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Abstract

Brain plastic changes in second language acquisition do not show a clear pattern with many brain regions shown inconsistently in different phases of learning. Here, we assessed changes in white matter microstructure along the first three months of learning German as a second language in an immersive context. In this first phase of L2 learning, item-based learning by chunks plays a fundamental role. Participants only acquire more creative skills including the internalisation of productive rules towards the end of this phase, which is rarely observed in immersion. Concurrently, we acquired well-established measures of language aptitude, and executive function, and the participants also took part in an L2 fMRI experiment. We found longitudinal changes in diffusivity measures not only in typical language regions, but also in subcortical grey matter and in the brain stem, with a clear right predominance. We also found the involvement of primary sensory and motor areas. In agreement with these results, the fMRI task also presents less lateralised brain activations. Besides, we found changes in areas responsible for cognitive control, together with areas in the auditory and visual pathways, and dopaminergic regions. Additionally, we observed that fluent sequential bilingualism previous to the study was associated with better word learning skills, being an overall good predictor of success. Skills involved in word learning were shown to be central in this early phase. Measures of proficiency obtained from spontaneous language production of the participants were furthermore the best indicators of brain changes, especially in typical language-related brain regions.