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

Abstract

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.

High-intensity transcranial magnetic stimulation reveals differential cortical contributions to prepared responses

“…It is also possible that the shorter delays seen in startle trials were due to an incomplete, or "partial," response being triggered by the SAS; however, analysis of EMG data revealed agonist muscle bursts (iEMG 30 ) were in fact larger in SAS trials. Small to moderate increases in the integrated EMG of the prime mover have been previously reported in studies employing an SAS (Carlsen et al 2013;Siegmund et al 2001;Smith and Carlsen 2018), with the interpretation that this could be the result of startle-related drive summing with the voluntary activation. Indeed, the startle reflex typically exhibits as a flexor-dominant response (Landis et al 1939), so increased startle-related activation may be expected.…”

“…As such, TMS applied more than 50 ms before agonist EMG burst onset in a RT paradigm can be used to investigate the involvement of voluntary cortical drive to movement preparation and initiation. With RTs of~130 ms commonly seen for targeted wrist movements in response to an auditory go signal (Maslovat et al 2014;Smith and Carlsen 2018), TMS applied 80 ms following the go signal falls within the expected time interval for cortical suppression.…”

“…In SAS trials with TMS, TMS was applied 30 ms following the SAS (170 ms before the imperative go signal). RTs of~80 -90 ms are often seen in response to an SAS for targeted wrist extension movements (Maslovat et al 2014;Smith and Carlsen 2018;Stevenson et al 2014); thus this timing of TMS application occurred~50 ms before the expected agonist EMG burst onset, allowing investigation of cortical involvement in StartReact responses.…”

Response preparation of a secondary reaction time task is influenced by movement phase within a continuous visuomotor tracking task

“…MEP amplitudes were analysed using similar analysis but using only Phase (Start, Middle, End) as a fixed factor. Premotor RT from TMS trials was not analysed or compared with control or SAS RT because of the confounding facilitatory effect of TMS on RT when applied close to the go‐signal (Pascual‐Leone et al, 1992; Smith & Carlsen, 2018; Soto et al, 2010). SCM proportion was analysed using a repeated measures analysis of variance (RM‐ANOVA) to examine for differences in SCM activation dependent on Phase (Start, Middle, End).…”

“…TMS can have a confounding and/or facilitatory effect on RT when applied near or at the presentation of a go‐stimulus (Pascual‐Leone et al, 1992; Smith & Carlsen, 2018; Soto et al, 2010). As such, premotor RT from the TMS trials was not analysed or compared with RT collected from control and SAS trials in the present experiment.…”