The Dorsal Rather than Ventral Pathway Better Reflects Individual Syntactic Abilities in Second Language

Modality-Dependent Brain Activation Changes Induced by Acquiring a Second Language Abroad

The dynamic nature of cortical activation changes during language acquisition, including second-language learning, has not been fully elucidated. In this study, we administered two sets of reading and listening tests (Pre and Post) to participants who had begun to learn Japanese abroad. The two sets were separated by an interval of about 2 months of Japanese language training. We compared the results of longitudinal functional MRI experiments between the two time-points and obtained the following major findings. First, the left-dominant language areas, as well as bilateral visual and auditory areas, were activated, demonstrating the synergistic effects of multiple modalities. There was also significant activation in the bilateral hippocampi, indicating the expected involvement of memory-related processes. Second, consistent with the behavioral improvements from Pre to Post, the brain activations decreased in the left inferior and middle frontal gyri during the listening tests, as well as in the visual areas (the bilateral inferior and superior parietal lobules, and left inferior and middle occipital gyri) during the reading tests, while activations in the right superior and middle temporal gyri increased during the listening tests. These modality-dependent activation changes could not be explained by domain-general cognitive factors, such as habituation or familiarization, because we used completely different test sets for Pre and Post. Third, the posterior hippocampus showed a main effect of the hemisphere, whereas the anterior hippocampus showed a significant main effect of the event (i.e., specific to first listening events), reflecting initial encoding of auditory information alone. In summary, activation changes from Pre to Post indicate functional changes in modality-dependent networks over a short period of staying abroad, which would enable effective acquisition of a second language.

Differential Signatures of Second Language Syntactic Performance and Age on the Structural Properties of the Left Dorsal Pathway

In adult second language (L2) acquisition, individual differences are considerably large even among people with similar experiences. The neural mechanisms underlying this variability would include structural plasticity of language-related pathways. To elucidate such neuroplasticity, we focused on the transitional period of adolescence, which is associated with certain plasticity toward maturation following the sensitive period of language acquisition (≤12 years old). The adolescent brain would thus be influenced by age-dependent factors, as well as performances in L2. Here, we examined individual differences in L2 performances controlling the duration of experience to reveal the differential signatures of performances and age on the plasticity of structural properties in major language-related pathways. We recruited Japanese students at two ages, i.e., junior (age: 13–14) and senior (age: 16–17) high-school students, all of whom started to expose to English at age 12 or 13. We divided them into subgroups, so that either L2 performance [Junior (High)/Senior (Low)] or age [Senior (Low)/Senior (High)] was matched in group comparisons; the duration of L2 experience was also controlled between the Senior (Low) and Senior (High) groups. We then examined the thickness and fractional anisotropy (FA) of the dorsal and ventral pathways, i.e., the arcuate fasciculus (Arcuate) and inferior fronto-occipital fasciculus (IFOF), respectively, using semi-automatic methods for selecting regions without branches. Regarding FA in the left Arcuate, the Senior (High) group showed significantly higher FA than the other two groups, indicating performance-related group differences. Further, FA in the left Arcuate was selectively correlated with the accuracy of a syntactic task. Regarding the thickness of the left Arcuate, the Senior (High) and Senior (Low) groups showed significantly larger thickness than the Junior (High) group, indicating age-related group differences. These differential performance-related and age-related signatures were evident on the left Arcuate alone, in contrast to the right Arcuate that showed only mild differences in thickness, and to the bilateral IFOF that lacked either signature. Our results suggest that the left dorsal pathway continued to develop to adolescence, and that performance differences in a syntactic task can be predicted by its FA, independent of age and the duration of experience.