Abstract

This dissertation presents findings that address several questions with respect to research demonstrating protection from age-related cognitive decline and dementia in older bilinguals relative to monolinguals. Manuscript 1 (Chapter 4) reports research investigating the contribution of bilingualism to cognitive reserve by examining the clinical and neurophysiological manifestations of dementia in monolingual and multilingual patients with mild cognitive impairment (MCI) and Alzheimer disease (AD). Neuropsychological assessment data, demographic information, cortical thickness, and gray matter tissue density are compared between monolinguals and bilinguals. Results in AD patients indicated lower grey matter density in the posterior parahippocampal gyri (and similar directional trends for the rhinal cortices) for multilinguals compared to monolinguals, despite the fact that the groups were matched for functioning on two episodic memory tests. Results in both MCI and AD patients showed thicker cortex and greater tissue density in a number of regions related to bilingualism in multilinguals compared to monolingual. Additionally, this study also found significant correlations between brain regions related to language and cognitive control and episodic memory measures, for multilingual patients but no monolingual patients. This provides evidence towards our hypothesis that for multilingual patients, greater brain matter in cognitive control regions may form part of compensatory memory network.
Manuscript 2 (Chapter 5) reports research investigating functional differences in the brain activity of younger and older monolinguals and bilinguals while completing cognitive control tasks (i.e., Stroop, Simon, and Eriksen flanker tasks). Previously collected and published data (Kousaie & Phillips, 2012b; 2017) are re-analysed using novel electrophysiological measures to investigate whether bilingualism contributes to differences in brain responses between monolinguals and bilinguals, and whether these effects vary as a function of aging. As was seen in the previously published research, neither the younger nor the older participants show conflict-specific language-group differences in behavioural results (with the exception of the Stroop task for the older adults). However, differences are seen in electrical brain activity between the four groups suggesting differences in cognitive control processing. Broadly, we found an overall age difference in power (with older adults lower higher power in the alpha and theta frequency bands, and more suppression in the beta frequency band than younger adults), and some evidence for conflict-specific language-group differences (with younger and older bilinguals showing larger conflict effects in power than their monolingual counterparts). We also found that induced activity was a better marker of conflict processing than evoked activity and that the locus of the conflict differed across the three tasks with respect to the manifestation of trial type differences in event-related power.