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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We investigated the independent contributions of second language (L2) age of acquisition (AoA) and social diversity of language use on intrinsic brain organization using seed-based resting-state functional connectivity among highly proficient French-English bilinguals. There were two key findings. First, earlier L2 AoA related to greater interhemispheric functional connectivity between homologous frontal brain regions, and to decreased reliance on proactive executive control in an AX-Continuous Performance Task completed outside the scanner. Second, greater diversity in social language use in daily life related to greater connectivity between the anterior cingulate cortex and the putamen bilaterally, and to increased reliance on proactive control in the same task. These findings suggest that early vs. late L2 AoA links to a specialized neural framework for processing two languages that may engage a specific type of executive control (e.g., reactive control). In contrast, higher vs. lower degrees of diversity in social language use link to a broadly distributed set of brain networks implicated in proactive control and context monitoring.

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Of current interest is how variations in early language experience shape patterns of functional connectivity in the human brain. In the present study, we compared simultaneous (two languages from birth) and sequential (second language learned after age 5 years) bilinguals using a seed-based resting-state MRI approach. We focused on the inferior frontal gyrus (IFG) as our ROI, as recent studies have demonstrated both neurofunctional and neurostructural changes related to age of second language acquisition in bilinguals in this cortical area. Stronger functional connectivity was observed for simultaneous bilinguals between the left and right IFG, as well as between the inferior frontal gyrus and brain areas involved in language control, including the dorsolateral prefrontal cortex, inferior parietal lobule, and cerebellum. Functional connectivity between the left IFG and the right IFG and right inferior parietal lobule was also significantly correlated with age of acquisition for sequential bilinguals; the earlier the second language was acquired, the stronger was the functional connectivity. In addition, greater functional connectivity between homologous regions of the inferior frontal gyrus was associated with reduced neural activation in the left IFG during speech production. The increased connectivity at rest and reduced neural activation during task performance suggests enhanced neural efficiency in this important brain area involved in both speech production and domain-general cognitive processing. Together, our findings highlight how the brain's intrinsic functional patterns are influenced by the developmental timeline in which second language acquisition occurs.

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