direct or simple correspondence between behavioral asymmetries and hemispheric asymmetry: Many artifactual, nonhemispheric factors have been shown to affect the size and even the direction of behavioral asymmetries. In their initial enthusiasm with the availability of noninvasive methods to infer hemispheric lateralization, researchers neglected to search for and rule out noncerebrally based explanations, tending to ascribe any observed asymmetry to a neural source or to interpret any lack of behavioral asymmetry in terms of bilateral hemispheric involvement. The danger in this kind of reasoning is magnified when one is additionally claiming group differences in degree of lateralization. A standard criticism of many bilingual laterality studies has been that in the absence of objectively assessed, systematic criteria for subject selection, such as language and task proficiency, group differences could simply reflect floor or ceiling effects mistakenly attributed to bilateral hemispheric involvement.

2. The Conceptualization Issue: What Aspects of Language Are Lateralized in Each Hemisphere?

It is now known that language is not uniformly subserved by the LH. When applied to bilinguals, the question of differential lateralization of language becomes one of interpreting what a greater RH effect in bilinguals might mean. Does it mean that aspects of language normally subserved in the LH in monolin-guals are bilaterally mediated in bilinguals? Or does it mean that in both monolinguals and bilinguals language lateralization is the same; what differs is the relative reliance of the groups on those aspects of language thought to be under RH control. Although the latter view would appear to be more parsimonious, the research to date is still not at a point where these possibilities can be separated and tested. One can try and disentangle these possibilities by designing studies with processing components presumably mediated by one or the other hemisphere to determine if hemispheric differences will differ by task. In some work that attempted to do this, Vaid found that when a task calls for phonetic processing, a LH superiority was found in both languages of bilinguals, whether early or late bilinguals, as well as in monolinguals. Similarly, when the task called for syntactic processing, a LH superiority was again observed in all groups. On semantic tasks, group differences emerged, with monolinguals and late bilinguals showing a LH superiority and early bilinguals showing either no hemisphere differences or a RH superiority in both their languages.

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