Task paradigm used to assess corticospinal excitability. A. Experiment I. In both grating and smooth trials, participants were trained to produce a single gentle downwards right index finger movement in sync with the sound of a tone lasting for 1.8 s. During the smooth trials, the finger moved downwards to touch a relatively smooth grating dome (0.5 mm spatial period) with no discrimination requirement. During the grating trials, participants touched a coarse grating dome (1.5 or 3 mm spatial period) and reported whether the grooves ran along or across the fingertip. In each trial, the TMS pulse was set to trigger over the left motor cortex 1.5 s after the tone, corresponding to the time point in the trial when the finger was touching the dome and the participant was actively discriminating during the grating trials. Contralateral MEPs were recorded in the task hand. B. Experiment II. The task conditions were identical to Experiment I except that both the right and left hands were tested in Experiment II, and contralateral MEPs were always recorded in the resting hand instead of the task hand.