Premovement suppression of corticospinal excitability is modulated by reaction time task requirements

The amplitude of motor-evoked potentials (MEPs) elicited using transcranial magnetic stimulation (TMS) has been shown to decrease in the short interval prior to response initiation. The cause of this premovement MEP suppression is currently unclear and has been attributed to various processes such as preparation-related inhibition preventing the premature release of planned action or increasing signal-to-noise ratio to facilitate rapid response initiation. The present study explored whether the decrease in MEP amplitude is affected by the task requirements, using reaction time (RT) paradigms that differ in the timeline of preparation and initiation of a motor response. Participants completed simple RT (SRT), choice RT (CRT), and go/no-go (GNG) tasks, 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 during the SRT and GNG tasks, as these paradigms encourage advance preparation and response inhibition. Conversely, if the reduction in corticospinal excitability is associated with facilitating response initiation processes, then suppression would be expected for all tasks, including the CRT paradigm in which preparation does not occur until presentation of the go-signal. Results showed MEP amplitudes decreased for all tasks as the go-signal approached; however, both the SRT and GNG had significantly greater MEP suppression 50 ms prior to, and coincident with the go-signal. These results indicate that the nature and origin of the suppression is likely multifactorial and relates to both preparatory and initiation-related processes, with the timeline and magnitude of suppression dependent on the nature of the task being executed.