Bilingual experience affects white matter integrity across the lifespan.

Bilingualism has been associated with increases in compensatory mechanisms to age-related neurocognitive decline thus delaying dementia symptom onset and leading to a more favorable trajectory of neurocognitive aging. However, most research to date has examined bilingualism-induced effects on neurocognition within older age ranges or young adults - with middle-aged individuals typically not being a population of interest. Furthermore, bilingualism is often treated as a dichotomous variable, despite it being a heterogeneous experience on an individual level. In the present study, we employed diffusion tensor imaging (DTI) to examine whether bilingualism, and the degree of engagement in bilingual experience, modulates the nature or rate of white matter decline associated with aging. DTI data and language history data were collected from a cohort of monolingual and bilingual individuals spanning a wide age range. Two separate analyses were run. First, generalized additive models were run on matched monolingual and bilingual samples, examining effects of age on the trajectory of white matter integrity and how bilingualism modulates this effect. This analysis revealed a significant effect of age within the monolingual group for fractional anisotropy values in the right superior longitudinal fasciculus. However, the age effect within the bilingual group was not significant, indicating a faster decline in white matter integrity within the monolingual cohort. Second, general linear models were run on the entire participant sample, examining an interaction between age and degree of bilingual engagement on white matter integrity. Results from this analysis indicate that increased engagement in bilingual language use across the lifespan correlates with a slower decline in white matter integrity with age. Together these results indicate bilingualism, and specifically degree of bilingual engagement, impacts the trajectory of age-related decline in white matter integrity across the lifespan.