How bilingualism protects the brain from aging: Insights from bimodal bilinguals

Abstract

Bilingual experience can delay cognitive decline during aging. A general hypothesis is that the executive control system of bilinguals faces an increased load due to controlling two languages, and this increased load results in a more ‘tuned brain’ that eventually creates a neural reserve. Here we explored whether such a neuroprotective effect is independent of language modality, i.e., not limited to bilinguals who speak two languages but also occurs for bilinguals who use a spoken and a signed language. We addressed this issue by comparing bimodal bilinguals to monolinguals in order to detect age-induced structural brain changes and to determine whether we can detect the same beneficial effects on brain structure, in terms of preservation of gray matter volume (GMV), for bimodal bilinguals as has been reported for unimodal bilinguals. Our GMV analyses revealed a significant interaction effect of age × group in the bilateral anterior temporal lobes, left hippocampus/amygdala, and left insula where bimodal bilinguals showed slight GMV increases while monolinguals showed significant age-induced GMV decreases. We further found through cortical surface-based measurements that this effect was present for surface area and not for cortical thickness. Moreover, to further explore the hypothesis that overall bilingualism provides neuroprotection, we carried out a direct comparison of GMV, extracted from the brain regions reported above, between bimodal bilinguals, unimodal bilinguals, and monolinguals. Bilinguals, regardless of language modality, exhibited higher GMV compared to monolinguals. This finding highlights the general beneficial effects provided by experience handling two language systems, whether signed or spoken.

Effects of bilingualism on white matter integrity in older adults

“…For example, proponents of the cognitive reserve perspective (e.g., Craik et al, 2010; Perani et al, 2017; Schweizer et al, 2012) usually include Alzheimer’s disease patients in their studies whereas proponents of the neural reserve perspective typically recruit healthy older adults without disease progression (e.g., Abutalebi et al, 2015b; Li et al, 2017; Li et al, 2014; Olsen et al, 2015). Therefore, the theories largely describe different populations.…”

Understanding structural plasticity in the bilingual brain: The Dynamic Restructuring Model

“…Specifically, evidence from cortical, subcortical and cerebellar grey and white matter adaptations will be presented, divided into sections separating the populations based on the quantity and type of their language learning and switching experiences. It is worth noting here that this review will not attempt to differentiate between different indices of plasticity; in other words, evidence from methods looking at cortical thickness, volume and surface extent will be presented and treated equally as evidence for structural adaptations (for a discussion on the differences between these approaches, see Li, Abutalebi, Emmorey, Gong, Yan, Feng, Zou & Ding, 2017). None of these methods can confidently describe the changes that happen at the microstructural level though, so any suggestions will remain speculative based on the predictions by models such as the EPH.…”

Neural basis of bilingual language control

“…Gray matter differences might also be predicted between elderly bilingual and monolingual speakers potentially leading to the so‐called neural reserve. Indeed, bilateral volumes in the temporal poles and the orbitofrontal cortex (OFC) were greater for aging bilinguals relative to age‐matched monolinguals, with equivalent scores on education, socio‐economic status, and cognitive performance . Interestingly, gray matter volumes in the left anterior temporal pole increased with proficiency in the L2 suggesting that language control demands continue to shape the structure of the regions they target.…”