Transcranial Alternating Current Stimulation Online and Offline Effects on the Primary Motor Cortex Shown by TMS-Induced Motor Evoked Potentials

Transcranial alternating current stimulation (tACS) is non-invasive brain stimulation method that allows to stimulate the brain at specific frequency. It has potential in clinical application (as potential drug-free treatment of neurological and psychiatric disorders), for cognitive enhancement in healthy individuals and as research tool to study brain rhythms. However, physiological effects of tACS are not understood yet. TACS was shown to be efficient to modulate primary motor cortex in frequency-dependent and state-dependent way during stimulation. Studies on tACS aftereffects on primary motor cortex excitability measured by TMS-induced motor evoked potentials (MEPs) revealed mixed results. The primary goal of this study is investigation of 10 Hz (alpha) and 20 Hz (beta) tACS and sham aftereffects on MEPs peak-to-peak amplitude in different time windows (seven blocks of MEPs with 5 min break between them) to reveal the dynamics of cortical excitability after stimulation. TACS was applied for 15 min with 1 mA intensity over primary motor cortex (M1) measured with navigated TMS as hotspot for first dorsal interosseous (FDI) muscle stimulation. Reference electrode was placed on ipsilateral shoulder. We found no significant aftereffects of tACS on MEPs amplitude for all three conditions. The present findings suggest that beta entrainment of primary motor cortex by tACS falls down rapidly after the end of tACS delivering.