Cortico-cortical associative stimulation induced increases in the excitability of corticospinal projections in humans

Bursts of tACS (500 ms; 140Hz; 1mA) were associated (180 stimulus pairs) with single magnetic stimuli (120% resting motor threshold rMT) delivered over the opposite (left) M1. The tACS ended 6 ms prior to the TMS. In a separate condition, TMS (55% rMT) was delivered to right M1 6 ms before (120% rMT) TMS was applied over left M1. In a sham condition, TMS (120% rMT) was delivered to left M1 only. Prior to each intervention (Pre), a MEP recruitment curve was obtained by delivering TMS over the left hemisphere at 10% increments of intensity between 90% and 160% of the rMT. Six stimuli were delivered at each level of intensity. A further twelve stimuli were delivered at 120% rMT. Equivalent sets of stimuli were delivered immediately following the intervention (Post0) and at 10 (Post10), 20 (Post20) and 30 (Post30) minutes thereafter