Beta tACS of varying intensities differentially affect resting-state and movement-related sensorimotor power

Individuals who face difficulties with voluntary movement experience considerable challenges in performing everyday tasks, significantly compromising their sense of autonomy. Transcranial alternating current stimulation (tACS) holds promise in modulating sensorimotor beta oscillations, which underscore voluntary movement. However, the exact effect of beta tACS on oscillatory power is still largely elusive. This study aimed to examine the effect of different intensities of beta tACS (20 Hz) on both resting-state and event-related sensorimotor oscillations. Twenty-one healthy young adults (13 female; mean age 24.30 ± 4.84 years) received four separate 20 min sessions of tACS at different intensities (sham, 0.5 mA, 1.0 mA, or 1.5 mA, peak-to-peak), targeting the left primary motor cortex during rest. Electroencephalography (EEG) was recorded before and after stimulation, during both resting state and a self-paced right index finger button press task. Changes in sensorimotor beta power (13-30 Hz) were analyzed. For the resting-state, none of the real stimulation intensities induced significant changes in beta power relative to sham. For event-related activity, we observed intensity-dependent changes in bilateral broadband power (4-90 Hz): during movement preparation, 1.0 mA stimulation increased power; during movement termination, 0.5 mA stimulation decreased power while 1.0 mA and 1.5 mA stimulation induced comparable increases in power. While none of the stimulation intensities induced changes in broadband power during movement execution, 1.0 mA stimulation shifted participants' peak beta frequency toward the tACS frequency. Interestingly, changes in power during movement preparation and execution following 1.0 mA stimulation were negatively associated with participants' pre-tACS peak beta frequency. Together, these findings contribute to our understanding of the sensorimotor response to beta tACS, as well as the effect of stimulation intensity on tACS-induced neuromodulation, which has important implications for research and clinical settings.