Abstract
Mechanical perturbations delivered to the upper limbs elicit reflexive responses in stretched muscle at short- (M1:25-50 ms) and long- (M2:50-100 ms) latencies. When presented in a simple reaction time (RT) task, the perturbation can also elicit a preprogrammed voluntary response at a latency (premotor RT values ~70 ms) that overlaps the M2 response. This early elicitation of the voluntary response by a perturbation has been called a triggered reaction (Houk 1978). Recent work has proposed that unexpected mechanical perturbations may also elicit a reflexive startle response and therefore the StartReact effect underlies initiation of triggered reactions (Ravichandran et al. 2013). The present study investigated whether triggered reactions can also be elicited at short-latency in the absence of a startle response. Twelve participants performed ballistic wrist extension movements following an expected wrist extension perturbation imperative signal. The perturbation elicited stretch responses (M1/M2) in wrist flexors and the preprogrammed voluntary response in wrist extensors. To make comparisons with the StartReact effect, a startling auditory stimulus (SAS) was also presented on random trials. While the SAS consistently elicited a startle response in orbicularis oculi and sternocleidomastoid on 68.2% of trials, the perturbation did not reliably elicit a startle response. Despite this, two-thirds of perturbation-only trials had premotor RTs of less than 100 ms and the earliest responses began at ~70 ms. These findings suggest that an overt startle response is not required for the early elicitation of a triggered reaction via a mechanical perturbation.Acknowledgments: NSERC