Early but discontinued exposure to a language exerts lasting effects on white matter architecture in the brain.

Language experiences influence later cognitive development, but how they shape brain organization remains unclear. We investigated four groups of participants who differed in their early linguistic environments: International Adoptees (IA) from China, who discontinued their first language (L1) Mandarin in the first two years of life and only speak French since adoption; L1-Mandarin speakers with early acquisition and continued use of French; L1-French speakers; and English-French bilinguals. We used diffusion-weighted imaging to explore group differences in two white-matter tracts in the brain that play a role in language processing, the arcuate fasciculus and superior longitudinal fasciculus. Our findings revealed that despite discontinuing Mandarin early in life, IA participants exhibited structural features resembling those of native Mandarin speakers, including smaller tract volumes in the left hemisphere and more balanced tract volumes across hemispheres. In contrast, English-French bilingual and L1-French groups exhibited similar white-matter characteristics, which were distinct from the two tone-language exposed groups, suggesting a structural organization specific to tonal language experience, and not bilingualism in general.