Neural circuitry of the bilingual mental lexicon: effect of age of second language acquisition.

Numerous studies have proposed that changes of the human language faculty caused by neural maturation can explain the substantial differences in ultimate attainment of grammatical competences between first language (L1) acquirers and second language (L2) learners. However, little evidence on the effect of neural maturation on the attainment of lexical knowledge in L2 is available. The present functional magnetic resonance study addresses this question via a cross-linguistic neural adaptation paradigm. Age of acquisition (AoA) of L2 was systematically manipulated. Concrete nouns were repeated across language (e.g., French-German, valise(suitcase)-Koffer(suitcase)). Whereas early bilinguals (AoA of L2<3years) showed larger repetition enhancement (RE) effects in the left superior temporal gyrus, the bilateral superior frontal gyrus and the right posterior insula, late bilinguals (AoA of L2>10years) showed larger RE effects in the middle portion of the left insula and in the right middle frontal gyrus (MFG). We suggest that, as for grammatical knowledge, the attainment of lexical knowledge in L2 is affected by neural maturation. The present findings lend support to neurocognitive models of bilingual word recognition postulating that, for both early and late bilinguals, the two languages are interconnected at the conceptual level.

Word order processing in the bilingual brain.

One of the issues debated in the field of bilingualism is the question of a "critical period" for second language acquisition. Recent studies suggest an influence of age of onset of acquisition (AOA) particularly on syntactic processing; however, the processing of word order in a sentence context has not yet been examined specifically. We used functional MRI to examine word order processing in two groups of highly proficient German-French bilinguals who had either acquired French or German after the age of 10, and a third group which had acquired both languages before the age of three. Subjects listened to French and German sentences in which the order of subject and verb was systematically varied. In both groups of late bilinguals, processing of L2 compared to L1 resulted in higher levels of activation mainly of the left inferior frontal cortex while early bilinguals showed no activation difference in any of these areas. A selective increase in activation for late bilinguals only suggests that AOA contributes to modulating overall syntactic processing in L2. In addition, native speakers of French showed significantly higher activation for verb-subject-order than native German speakers. These data suggest that AOA effects may in particular affect those grammatical structures which are marked in the first language.