An examination of the startle response during upper limb stretch perturbations.
Unexpected presentation of a startling auditory stimulus (SAS>120 decibels) in a reaction time (RT) paradigm results in the startle reflex and an early release (<100ms) of the preplanned motor response (StartReact effect). Mechanical perturbations applied to the upper limbs elicit short- (M1) and long-latency (M2) stretch reflexes and have also been shown to initiate intended motor responses early (<100ms). Ravichandran et al. (2013) recently proposed that unexpected delivery of a perturbation could also elicit a startle response and therefore the StartReact effect may be responsible for the early trigger of a preplanned response. To investigate this further, we examined startle incidence, RT, and stretch reflex modulation for both expected and unexpected perturbations. In Experiment 1, participants performed active (ACT) and passive (DNI) conditions to an expected large perturbation (similar to previous studies examining M2). The startle response was not observed; however, the perturbation still elicited the voluntary response at short latency (<100ms) and goal-dependent modulation of the M2 response was observed. In Experiment 2, participants performed ACT and DNI conditions to a weak auditory stimulus or a small wrist perturbation. On unexpected trials we probed startle circuitry with a large perturbation or SAS. The SAS consistently elicited a startle response in both ACT and DNI conditions, but startle-like activity was only observed on 17.4% of ACT perturbation probe trials. Our findings suggest that while unexpected upper limb perturbations can be startling, startle triggering of the preplanned voluntary response is not the primary mechanism responsible for goal-dependent modulation of the M2 response.
School of Kinesiology, University of British Columbia, Canada.
Christopher J
CJ
Forgaard
School of Kinesiology, University of British Columbia, Canada.
Ian M
IM
Franks
School of Kinesiology, University of British Columbia, Canada.
Dana
D
Maslovat
School of Kinesiology, University of British Columbia, Canada.
Nicolette J
NJ
Gowan
School of Kinesiology, University of British Columbia, Canada.
Jonathan C
JC
Kim
School of Kinesiology, University of British Columbia, Canada. Electronic address: Romeo.Chua@ubc.ca.
Romeo
R
Chua
D000293:Adolescent; D000328:Adult; D004576:Electromyography / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D009068:Movement / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D013216:Reflex, Startle / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology*; D034941:Upper Extremity / Q000502:physiology*; D014953:Wrist / Q000502:physiology*; D055815:Young Adult
M2; StartReact effect; long-latency reflex; reaction time; startle reflex