Understanding white matter integrity stability for bilinguals on language status and reading performance.

Recent studies using diffusion tensor imaging (DTI) have described overall white matter integrity in bilinguals but have not related structural neural pathways to language functions. The current study examined white matter integrity and its relationship to reading skill in monolingual English and bilingual Chinese-English speakers. Eleven monolingual speakers (mean age 28.5 years) and 13 bilingual speakers (mean age 24.2 years; English as a second language was acquired post 5 years of age) participated. Behavioural response times and accuracy rates to name regular and exception words were recorded. Participants were then scanned using a standardized DTI protocol. Fractional anisotropy (FA) and mean diffusivity values were derived from a voxelwise statistical analysis for comparisons between participant groups. Tests for relationships between response time and FA were also conducted. Our results show minimal regions of higher FA for monolinguals when compared to bilinguals and no regions of higher FA for bilinguals when compared to monolinguals, which indicates that white matter integrity may not stabilize in bilinguals until late adulthood. We do show several regions where an increase in FA is associated with faster response times. Interestingly, the FA-response time relationship varies between groups and between word types, which may reflect an increased processing demand for retrieval of difficult words (e.g., exception words). These results provide some support for the interference control and reduced frequency hypotheses outlined by Jones et al. (Cerebr Cortex 22:892-902, 2012). The current findings advance our understanding of the underlying cortical networks associated with language status and reading skill in monolingual and bilingual adults.