Abstract
Presenting a startling acoustic stimulus (SAS) during a simple reaction time (RT) task significantly decreases RT; however, the neural mechanism behind this early response triggering is not fully understood. One hypothesis is that a SAS leads to an involuntary increase in initiation-related activation processes via a sub-cortically mediated pathway, triggering a cortically stored response without engaging the usual cortical processing. Subthreshold transcranial magnetic stimulation (TMS) applied over motor cortex has been shown to increase cortical excitability in the time period 6-30ms after the pulse (termed intracortical facilitation [ICF]). ICF protocols have also been shown to reduce simple RT. The purpose of this experiment was to examine whether subthreshold TMS applied to motor cortex early in the RT interval following a SAS would result in a further decrease in RT, providing evidence the cortex is involved in SAS-triggered responses. Participants completed a simple RT task where a SAS was occasionally presented concurrent with the go-signal. On selected trials TMS (or sham TMS) was applied in the first 30 ms following the go-signal during control and SAS trials. Results from control trials revealed that TMS led to a significant decrease in RT (~30 ms), with sham TMS showing a decrease of smaller magnitude (~15 ms). Furthermore, RT on SAS trials was significantly faster than on control trials; however, no additional decrease in RT was observed following real or sham TMS. These results suggest that either cortical involvement is limited/bypassed during SAS trials or that startle leads to a ceiling effect on RT facilitation.Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation