The neural substrates of bilingual language production: How proficiency and age of acquisition affect neural segregation and lateralization.

The neural organization of language processing in bilinguals is a topic of considerable debate. Conflicting findings have been reported from neuropsychology, psycholinguistics, and neuroimaging. Some researchers have claimed that each of a bilingual's language capabilities are localized to distinct regions of cortex, while others have suggested that these neural representations are shared. Similarly, some have argued that L2 functioning is more bilaterally represented than L1, while others have argued that both languages are highly left lateralized. Such contradictory findings suggest that conclusions drawn from patients with neurological damage and behavioral lateralization experiments may be unreliable. The current studies used fMRI to investigate the neural organization of language production in four groups of bilinguals: high proficiency late bilinguals, high proficiency early bilinguals, low proficiency late bilinguals, and low proficiency early bilinguals. Subjects were scanned while engaging in a silent production task. Subjects were instructed to think about what they did at a particular time yesterday in one of their languages. All four groups displayed highly overlapping neural representations of L1 and L2. Three late bilinguals (two low proficiency, one high) appeared to show some evidence of segregation. In general, however, the activity produced by one language was a subset of that produced by the other language. In terms of lateralization, only highly proficient late bilinguals showed significantly decreased lateralization of L2, compared to L1. Low proficiency bilinguals showed significantly more left inferior frontal gyrus activity during L2, possibly indicative of inhibitory mechanisms. Late bilinguals showed a significantly larger increase in right hemisphere activity from L1 to L2, compared to early bilinguals.PhDBiological SciencesCognitive psychologyNeurosciencesPhysiological psychologyPsychologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123790/2/3106017.pd