Traditional tenets about the hard-wired nervous system have long been dispelled with mounting evidence for activity-dependent plasticity throughout the CNS. Fascinating results from animal, and more recently, human studies have shown that even the “simplest” spinal cord reflex, the stretch reflex pathway or its electrical analog, the H-reflex, can be altered to increase or decrease in size through operant conditioning (Wolpaw, 2010). In animals, a reward is provided whenever the H-reflex amplitude is above or below a threshold value. Through modulation of descending influence, the animal can gradually learn to maintain its H-reflex amplitude at a certain level. Humans can also learn to increase or decrease the size of the soleus H-reflex (Thompson et al., 2009). Some gait impairments following SCI could be associated with hyperreflexia and abnormal reflex responses in the ankle plantarflexors (Dietz & Sinkjaer, 2007). The possibility that H-reflex amplitude could be down-conditioned raises the compelling question of whether such protocols may benefit individuals with SCI who present with spastic gait disorder.
This idea was recently tested in a group of 13 individual with chronic (>8 months) motor-incomplete SCI who all were ambulatory and presented with spasticity (e.g. ³1 on Modified Ashworth Scale) and weak ankle dorsiflexion (Thompson et al., 2013). Participants were randomly assigned at a 2:1 ratio to the down-conditioning (DC) group (n=9) or the unconditioned (UC) group (n=4). Each participant completed 6 baseline sessions followed by 30 sessions (3 sessions/week) of control (UC group) or conditioning (DC group). Visual feedback was provided to the DC group to inform them of whether they were successful in reducing their H-reflex amplitude to within the target range. In the UC group, each session involved H-reflex recordings without any visual feedback or instructions about H-reflex amplitude. Note that in this study, no locomotor training was provided; training sessions consisted of only the H-reflex down-conditioning (or control protocol).