Sub-threshold transcranial magnetic stimulation does not facilitate reaction time for complex movements beyond speeded sensory effects


Presentation of a startling acoustic stimulus (SAS) during a simple reaction time (RT) task significantly reduces RT, thought to be due to a SAS involuntarily triggering the release of a cortically stored motor program. In a previous study we applied sub-threshold transcranial magnetic stimulation (TMS), which increases excitability of motor cortex, in the RT interval following a SAS. TMS significantly reduced control RT, but had no effect on startle RT, suggesting that cortex has limited involvement in startle-triggered movements; however, it is also possible the simple wrist movement used led to a floor effect of startle RT. In the present experiment a more complex movement was used to investigate cortical involvement in startle-facilitated RTs, as increased complexity results in longer control and startle RTs, providing a greater available range for RT facilitation by sub-threshold TMS in the startle condition. Participants completed a simple RT task requiring a three-key press movement in response to an auditory go-signal, which was randomly replaced by a SAS on 25% of trials. On a subset of trials real or sham TMS was applied over motor cortex 30ms following the go-signal or 15ms following the SAS. Results showed that following the control go-signal, both sham and real TMS significantly reduced RT; however, there was no difference between the conditions or between any of the SAS conditions. These results suggest that complex movements are not susceptible to RT facilitation by sub-threshold TMS in the same manner as simpler movements, despite showing similar facilitation in response to a SAS.

Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation