Exploring the Effects of tACS Duration on Resting-State EEG: An Exploratory Within-Subject Study in Healthy Volunteers.

Transcranial alternating current stimulation (tACS) is a noninvasive brain stimulation method that modulates neural activity by applying low-intensity alternating current to the scalp. Although tACS has shown promise in enhancing cognitive and motor functions and alleviating neuropsychiatric symptoms, variations in stimulation parameters led to inconsistent outcomes. While stimulation frequency and electrode montage have been extensively explored, systematic analyses focusing on stimulation duration remain limited. Resting-state EEG, recorded under relaxed conditions without specific tasks, minimizes variability due to individual performance and external factors, thus providing a stable measure of tACS-induced neuromodulation. Therefore, we aimed to investigate the differences in neuromodulatory effects between three tACS durations for effective neuromodulation using resting-state electroencephalography (EEG). Ten participants completed three randomized tACS sessions on different days, each with a duration of 10, 20, or 30 min. Resting-state EEG was recorded before and after stimulation under eyes-open (EO) and eyes-closed (EC) states. Power spectral density (PSD) and network indices were analyzed for neuromodulatory effects. The omnibus analysis revealed no significant main effect of stimulation duration on neuromodulatory outcomes. However, within-condition analyses revealed significant increases in PSD in the post-tACS EO recording after 10 min (r = 0.79) and 20 min (r = 0.66), whereas in the post-tACS EC recording significant increases were observed only at 10 min (r = 0.63). Network efficiency also increased significantly in the EO state after 10 (r = 0.79) and 20 min (r = 0.85) for clustering coefficient and after 10 (r = 0.73) and 20 min (r = 0.85) for path length, respectively. Moreover, the observed patterns differed between brain states, with more consistent effects observed in the EO state. These findings suggest that neuromodulatory responses may vary depending on both tACS duration and brain state, highlighting the importance of considering both factors in the design of tACS protocols and interpretation of neuromodulatory effects.