Anticipation and long-latency reflex modulation


Perturbations applied to the upper limbs elicit short (M1: 25-50 ms) and long-latency (M2: 50-100 ms) reflexes 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. This reflexive activity is followed shortly thereafter by a voluntary response. While many studies have examined modulation of M2 between passive and active conditions, through the use of constant foreperiods, it has also been shown that M2 size in a passive condition can change based on factors such as habituation and anticipation of perturbation delivery (Rothwell et al., 1986). The purpose of the present study was to further examine the influence of temporal anticipation on M2 modulation. Fifteen participants performed active and passive responses to a perturbation which 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 conditions for M1 (all p values >.10), and M2 was larger (p=.005) in the active rather than passive conditions. Interestingly, within the two passive conditions, the long variable foreperiods resulted in a larger M2 (p=.045) than the trials with short foreperiods. These results suggest that perturbation predictability, even when using a variable foreperiod, can influence excitability of the pathway(s) contributing to the long-latency reflex. 

Acknowledgments: This research was supported by NSERC