Two types of TMS-induced movement variability after stimulation of the primary motor cortex

Using transcranial magnetic stimulation, we studied the role of the primary motor cortex (M1) in repetitive movements, examining whether the functional contribution of this region is associated with controlling response timing, response implementation, or both. In two experiments, participants performed a rhythmic tapping task, attempting to produce isochronous intervals (range of 350-550 ms) while stimulation was applied over M1 or a control site. M1 stimulation was associated with increased variability of the inter-tap intervals (ITI), and, by manipulating stimulation intensity, we identified two distinct changes in performance: a generalized increase in ITI variability and a delay in the subsequent response when the pulse fell within a restricted window prior to movement onset. Using a series of simulations, we demonstrate that the general increase in variability and the temporally specific delay reflect disruption of response implementation processes rather than an increase in noise associated with response timing.