Boosting brainpower in ageing: Task-specific and transfer effects of tDCS-enhanced working memory training.

Ageing is associated with neural alterations that impair cognitive abilities, particularly working memory (WM), which is the capacity to temporarily store and manipulate information essential for daily functioning. Transcranial direct current stimulation (tDCS) has shown promise in enhancing WM in older adults by modulating cortical excitability and promoting neuroplasticity. However, findings on the combined effects of tDCS and WM training remain inconsistent, especially regarding transfer of training gains to nontrained WM tasks (near-transfer effects). This study investigated the behavioural and neural effects of tDCS across distinct WM phases (encoding, retention/manipulation, and recall) in older adults. Participants were divided into two groups, receiving either active or sham tDCS targeting the left dorsolateral prefrontal cortex (DLPFC) during six sessions of adaptive WM training conducted over three weeks. Electroencephalographic (EEG) data were collected, focusing on theta and alpha oscillatory activity. Pre-, post-, and follow-up assessments included modified Letter Span and Corsi Tests, each featuring retention and manipulation conditions. Results revealed no additional behavioural or neural benefits of tDCS. Nonetheless, both groups exhibited task-specific performance improvements posttraining, observed exclusively in manipulation conditions, which were sustained at 1-month follow-up. These improvements were likely driven by the Operation Span (OSPAN) task, which engaged executive processes central to manipulation. Neural findings revealed distinct oscillatory changes and behaviour-neural associations, with correlations unique to the tDCS group that may indicate subtle modulatory effects. However, these results remain tentative given the limited sample size and inter-individual variability. Overall, the findings support adaptive WM training in older adults and underscore the importance of developing personalised tDCS protocols to optimise cognitive and neurophysiological outcomes.