Mean diffusivity reveals white matter microstructural differences between monolinguals and bilinguals

Abstract

Recent hypotheses about the effect of second language experience on neuroplasticity have suggested possible differences in white matter microstructure between bilinguals and monolinguals. In this context, mean diffusivity (MD) has emerged as a sensitive measure of neural plasticity. Given that neural plasticity is an important consequence of bilingualism (Green & Abutalebi, 2013), the primary objective of this study was to use MD and associated components axial and radial diffusivity to compare white matter microstructure of bilinguals with monolinguals based on a prediction that exposure to a second language leads to neuroplasticity connecting specific brain regions. Eighteen Hindi-English bilinguals (mean age & SD: 23.94±1.5years) and 18 Italian monolinguals (mean age & SD: 23.45±2.1 years) closely matched for age, levels of education, literacy and SES (Socio Economic Status) were examined. Language proficiency assessments revealed that bilingual subjects were equally proficient in Hindi and English. Diffusion weighted images were acquired using a transverse multi-slice, single-shot, spin echo-planar imaging (EPI) sequence with 35 gradient directions at b-value = 1000 s/mm2 and one b = 0 reference image. After preprocessing, diffusion tensor was fitted onto the corrected images using FMRIB’s FDT toolbox (http://www.fmrib.ox.ac.uk/fsl/) which generated diffusion image maps (eigen values (λ1, λ2, and λ3)), fractional anisotropy (FA) and MD, AD λ|| = λ1, RD (λ┴ = (λ2 +λ3)/2)). Voxel-wise statistics were carried out at major tract centres common to all participants using Tract Based Spatial Statistics (TBSS),(Smith et al., 2006).In order to generate additional preliminary insights into the geometry of the underlying white matter structure in bilingual speakers, distributions for mean diffusivity values for bilinguals and monolinguals were also estimated. Higher mean diffusivity (MD) for bilinguals in white matter tracts such as forceps minor and bilateral superior longitudinal fasciculus (p=0.05) were observed. There was a high degree of concordance with the tracts seen in MD and RD indicating that the increased mean diffusivity was due to a higher rate of diffusion along the perpendicular as compared to the parallel direction.MD and RD distributions of bilateral SLF in monolinguals and bilinguals showed similar trends as the FA distribution comparison. Our results demonstrate for the first time changes in mean and radial diffusivity, specifically in the right superior longitudinal fasciculus and the forceps minor in bilinguals as compared to monolinguals. We attribute these results to structural neuroplasticity in the bilingual brain as a consequence of consistent and daily use of cognitive systems involved in language control. We speculate that the structural differences found in the SLF reflect to the effects of richer articulatory repertoire required from bilinguals.