Age-related cognitive decline is a common phenomenon characterised by changes in cognitive functions such as working memory capacity, attentional control, and inhibition. However, there is variability in the extent and impact of these declines between individuals. This variability may be attributed to differences in functional neuroplasticity, the ability to reorganise neural networks and recruit alternative brain regions to compensate for age-related changes. Bilingualism offers a unique lens through which to understand cognitive aging and functional neuroplasticity. Research suggests that bilingualism may modify the trajectory of cognitive changes in older adults, with bilingual older adults outperforming monolinguals on non-verbal executive function tasks related to inhibitory control, task switching and working memory. However, the specific role of functional neuroplasticity in driving these language group differences remain unclear. This thesis investigates how bilingualism may impact age-related changes in cognitive control and attentional processes using two non-verbal executive function tasks. Additionally, the influence of task demands on the observation of these differences is examined. Understanding how bilingualism may impact brain activity regulation can offer insight into the mechanisms behind experience-induced plasticity in older adulthood.
Experiment one employed a visual Go/NoGo paradigm to investigate the impact of bilingualism on age-related changes in conflict monitoring and attentional processes under low and high task demands. Results revealed that when conflict monitoring demands were low, both older adult monolinguals and bilinguals regulated conflict monitoring processes akin to younger adults, indexed by the presence of an N2-effect. However, under high task demands, older adult monolinguals over-recruited conflict monitoring processes, reflected in an attenuation of the N2-effect. Given the lack of behavioural differences between the two language groups, it is possible that this over-recruitment observed in older adult monolinguals reflected a compensatory process, and the absence of this over-recruitment in older adult bilinguals was reflective of more efficient cognitive control.
In Experiment two, the effect of bilingualism on age-related changes in the inhibition of interference from irrelevant information was investigated using a visual Simon task. Older adults exhibited increased interference to irrelevant stimuli relative to younger adults, indexed by larger Simon effects in reaction time. No behavioural differences were observed between monolinguals and bilinguals in either age group. However, at the neural level, older adult monolinguals exhibited greater interference from irrelevant stimuli than older adult bilinguals, indicated by a larger N2-effect, while the younger adult sample exhibited comparable N2 amplitudes between the language groups. These results suggest that language experience impacts the ability to inhibit irrelevant interference, but this effect may only be observable in older adulthood on this paradigm, possibly due to low task demands.
The third empirical chapter revealed no association between the Simon and Go/NoGo task on either behavioural or electrophysiological measures. This finding supports the lack of convergent validity between executive function tasks assumed to measure common EF constructs. This suggests that performance on these tasks could be underpinned by two different kinds of inhibitory control mechanisms.
Overall, this thesis contributes to our understanding of how bilingualism may impact functional changes in cognition during aging. It highlights how bilingual older adults are better able to adapt cognitive control processes to meet task demands especially under high task demands, and they experience less interference from irrelevant stimuli than monolinguals. Furthermore, the research presented here highlights the importance of considering the specificity of executive function tasks as performance may be underpinned by distinct aspects of inhibitory and monitoring processes.
Future work should employ extensive ladder design paradigms to further examine the impact of cognitive load on language group differences in cognitive control and attentional processes. Additionally, employing a multi-paradigm approach alongside latent variable analysis would enhance our understanding of executive function and the role of attention in explaining these differences.