Long-term effects of bifrontal transcranial alternating current stimulation on verbal working memory and episodic memory in young healthy adults.

The current randomized controlled trial (RCT) examined both immediate and long-term effects of five consecutive bifrontal tACS sessions on verbal WM functioning in 30 healthy adult participants. WM performance and event-related quantitative electroencephalography (qEEG) parameters (mean power of frontal theta oscillations during three different WM events) were noted at baseline, immediately after tACS intervention, and at four-week post intervention. In comparison to the sham-tACS group, WM performance in active tACS group was improved following a four-week post-intervention period across all WM events, but not immediately after tACS intervention. Left prefrontal theta power in the active group decreased immediately after the intervention (in comparison to baseline) and remained low at four weeks, while the sham group returned to high theta-power baseline levels. Additionally, at four weeks post-intervention, participants in the active group demonstrated higher episodic memory accuracy compared to the sham group, which was associated with lower frontal theta power during WM encoding events. These findings are consistent with the hypothesized long-term plasticity effects. The current results highlight the role of event-related prefrontal theta power modulation in enhancing verbal WM and episodic memory retrieval in humans acutely, and over time.