Perturbation Predictability Can Influence the Long-Latency Stretch Response

Perturbations applied to the upper limbs elicit short (M1: 25-50 ms) and long-latency (M2: 50-100 ms) responses in the stretched muscle. M1 is produced by a spinal reflex loop, while M2 receives contribution from a longer trans-cortical pathway and is susceptible to intention. Thus when the participant is asked to counteract the perturbation, M1 is usually unaffected while M2 increases in size. While many studies have examined modulation of M2 between passive and active conditions, through the use of constant foreperiods (interval between warning signal and a perturbation), it has also been shown that the magnitude of M2 response in a passive condition can change based on factors such as habituation and anticipation of perturbation delivery. To prevent anticipation of a perturbation, most studies have used variable foreperiods; however the range of possible foreperiod duration differs between experiments. The present study examined the influence of different variable foreperiods on modulation of the M2 response. Fifteen participants performed active and passive responses to a perturbation that stretched wrist flexors. Each block of trials had either a short (2.5-3.5 seconds; high predictability) or long (2.5-10 seconds; low predictability) variable foreperiod. As expected, no differences were found between any conditions for M1, while M2 was larger in the active rather than passive conditions. Interestingly, within the two passive conditions, the long variable foreperiods resulted in a larger M2 (p=.006) than the trials with short foreperiods. These results suggest that perturbation predictability, even when using a variable foreperiod, can influence excitability of the circuitry contributing to the long-latency stretch response.