Distinct roles of left inferior frontal regions that explain individual differences in second language acquisition.

Second language (L2) acquisition is more susceptible to environmental and idiosyncratic factors than first language acquisition. Here, we used functional magnetic resonance imaging for L2 learners of different ages of first exposure (mean: 12.6 and 5.6 years) in a formal school environment, and compared the cortical activations involved in processing English sentences containing either syntactic or spelling errors, where the testing ages and task performances of both groups were matched. We found novel activation patterns in two regions of the left inferior frontal gyrus (IFG) that correlated differentially with the performances of the late and early learners. Specifically, activations of the dorsal and ventral triangular part (F3t) of the left IFG correlated positively with the accuracy of the syntactic task for the late learners, whereas activations of the left ventral F3t correlated negatively with the accuracy for the early learners. In contrast, other cortical regions exhibited differential correlation patterns with the reaction times (RTs) of the syntactic task. Namely, activations of the orbital part (F3O) of the left IFG, as well as those of the left angular gyrus, correlated positively with the RTs for the late learners, whereas those activations correlated negatively with the RTs for the early learners. Moreover, the task-selective activation of the left F3O was maintained for both the late and early learners. These results explain individual differences in L2 acquisition, such that the acquisition of linguistic knowledge in L2 is subserved by at least two distinct inferior frontal regions of the left F3t and F3O.

Correlated functional changes of the prefrontal cortex in twins induced by classroom education of second language.

As the neural substrates of second language (L2) acquisition are largely unknown, they may or may not be similar to those of first language (L1) representation. We used functional imaging to examine whether training in the conjugation of English verbs from present to past tense alters brain activations in 13-year-old twins. A novel experimental design contrasted past tense verb identification and verb matching, which were tested in either English (L2) or Japanese (L1). After 2 month classroom training in the past tense using bingo games, the two individuals in each pair exhibited significantly correlated performances. When pre- and post-scanning sessions were compared with each other for L2, the left dorsal inferior frontal gyrus (IFG) exhibited significantly correlated activation increases within each pair of twins and the increases were positively correlated with individual performance improvements. Moreover, the cortical plasticity for L2 acquisition was guided toward the L1 specialization of the left dorsal IFG, in spite of notable differences between L1 and L2 in the students' linguistic knowledge and in their performance in making past tense forms. These findings suggest a cortical mechanism underlying L2 acquisition, which critically depends on shared genetic and environmental factors for each twin in a surprisingly predictive manner.