Abstract
Working memory (WM) is critical for maintaining and manipulating information in support of goal-directed behaviour. As individuals age, WM function declines, accompanied by changes in underlying neural dynamics. This thesis explored the neural mechanisms of WM ageing and the potential of neuromodulation and cognitive training to support cognitive function in older adults. A particular focus was placed on theta and alpha oscillatory activity, compensatory mechanisms, and the causal role of the dorsolateral prefrontal cortex (DLPFC) in executive processing.In the first empirical study, electroencephalography (EEG) was used to examine oscillatory correlates of verbal (Letter Span) and visuospatial (Corsi) WM tasks under high cognitive load in younger and older adults. While older adults maintained performance in verbal WM, they showed deficits in visuospatial conditions. Task-dependent theta and alpha modulations revealed domain-specific compensatory strategies, with younger adults showing stronger mid-frontal theta power and older adults relying more on alpha-based gating mechanisms during maintenance and recall phases.
The second study assessed the behavioural and neural effects of combining transcranial Direct Current Stimulation (tDCS) with adaptive WM training in older adults. Over six sessions, participants received either active or sham tDCS to the left DLPFC while completing the Operation Span (OSPAN) task. Despite the absence of additive tDCS effects, near-transfer improvements emerged in manipulation conditions of the Letter Span and Corsi Tests, sustained at one-month follow-up. EEG analyses highlighted training-induced oscillatory changes and subtle group-specific neural–behavioural associations, suggesting potential but individualised effects of tDCS.
The final study used continuous Theta Burst Stimulation (cTBS) to provide causal evidence for DLPFC involvement in WM. Older adults completed verbal and visuospatial N-back tasks following stimulation to either the left DLPFC or a control site (vertex), with concurrent EEG recordings. While behavioural performance was unaffected by stimulation, event-related potential (ERP) analyses revealed latency shifts (notably in P200 and N200 components) linked to early attentional and cognitive control processes, offering evidence for the DLPFC’s causal role in top-down regulation.
Together, these studies highlight the complexity of WM ageing and the dynamic interplay of oscillatory mechanisms, stimulation-induced changes, and task demands. Alpha and theta modulations in older adults reflected compensatory recruitment and strategic adjustments across tasks, phases, and modalities. While tDCS effects were limited, the findings suggest the need for personalised stimulation protocols tailored to individual neurocognitive profiles. Moreover, the DLPFC was reaffirmed as a critical hub for executive control, warranting continued investigation as a neuromodulation target.
Overall, this thesis extends theoretical models of cognitive ageing (e.g. CRUNCH, HAROLD, gating theories, and Baddeley’s WM model) by incorporating oscillatory, behavioural, and causal evidence. It underscores the potential of cognitive training to improve WM performance and calls for more targeted, neurobiologically-informed interventions to support healthy cognitive ageing.
| Date of Award | 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | STERGIOS MAKRIS (Supervisor), ALEX BAHRAMI BALANI (Supervisor), ADAM QURESHI (Supervisor) & DOROTHY TSE (Supervisor) |