Lifelong bilingualism and mechanisms of neuroprotection in Alzheimer dementia

Lifelong bilingualism is associated with delayed dementia onset, suggesting a protective effect on the brain. Here, we aim to study the effects of lifelong bilingualism as a dichotomous and continuous phenomenon, on brain metabolism and connectivity in individuals with Alzheimer's dementia. Ninety‐eight patients with Alzheimer's dementia (56 monolinguals; 42 bilinguals) from three centers entered the study. All underwent an [18F]‐fluorodeoxyglucose positron emission tomography (PET) imaging session. A language background questionnaire measured the level of language use for conversation and reading. Severity of brain hypometabolism and strength of connectivity of the major neurocognitive networks was compared across monolingual and bilingual individuals, and tested against the frequency of second language life‐long usage. Age, years of education, and MMSE score were included in all above mentioned analyses as nuisance covariates. Cerebral hypometabolism was more severe in bilingual compared to monolingual patients; severity of hypometabolism positively correlated with the degree of second language use. The metabolic connectivity analyses showed increased connectivity in the executive, language, and anterior default mode networks in bilingual compared to monolingual patients. The change in neuronal connectivity was stronger in subjects with higher second language use. All effects were most pronounced in the left cerebral hemisphere. The neuroprotective effects of lifelong bilingualism act both against neurodegenerative processes and through the modulation of brain networks connectivity. These findings highlight the relevance of lifelong bilingualism in brain reserve and compensation, supporting bilingual education and social interventions aimed at usage, and maintenance of two or more languages, including dialects, especially crucial in the elderly people.
We investigated the neural mechanisms underlying the ability of bilingual senior individuals to better cope with pathology and symptoms of Alzheimer dementia, as compared to matched monolingual individuals with Alzheimer dementia. By using FDG‐PET to measure brain glucose metabolism and connectivity, we found evidence of neural reserve on brain hypometabolism and compensatory effects on brain metabolic connectivity, predominantly in the left hemisphere. In both cases, effects of lifelong bilingualism on the brain were linearly associated with the degree of second language use, that is, the higher the second language use, the higher the neural reserve/compensatory effect on brain metabolism and connectivity, respectively.