Experience-dependent neuroplasticity in the hippocampus of bilingual young adults.

Abstract

Models of experience-dependent neuroplasticity predict that the acquisition and extensive use of a new skill trigger a nonlinear trajectory of neurostructural modifications, where initial expansion of relevant brain areas subsequently (once the skill is acquired) gives way to volumetric re-normalization. Such predictions also apply in the domain of language during learning and/or simultaneous management of two (or more) linguistic systems. In a sample of 69 young adult Russian-English bilinguals, we tested the hypothesis that individual differences in bilingual engagement non-linearly correlate with normalized volume of the hippocampus - a key learning-related brain region particularly amenable to experience-dependent plasticity. Results revealed an inverted-U shape association between second language engagement and left hippocampal gray matter volume. The present results replicate and expand the findings from aging populations, showing a non-linear pattern of structural hippocampal plasticity in healthy young adults. These findings support the role of bilingualism as a promoter of experience-dependent neuroplasticity.Significance statement Bilingual experience has been associated with neurocognitive adaptations and linked to more favorable cognitive aging. The hippocampus, crucial in aging, has been previously shown to exhibit volumetric increases in response to language learning with some reports of non-linear adaptations linked to bilingual experience. General models of neuroplasticity related to skill acquisition and bilingualism-specific models predict a morphological trajectory of volumetric expansions followed by renormalization of hippocampal volumes along the bilingual experience continuum. In this cross-sectional study we, for the first time, empirically tested this prediction in a sample of young individuals. In line with model predictions, our findings revealed an inverted-U shape relationship between second language engagement and left hippocampal volume, suggesting bilingualism as a source of experience-dependent neuroplasticity.