Effects of bilingualism on white matter integrity in older adults

Abstract

Bilingualism can delay the onset of dementia symptoms and has thus been characterized as a mechanism for cognitive or brain reserve, although the origin of this reserve is unknown. Studies with young adults generally show that bilingualism is associated with a strengthening of white matter, but there is conflicting evidence for how bilingualism affects white matter in older age. Given that bilingualism has been shown to help stave off the symptoms of dementia by up to four years, it is crucial that we clarify the mechanism underlying this reserve. The current study uses diffusion tensor imaging (DTI) to compare monolinguals and bilinguals while carefully controlling for potential confounds (e.g., I.Q., MMSE, and demographic variables). We show that group differences in Fractional Anisotropy (FA) and Radial Diffusivity (RD) arise from multivariable interactions not adequately controlled for by sequential bivariate testing. After matching and statistically controlling for confounds, bilinguals still had greater axial diffusivity (AD) in the left superior longitudinal fasciculus than monolingual peers, supporting a neural reserve account for healthy older bilinguals.

Redefining bilingualism as a spectrum of experiences that differentially affects brain structure and function

“…Diffusion tensor imaging (DTI) data were preprocessed using the top-up (41), and eddy (42) pipelines within FSL. WM integrity was assessed via several measurements, including FA, MD, RD, and AD values (43,44). These values were calculated using the FDT and DTIFIT (45) pipelines.…”

Neural basis of bilingual language control

“…Pioneering research on white matter tracts by Luk et al ., for example, revealed that bilingual speakers report greater integrity of long‐range tracts in the language network (e.g., the bilateral superior longitudinal fasciculi, right inferior fronto‐occipital fasciculus, and uncinate fasciculus). Anderson et al . showed increased white matter integrity specifically in the left superior longitudinal fasciculus for such older life‐long bilinguals.…”

The Many Shades of Bilingualism: Language Experiences Modulate Adaptations in Brain Structure

“…These studies report monolingual and bilingual group differences, in a way where healthy older bilingual population exhibit resilience to age‐related neurodegeneration. This resilience is expressed as preservation of grey matter in brain areas implicated in language acquisition and control, such as the dorsolateral prefrontal cortex, anterior cingulate cortex and ATL (Abutalebi et al., , ), the IFG and IPL (Borsa et al., ; Heim et al., ), and the hippocampus (Voits, Robson, Rothman, & Pliatsikas, ), and higher integrity in white matter tracts such as CC, ILF, and IFOF (Anderson et al., ; Gold, Johnson, & Powell, ; Luk et al., ), all regions central to the predictions of the dynamic restructuring model, especially at Stages 2 and 3. All of this adds to the idea of bilingualism as a contributing factor to a neural reserve (i.e., build‐up of structural scaffolding of neural tissue in the brain, supporting it against tissue or synaptic loss).…”