Second language use rather than second language knowledge relates to changes in white matter microstructure

Learning and learning to regulate more than one language is shown to have an impact on the structural connectivity of the brain in networks related to language processing and executive control. The available evidence remains however variable in terms of the occurrence, localization and extent of these effects. Variability likely depends on the fact that grouping heterogeneous linguistic profiles under a dichotomous condition (bilingualism vs. monolingualism) may obscure critical aspects of language experience underlying white matter changes. Here, we treated the main quantifiable features in which bilingual experience can be partitioned—that is, age of acquisition, proficiency and use of a second language—as continuous variables, and tested their effects on a sample of young adult participants. Findings indicate that the time spent using a second language, rather than the age of acquisition or knowledge of that language, significantly modulates white matter microstructure in a bilateral cingulo-frontal cluster encompassing structures primarily related to language control. Taken together, these data point to a usage-dependent remodeling of cingulo-frontal connections, and substantiate the conceptualization of bilingualism as a complex and dynamic experience.