The effect of tACS on cortical excitability measures. Average MEP amplitude (±SEM) values for MT_1mV (A), ICF (B), and SICI (C) at baseline (BL), T1 (5 min), and T2 (15 min) and after tACS for all sessions: beta, gamma, and sham. D, Individual subject data showing MEP amplitude values for SICI at BL, T1 and T2 in gamma (left) and sham sessions (right). MEP amplitudes are given relative to baseline (set as 100%). SICI amplitude was increased at T1 by gamma tACS compared with sham session (black asterisks) and compared with T2 in gamma session (pink asterisks). *p < 0.05; **p < 0.005.