Why The Bilingual Brain

For a long time, the topic of neurological substrates of bilingualism was simply not addressed in the neuro-psychological literature, or it was raised only in obscure outlets. As already noted, a comprehensive account of brain-behavior relations can no longer afford to ignore bilingualism, which represents a particularly prevalent and varied form of language experience. The inclusion of bilinguals broadens the scope of research questions, permitting a fuller examination of how language functioning is influenced both by biological parameters (such as the state of brain maturation at birth versus at puberty, with obvious implications for differences in the processing of languages acquired early versus late in life) and by cultural parameters (such as how a language is taught and in what context it is primarily used).

The earliest empirical research on the bilingual brain took the form of case reports of selective language loss or recovery following aphasia in speakers of two or more languages. This literature on polyglot aphasia, as it was called, sparked many questions and subsequent investigations of brain involvement in the language functioning of brain-injured and brain-intact bilin-guals using a diverse array of methods. Questions raised in the context of the polyglot aphasia literature (and subsequently tested using normative samples) include the following: What factors influence which language is recovered following brain injury in polyglots? To what extent is language impairment parallel or differential across languages? Does brain damage impair the ability to translate or code-switch between languages? Does it produce novel forms of language mixing? How are the different regions of the left hemisphere organized for language in bilinguals? What is the role of the right hemisphere in bilingual versus monolingual language functioning? How does the modality in which the language is learned or other factors in the context of language acquisition influence the pattern of brain involvement in language functioning?

Although there are now several hundred clinical reports ofaphasia in polyglots and bilinguals and more than 160 experimental studies on cerebral lateralization of language in brain-intact bilinguals, generalizations from this literature are fraught with difficulty. A major source of this difficulty is the fact that although theorizing about the bilingual brain has been shaped by prevailing conceptions about language, bilingualism, and the brain, each of these domains has undergone considerable changes in conceptualization. Thus, some of the earlier hypotheses examined in the bilingual neuropsychological literature are untenable because they rely on assumptions about the brain, language, or bilingualism that have not been supported. Similarly, some evidence acknowledged to be relevant to testing existing hypotheses has turned out to be inadequate, whether in the sense of being insufficient or unsystematic or in the sense of not providing a true test of the hypothesis under study. Moreover, some phenomena that have been interpreted predominantly in bilingual-specific, neuroana-tomical terms (e.g., translation or switching) may alternatively be interpreted in terms of more general cognitive attentional phenomena. Finally, it has become apparent that despite the size of the existing neuropsychological literature on bilingualism, many questions remain understudied.

In light of the complexity of the literature, this review uses a combination of a sociohistorical and a methodological approach, in which the aim is to situate the questions in terms of the prevailing zeitgeist and methodologies available at different points in the research enterprise.

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