AbstractMechanical perturbations to the arm elicit short- and long-latency reflexes (SLR 25-50 ms; LLR 50-100 ms) as well as a voluntary response (beginning at ~100 ms) in stretched muscles (Hammond 1956). It is well documented that the EMG activity during the LLR modulates based on behavioural goals similar to the subsequent voluntary activity (Scott 2016). Interestingly, it has been suggested that such goal-dependent "reflex" activity and perturbation-triggered voluntary reaction may be impervious to increased attentional demands (Schmidt 1987). The purpose of the current study was to investigate the influence of increased attentional demands on modulation of the LLR and onset latency of a voluntary response using a psychological refractory period (PRP) paradigm. Participants performed a verbal response (Response 1) to the onset of an auditory stimulus (S1), while also reacting to an extension perturbation of the wrist (S2) with a rapid flexion response (Response 2). The stimulus onset asynchrony (SOA) between S1 and S2 varied from 100-1000 ms. Results indicated that the voluntary response to the perturbation was susceptible to a PRP effect (delayed at short SOAs), however the amount of delay was much smaller than traditional PRP studies. By contrast, the magnitude of the LLR was facilitated to the greatest extent on these short SOA trials, possibly due to intersensory facilitation. We also found instances of response reversals at short SOAs whereby the voluntary response to S2 began prior to the vocal response to S1, which may be attributed to the fast RT associated with responding to a perturbation.
Acknowledgments: Natural Sciences and Engineering Research Council of Canada; BrainsCAN; Canada First Research Excellence Fund