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Dissociating language and word meaning in the bilingual brain

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How do bilingual (or multilingual) persons keep different languages apart and switch between them as needs arise? Crinion et al. have used an ingenious method to dissociate brain regions sensitive to word meaning from those sensitive to the combination of meaning and language. This work should stimulate further research examining the role of subcortical areas in language processing and in context-appropriate language production.

Introduction

The organization and functioning of the bilingual (or multilingual) brain continues to fascinate neuroscientists and lay persons, on account of the seeming ease with which bilingual persons can communicate in different languages with relatively little confusion. To date, most functional imaging studies on bilingualism have evaluated how different languages are represented in the brain [1]. This question has been probed using different languages and modalities, and at both the single-word and sentence levels. Much less work has addressed the question of how a bilingual can keep languages apart during reading and language production [2]. Crinion and colleagues [3] have filled this lacuna with a study that demonstrated language-dependent neuronal responses while single words were processed for meaning.

Section snippets

The adaptation paradigm

Adaptation refers to the observation that a pair of identical stimuli elicits a smaller neuronal response than two dissimilar or different stimuli. This underlies the alternative name for this technique, ‘repetition suppression’ [4]. By manipulating the stimuli along a dimension of interest, researchers can determine whether neurons that respond to a stimulus are sensitive to the probed dimension. Adaptation can be used to show that clusters of neurons lying within the same brain region exhibit

Differences in sensitivity to change in language and the role of the caudate in language processing

The most interesting feature of the work by Crinion et al. was the finding that language-dependent semantic priming selectively involved the left caudate (Figure 2). The authors cited clinical neuropsychological and electrical stimulation studies to support the notion that the caudate might function in the control of language production.

Subcortical structures are infrequently featured in discussions of language processing. A large clinical series of acute caudate vascular-lesion studies

Control of language production in bilinguals

Although a prior study showed bilateral caudate engagement when volunteers read words while the production language was switched [13], Crinion et al. have provided the first imaging study to suggest that the left caudate might be involved in determining the language used during production. Historically, abnormal switching of languages is thought to involve damage to the left supramarginal gyrus, an observation supported by at least one functional imaging study [13]. The lateral prefrontal [14],

Differences in sensitivity to change in word meaning

The temporal lobe is part of a network of brain regions involved in processing word meaning, irrespective of modality or language [17]. Extending this work, Crinion et al. found an anterior temporal region to be sensitive to word meaning without being sensitive to the language used. Semantic priming occurred whether or not the prime and target were in the subjects’ first language. Compared with two bilingual studies that used the adaptation paradigm 5, 18, the brain region that showed

Concluding remarks

The study by Crinion et al. highlights the value of the adaptation paradigm in language research and sets a high standard for experimental control. It should stimulate further research into the role of subcortical structures in language processing and emphasize the importance of understanding the neural basis of language selection in bilinguals. I suspect that the left caudate is likely to interact with the prefrontal cortex in this regard. Additional work needs to be carried out to evaluate

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