Preparatory suppression of the human primary motor cortex induced by repetition of simple and choice reaction time tasks: a transcranial magnetic stimulation study.

Transcranial magnetic stimulation was performed to investigate preparatory suppression of activity in the human primary motor cortex (M1) in relation to trial repetition of simple (SRT) and Go/NoGo choice RT (CRT) tasks. These tasks were performed in such a way that after a warning signal, the subjects (N=16) maintained 5% MVC isometric finger force against the force sensor to secure a facilitated state of M1. A response signal to generate pulsed force came at 2 s after the warning signal. TMS was given 1.5 s after the warning signal, and the amplitudes of motor-evoked potentials (MEPs) in the first dorsal interosseous muscle were evaluated during 30 repetitive trials over 3 sessions for each subject. For the SRT task, the MEP amplitude was significantly decreased from baseline values in all trials of the three sessions. For the CRT task, on the other hand, there was a clear decreasing trend of the MEP amplitude with trial at the first and second sessions. The mean MEP amplitude at the first session was clearly higher than the baseline while it decreased significantly and reached the value below the baseline at the third session. The findings indicate that active suppression of M1 activity is involved in the preparatory state for RT tasks and that the degree of this suppression can relate to trial experience. The effect is thus most likely a consequence of a rapid adaptive change with the central nervous system in optimizing the preparatory state of M1 for the upcoming motor response.