The effect of combined transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) on executive functions

Executive functions (EFs) such as working memory, cognitive flexibility, and semantic retrieval are crucial for adaptive behavior and higher-order cognition. Non-invasive brain stimulation techniques, particularly transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS), have been proposed as potential methods to enhance these cognitive functions by exogenously modulating ongoing functional oscillatory neural activity. The current randomized, double-blind, sham-controlled study investigated the effects of bifrontal 6 Hz tACS intervention, applied either alone or in combination with anodal tDCS, on executive functions in healthy adults. Sixty-six participants were randomly assigned to tACS, combined tACS+tDCS, or sham-stimulation intervention groups. Intervention was administered once daily (20 minutes a day) over five consecutive days. Executive fucntioing was assessed before and after the intervention using measures of working memory (Digit Span Forward and Backward), cognitive flexibility (Trail Making Test–B), and semantic fluency. Results indicated that theta-tACS selectively improved working memory capacity, as measured by Digit Span Forward, compared to sham stimulation. The combined tACS+tDCS protocol also resulted in a significant, albeit more moderate, improvement in working memory capacity relative to sham. No stimulation-related effects were observed for cognitive flexibility, semantic retrieval, or verbal working memory. In addition, changes in outcome measures revealed a significant main effect of Time, indicating general improvement from baseline to post-intervention across all groups, without differences between groups (tACS vs tDCS+tACS vs sham stimulation). Notably, although a significant Time × Group interaction was observed for TMT completion time, the pattern of change did not demonstrate a clear superiority of either active stimulation condition relative to sham, suggesting that part of the observed improvement may reflect practice or learning effects on TMT completion times. These findings suggest that theta-tACS over the dorsolateral prefrontal cortex (DLPFC) can selectively enhance passive-storage within working memory storage mechanisms. Importantly, the observed effect of the combined stimulation protocol highlights the potential of integrating tonic excitability- and oscillatory-neuromodulation approaches as a novel methodological avenue for targeting the PFC excitability to enhance EFs. The results have potential implications for educational (e.g., augmenting the trajectory of EF and academic success in children and teenagers) and clinical applications aimed at improving executive functioning (e.g., ASD individuals).