Abstract
Perturbations applied to the upper limbs can elicit multiple responses in the stretched muscle [e.g. short latency (M1:25-50ms) spinal reflex, longer-latency (M2:50-100ms) spinal/supra-spinal reflex, and voluntary response (>100ms)]. Recently, Ravichandran (2013) delivered limb perturbations simultaneous with an auditory go signal on a small proportion of unexpected RT trials. It was found that perturbations elicited a startle reflex that acted to trigger the voluntary response at significantly shortened latency (~70ms; StartReact effect), resulting in superimposition of the voluntary response onto M2. The present study examined the conditions under which limb perturbations might elicit a startle reflex and trigger the voluntary response. In Part 1, participants (N=8) performed in a RT wrist flexion task in response to an 80dB auditory stimulus or a small wrist perturbation (0.5Nm). On unexpected (20%) trials we delivered a large (1.5Nm) wrist perturbation or a 120dB startling acoustic stimulus (SAS). While the SAS evoked a startle reflex on 93% of trials, reflexive startle activation was only observed on 13% of perturbation trials. In Part 2, participants performed a RT task in response to an expected 1.5Nm perturbation and no incidence of a startle reflex was observed; however, the perturbation still elicited the prepared response at short latency (<100ms). The findings of this study suggest that upper limb perturbations can trigger rapid voluntary responses that superimpose onto M2. However, while unexpected upper limb perturbations can also evoke a startle reflex it is unlikely that the voluntary response is triggered by the same circuitry responsible for the auditory StartReact effect.
Supported by NSERC
