Abstract
The amplitude of motor-evoked potentials (MEPs) elicited using transcranial magnetic stimulation (TMS) has been shown to decrease in the brief interval (100-300ms) prior to response initiation in reaction time (RT) tasks. The underlying cause of this pre-movement MEP suppression is currently unclear. While some researchers suggest it is indicative of preparation-related inhibition processes preventing the premature release of planned action (i.e., a false start; Greenhouse et al. 2015), others have suggested it represents corticospinal suppression required to initiate a specific motor plan (Ibáñez et al. 2020). To differentiate between these proposed explanations, the present study explored whether the decrease in MEP amplitude leading up to a go-signal is affected by the preparation level of a motor response. Participants completed blocks of simple RT (SRT), choice RT (CRT), and go/no-go (GNG) tasks using a fixed 500ms foreperiod, while TMS was applied at various times between the warning signal and go-signal. It was hypothesized that if MEP suppression relates to preparation level, the greatest suppression would be observed prior to the SRT task since this paradigm allows for the largest amount of advance preparation (Donders, 1969). Results showed MEP amplitudes decreased for all tasks as the go-signal approached; however, both the SRT and GNG had smaller MEP amplitudes 50ms prior to, and coincident with the go-signal compared to the CRT (the task presumably allowing for the least amount of advance preparation). These results suggest the premovement suppression of MEP amplitude may be at least partially attributable to level of response preparation.
