Patellar tendon-tap intensity modulates response gain but not variability across cortical proprioceptive and stretch-reflex responses

Cortical proprioceptive processing to passive movements can be quantified with evoked and induced responses in magnetoencephalographic (MEG) signals. We examined how intensity of proprioceptive stimulation (i.e., tendon-tap evoked movement) scales cortical processing of proprioceptive afference from the knee joint, and whether it affects stimulus-to-stimulus response variability. Twenty-one healthy volunteers (28.2{+/-}5 yr, 10 females) underwent a total of 100 right patellar tendon taps (5.0--6.0s inter-stimulus interval) evoked by a novel MEG-compatible stimulator under higher and lower intensity conditions. Strength of cortical evoked and induced MEG and electromyographic responses from the vastus lateralis and medialis muscles were quantified, and their variability were assessed using a matched sliding-window coefficient of variation (CoV). Higher intensity stimulation elicited significantly stronger cortical evoked (~11%) and induced (~17% for beta suppression and ~18% for beta rebound) responses, and muscular response (~27%) than lower intensity stimulation (p<0.05). The stimulus intensity did not affect response variability, with comparable CoV values between higher and lower intensity condition (p>0.05), but the muscular responses showed greater variability than the cortical ones in both intensity conditions (CoV: ~30% vs. ~10%, p<0.001). In addition, the variability in cortical and muscular responses were not significantly correlated (p>0.05). Our results indicate that patellar tendon-tap intensity scaled cortical and muscular response gain, but did not measurably alter response variability. The distinct variability profiles of cortical evoked fields and stretch-reflex muscular response suggest that proprioceptive afference evoked by tendon-tap is transformed differently across cortical and spinal levels.