Left DLPFC involvement in verbal working memory in ageing: A cTBS-ERP study

Ageing is associated with cognitive decline, particularly in working memory (WM), a process linked to dorsolateral prefrontal cortex (DLPFC) functioning. Using continuous Theta Burst Stimulation (cTBS) to induce transient disruption, this study investigated the potential causal role of the left DLPFC in verbal and visuospatial WM performance in healthy older adults using behavioural and EEG measures. Twenty participants completed verbal and visuospatial N-back tasks under baseline, vertex, and left DLPFC stimulation conditions while EEG was recorded. Behavioural analyses showed learning-related improvement in verbal WM following vertex stimulation, whereas evidence for improvement following DLPFC stimulation was inconclusive, suggesting potentially attenuated learning-related effects. In the visuospatial task, evidence for behavioural improvement following vertex stimulation was inconclusive, whereas the DLPFC condition showed accuracy increases consistent with potential learning effects. ERP amplitudes were largely unaffected by stimulation, but several latency effects emerged across both tasks. In the verbal task, DLPFC stimulation was associated with prolonged P200 and P300 latencies, with shorter P200 latencies associated with greater accuracy improvements, suggesting a potential link between early attentional processing and learning-related behavioural change. In the visuospatial task, stimulation-related latency modulation was also observed, although behavioural findings were comparatively less consistent and exploratory analyses only suggested potential associations between N200 and P300 latency changes and behavioural improvement under specific cognitive loads. Together, these findings provide preliminary support for the continued involvement of the left DLPFC in verbal WM processing in older adults, particularly in the temporal coordination of attentional and cognitive control processes.