Sub-threshold transcranial magnetic stimulation applied after the go-signal facilitates reaction time under control but not startle conditions.

The presentation of a startling acoustic stimulus (SAS) in a simple reaction time (RT) task significantly reduces RT due to the involuntary early initiation of a prepared movement; however, the underlying neural mechanism remains unclear. It has been proposed that a SAS triggers a cortically stored motor program by involuntarily increasing initiation-related activation. Sub-threshold transcranial magnetic stimulation (TMS) can be used to investigate cortical processes, as it increases cortical excitability for 6-30 ms and significantly reduces RT. The purpose of the present experiments was to determine whether the application of sub-threshold TMS over motor cortex in close temporal proximity to a SAS would facilitate startle RT in the same manner as control RT, providing evidence for cortical involvement in startle-related RTs. Participants completed a simple RT task requiring targeted wrist extension in response to an auditory go-signal, which was randomly replaced by a SAS on 25% of trials. On a subset of trials, sub-threshold TMS was applied 30 ms following the go-signal in control trials or at -15, 0, +15 or +30 ms with respect to the SAS in startle trials. In all three experiments, sham and real TMS significantly reduced RT in control trials, with real TMS having a larger effect, but there was no effect of either real or sham TMS on startle-related RT. These results suggest that there may be limited cortical involvement in the initiation of movements in response to a SAS. As an alternative, startle may produce the fastest possible RTs, with little room for additional facilitation.