Abstracts

Rationale: Neural plasticity is a compensatory brain function after certain types of brain damage, such as stroke, trauma, and brain tumors. Neural plasticity can occur from primary to higher cerebral functions, but is limited with age. Although several hypotheses for neural plasticity have been considered including neurogenesis, changes in neuronal excitability, and development of subcortical fiber networks, no study has elucidated the fundamental mechanism. We hypothesized that the mechanism of speech when the bilinguals spoke the second language could be simulated the functional acquisition of speech after damage of the primary language areas. We investigated cortical activity during a passive listening sentence task of an individual’s second language with functional magnetic resonance imaging (fMRI) and differences of subcortical network with diffusion tensor imaging (DTI) in this study. Methods: Our participants of this study were Japanese monolinguals and Japanese-English bilinguals. We divided the bilinguals into simultaneous bilinguals who acquired English as the second language before the age of 7 years, and sequential bilinguals who started to learn English after the age of 7 years. The functional activating areas were assessed by blood oxygen level-dependent (BOLD) response during hearing sentence tasks in English and Japanese using 3T MRI. Data were analyzed by statistical method using MATLAB and SPM software. DTI and high-resolution T1-weighted gradient echo data were processed using the Connectome Mapping Toolkit. The edges were defined as connections between each region of interest according to the graph theory, and were quantitatively analyzed as the number of fibers (NOF). NOFs were compared among the three groups with one-way ANOVA with post-hoc analysis. Results: Japanese monolinguals showed narrow cortical activation area in left middle temporal gyrus when listening to a Japanese task. Simultaneous bilinguals showed activation in narrow regions in the bilateral superior temporal gyri during the second language task. In contrast, sequential bilinguals demonstrated large cortical activation areas, mainly language association areas in the frontal and temporal lobes during the second language task. Japanese monolinguals during the Japanese task and simultaneous bilinguals during the second language task showed similar negative BOLD responses in the areas which may correspond to key nodes of the default mode network. Connectome mapping analysis of bilinguals compared to Japanese monolinguals demonstrated that simultaneous bilinguals showed abundant NOFs between the right putamen and precentral gyrus. In contrast, sequential bilinguals showed fewer NOFs. Conclusions: White matter connectivity, especially from the right putamen to the cortex, was organized before the paradigm change in language acquisition, and wide cortical activation was utilized thereafter for understanding language. Funding: Scientific Research (KAKENHI) [grant number 15K10372]