The neural substrates underlying word generation: a bilingual functional-imaging study.

Abstract

We used positron emission tomography to investigate word generation in subjects whose first language was English but who were also proficient in French. These subjects performed three types of lexical search: rhyme generation based on phonological cues, synonym generation requiring a semantic search, and translation involving access to a semantic representation in the other language. Two control tasks required word repetition in each language. We investigated whether phonological and semantic wordgeneration activate similar regions and whether the same neural substrates subserve the second language as subserve the first. A series of cerebral blood flow increases, corresponding to Brodmann's areas 47, 46, 45, and 8, were observed in the left frontal cortex when the baseline repetition task was subtracted from each of the respective generation tasks. The results suggest that common neural substrates are involved in within-and across-language searches and that the left inferior frontal region is activated irrespective of whether the search is guided by phonological or semantic cues.The representation of multiple languages in the brain of the polyglot has been the subject of debate. Evidence from electrical stimulation of the cerebral cortex in conscious patients, from aphasia studies, and from experimental investigation of normal bilingual subjects has not been able to determine whether there is a common cortical substrate within which all languages operate (1), whether multiple languages may be represented by different, yet overlapping, cerebral regions within the language-dominant hemisphere (2), or whether cerebral representation of a second language differs fundamentally from that of the first (3).The use of positron emission tomography (PET) to measure activity-related changes in regional cerebral blood flow (CBF) provides possibilities to resolve the question of bilingual language organization. In a recent report using this technique (4), we found that similar brain regions supported word repetition in the first language (Li) and in the second language (L2), so that comparison of one of these conditions to the other yielded only a single CBF change, in the vicinity of the left putamen. We interpreted this finding as reflecting the role of the left basal ganglia in the complex motor timing involved in speaking a language that has been acquired later in life. In the present study, we examined the question of linguistic representation in the brain by investigating areas activated by higher-level cognitive tasks than word repetition, such as those that require semantic searches both within and across Li and L2. Most PET studies requiring meaningful analysis of verbal material have shown a unilateral activation in the left inferior frontal gyrus (LIFG), and debate has centered on whether activation of this region reflects semantic processing (5-8). One critical question arising from this past research is whether the LIFG is specialized for accessing or using word meaning or whether this same region is als...

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The role of age of acquisition and language usage in early, high‐proficient bilinguals: An fMRI study during verbal fluency

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Functional Specialization for Semantic and Phonological Processing in the Left Inferior Prefrontal Cortex

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