Abstract
Application of sub-threshold transcranial magnetic stimulation (TMS) over motor cortex has been shown to facilitate reaction time (RT), likely by increasing motor cortical activation. Yet, a similar effect was not observed for movements triggered early by a startling acoustic stimulus (SAS), supporting the hypothesis that a SAS acts by triggering a sub- cortically stored motor program without cortical involvement. An alternative explanation is that the simple movement used in previous experiments was already maximally speeded by the startle and no further RT facilitation was possible. The present experiment tested this by employing a more complex movement, which has previously resulted in increased startle RT. It was expected that this would provide a greater available range for sub-threshold TMS to facilitate RT during startle trials. In two experiments, participants performed a simple RT task requiring three keypresses with varying timing complexity following an auditory go-signal or a SAS. On a subset of trials, subthreshold TMS or sham TMS was applied over the motor cortex 30ms following the go-signal or 15ms following the SAS. Results showed that increased timing complexity increased RT in control trials, but in contrast to previous reports had no effect on startle RTs. Additionally, both real and sham TMS facilitated RT in control trials, whereas there was no effect of either stimulation condition on startle RT. As increased complexity did not increase RT for startle trials it cannot be concluded whether there is limited cortical involvement in SAS-triggered movements, or if an RT floor was reached.Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation and Science