Bilingualism as a Proxy of Cognitive Reserve

Previous studies have reported bilingualism to be a proxy of cognitive reserve (CR) based on evidence that bilinguals express dementia symptoms ~4 years later than monolinguals yet present with greater neuropathology at time of diagnosis when clinical levels are similar. This dissertation presents two studies that provide further evidence for the contribution of bilingualism to CR. The first study uses a novel brain health matching paradigm. Forty cognitively normal bilinguals with diffusion-weighted magnetic resonance images recruited from the community were matched with monolinguals drawn from a pool of 165 individuals in the Alzheimers Disease Neuroimaging Initiative (ADNI) database. White matter integrity was calculated for all participants using fractional anisotropy, axial diffusivity, and radial diffusivity scores. Propensity scores were obtained using white matter measures, sex, age, and education as predictive covariates, and then used in one-to-one matching between language groups, creating a matched sample of 32 participants per group. Matched monolinguals had poorer clinical diagnoses than that predicted by chance from a theoretical null distribution, and poorer cognitive performances than matched bilinguals as measured by scores on the MMSE. The findings support the interpretation that bilingualism acts as a proxy of CR such that monolinguals have poorer clinical and cognitive outcomes than bilinguals for similar levels of white matter integrity even before clinical symptoms appear. The second study examines the role of biomarkers and genetic factors associated with Alzheimer disease in a sample of 641 individuals from the ADNI database. Gradient boosted regression modelling was used to examine the influence of 10 predictive factors on clinical diagnosis in 3 different models. Weighted propensity scores were applied to analyses of white matter integrity and cognitive performance between clinical groups in two models and between language groups in one model. Analyses revealed a strong influence of biomarkers and genetic factors on clinical diagnosis in monolingual participants, but underrepresentation of bilingual participants in the sample limited interpretations of the findings between language groups. The results of the second study indicate that information about biomarkers and genetic factors improves analyses exploring the role of CR on dementia outcomes.