Abstract
Previous studies have shown that corticospinal excitability (CSE) decreases during the foreperiod of a simple reaction time (RT) task, just prior to the go-signal. The underlying mechanism for this pre-movement suppression of CSE remains debated. Some researchers propose it may be related to movement preparation/suppression processes, while others argue it is tied to processes associated with movement initiation. To examine these possibilities, we used single-pulse transcranial magnetic stimulation (TMS) to probe CSE via motor-evoked potential (MEP) amplitude at various time points during the foreperiod of a simple RT task. On most trials (control trials, 86%), the duration of the foreperiod was fixed at 500ms, whereas on the remaining trials (delay trials), the foreperiod was extended by 500ms (i.e., 1000ms total) to introduce an unexpected delay. As expected, results showed that on control trials, MEP suppression occurred just prior to the actual movement. Critically, it was hypothesized that if CSE suppression is linked to movement initiation, MEP decreases would also be observed just prior to movement onset on delay trials. However, contrary to this prediction, on delay trials MEP suppression was time-locked to the expected (500ms), rather than the actual (1000ms), timing of the go-signal, and was sustained until movement onset. The temporal dissociation between MEP suppression and movement onset on delay trials suggests that pre-movement suppression may be more linked to movement preparation than movement initiation processes.
