Response preparation of synchronous hand-foot movements differs depending on the mode of control


Synchronizing hand and foot movements under reactive versus predictive control results in differential timing structures between the responses. Under reactive control, where the movement is externally triggered, the EMG responses are co-activated, resulting in the hand displacement preceding that of the foot. Under predictive control, where the movement is self-paced, the commands are organized such that the displacement onset is synchronous, rather than the EMG. The current study used a startling acoustic stimulus (SAS), which is known to involuntarily trigger a prepared response to investigate whether these results are due to differences in the pre-programmed initiation timing structure of the responses. Participants (n=17) were asked to simultaneously raise both their right heel and right hand under both reactive and predictive modes of control. In the reactive condition, participants were required to perform the synchronous movement as quickly as possible in response to a visual go-signal. The predictive condition involved an anticipation-timing task where participants were required to coincide their synchronous movement with a sweeping clock hand reaching a target. On a subset of trials, a SAS (114 dB) was presented 150 ms prior to the imperative stimulus. Results showed that while response initiation latencies were significantly shorter on SAS trials, the differential timing structures between the hand-foot responses seen under reactive or predictive conditions was maintained when involuntarily triggered by the SAS. These results suggest that the timing between the individual responses, which differs between the reactive versus predictive mode of control, is pre-programmed and executed as a single command.

Acknowledgments: Acknowledgements: Supported by NSERC