Abstract
Perturbations of the arm produce short (M1; 25-50 ms) and long-latency (M2; 50-100 ms) responses in stretched muscle. In tasks requiring an intended movement, the voluntary response is also elicited at a short enough latency (<100 ms) to overlap onto the M2 response and contribute to goal-dependent M2 modulation. It has also been proposed that mechanical perturbations elicit a startle response (Ravichandran et al. 2013). During situations in which advance preparation can occur, the presentation of a startling stimulus not only activates startle circuitry, which can be observed in the sternocleidomastoid (SCM), but also results in a hastened onset of the preprogrammed response (termed the StartReact effect). The implication is that if a perturbation is startling, the StartReact effect could be responsible for the fast voluntary response and subsequent goal-dependent M2 modulation. However, it has also been suggested that the observed SCM activation following a perturbation may not be startle-related but instead be a result of a postural response (Forgaard et al. 2016). In order to differentiate between these alternatives, the current study employed a PPI stimulus, which is known to attenuate the startle response but would not be expected to affect postural SCM activation. Participants performed a compensate task against a perturbation and on random trials, a PPI stimulus was delivered 100 ms before the perturbation. Rather than attenuating SCM activity, the SCM responses following the perturbation were advanced on trials with a PPI stimulus. Our findings confirm that perturbation SCM activity is the result of a postural response and not the startle response.Acknowledgments: NSERC