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. Hum Brain Mapp 38:4109-4124, 2017. © 2017 Wiley Periodicals, Inc.

Keywords: bimodal bilinguals; cortical surface area; gray matter volume; neural reserve; neuroprotection.

Figure 1

Interaction effect of age × group on gray matter volume. On the left are brain images from lateral and ventral views showing the regions with positive interaction effect. No negative interaction effect was found. Four scatter diagrams of age vs. GMV are presented on the right for each region. *Significance of negative correlation between age and GMV only for the monolingual group in the simple‐effect analyses. GMV: gray matter volume; L: left; R: right; ATL: anterior temporal lobe; HC: hippocampus. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

Interaction effect of age × group on surface area (A) and cortical thickness (B) within the mask (black area). Within the two subplots (A) and (B), on the left are brain images from lateral and ventral views showing the regions with positive interaction effect, while the scatter diagrams of age vs. surface area or cortical thickness are displayed on the right. No negative interaction effect was found. *Significance of negative correlation between age and surface area only for the monolingual group in the simple‐effect analyses. L: left; R: right; ATL: anterior temporal lobe. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Proportion of GMV variation explained by cortical thickness and surface area in the multiple regression models for three ROIs. * and the affiliated value indicate significance and the proportion of additive contribution of the variables entered the models. GMV: gray matter volume; L: left; R: right; ATL: anterior temporal lobe. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 4

Cross‐group comparisons of GMV between bimodal bilinguals, monolinguals, and unimodal bilinguals in the three brain regions exhibiting significant interaction effect of age × group on GMV, and in the occipital lobe which was regarded as a control region. GMV: gray matter volume; L: left; R: right; ATL: anterior temporal lobe. *P < 0.05; **P < 0.01; ***P < 0.001; ns: no significance. [Color figure can be viewed at http://wileyonlinelibrary.com]