Bilingualism modulates baseline brain-cognition relationships in aging: Evidence from the HABS-HD cohort

Bilingualism has been proposed to confer advantages in cognitive aging, yet the neural mechanisms underlying any such advantages remain debated. In a pre-registered analysis of cross-sectional and longitudinal data from elders in the HABS-HD cohort, we asked whether bilingualism is associated with three measures of brain structure (total brain volume, hippocampal volume and white-matter lesions) and/or moderates the relationship between brain structure and two measures of cognition (executive function and episodic memory). Overall, we found no significant differences between bilinguals and monolinguals in their baseline brain structure or subsequent change in brain structure, suggesting that a life of bilingualism was not clearly associated with Brain Reserve or Brain Maintenance in old age. In baseline analyses, bilingualism moderated the relationship between total brain volume and executive function, and between hippocampal volume and episodic memory, such that these cognitive functions were less dependent on these brain measures in bilinguals, consistent with the concept of Cognitive Reserve. However, any effect of bilingualism on longitudinal associations between brain-change slopes and cognitive-change slopes did not reach significance, suggesting that bilingualism-related Cognitive Reserve may be limited to baseline differences. Further, exploratory analyses showed that the effects of bilingualism on hippocampal volume, and on its relationship with episodic memory, might differ according to clinical dementia status. In summary, our findings suggest that bilingualism is associated with altered baseline brain-cognition relationships, rather than with preservation of brain structure.