It has been demonstrated that endogenous brain oscillations can become entrained to rhythmic stimulation. Four studies were conducted to investigate the possibility of modulating endogenous frequency peaks, through entrainment, from stimulation delivered at off-peak frequencies. Study 1 utilised EEG recordings to establish whether off-peak stimulation within the alpha frequency range could successfully shift individual alpha peak frequencies in the direction of stimulation. Stimulation was delivered at 2Hz above and below the individual alpha peak frequency of each participant. EEG recordings taken immediately after stimulation found no significant modulation of individual alpha peak frequencies. In light of this, the subsequent studies assessed the effects of off-peak stimulation during the stimulation rather than immediately following it. These studies used a working memory task and were based on the theory that differences in theta and gamma frequency peaks causally effect working memory capacity. Studies 2 and 3 delivered tACS at off-peak theta frequencies (4Hz and 7Hz) and study 4 delivered tACS at off-peak gamma frequencies (40Hz and 70Hz) during a working memory task. This was done to ascertain whether such stimulation could alter working memory capacity, which would indicate that peak frequencies had been shifted. Studies 2 and 3 utilised two different electrode configurations (P4/Cz and P4/right supraorbital respectively). In study 3, working memory capacity was enhanced and impaired in the 4Hz and 7Hz conditions respectively. In studies 2 and 4, no significant changes in working memory capacity were found. The findings from study 3 indicate that off-peak entrainment can be used to shift frequency peaks in the direction of the stimulation frequency and that such shifts can have observable cognitive effects. The contrast between studies 2 and 3 highlights the importance of electrode placement in tACS studies
