Theta transcranial alternating current stimulation is not effective in improving working memory performance

Extensive research has established a significant relationship between frontal midline theta (FMT) activity in the 4-8 Hz range and working memory (WM) performance. Transcranial alternating current stimulation (tACS) is recognized for inducing lasting changes in brain oscillatory activity. Across two experiments, we tested the possibility that WM could be improved through tACS stimulation of dorsomedial prefrontal cortex (dmPFC) and anterior cingulate cortex (ACC), by affecting executive control networks associated with FMT. In Experiment 1, following either a 20-minute verum or sham stimulation applied to Fpz-CPz at 1 mA and 6 Hz, 31 participants performed WM tasks, while EEG was recorded. The task required participants to either mentally manipulate memory items or retain them in memory as they were originally presented. No significant effects were observed in behavioral performance, and we found no change in theta activity during rest and task following stimulation. However, alpha activity during retention or manipulation of information in WM was less strongly enhanced following verum stimulation as compared with sham. In Experiment 2 (N = 25), tACS was administered during and after the task in two separate sessions. Here, we changed the order of the stimulation blocks: a 25-minute task block was either accompanied first by sham stimulation and then by verum stimulation, or vice versa. Taken together, our results show no improvements in WM performance through tACS after-effects or online stimulation and demonstrate that theta frequency tACS applied at the midline is not an effective method for enhancing WM performance.