Evidence for Subcortical Plasticity After Non-invasive Paired Stimulation Protocols

Most current non-invasive plasticity protocols target the motor cortex and its corticospinal projections. Approaches for inducing plasticity in sub-cortical circuits and alternative descending pathways are less well developed. We used a novel associative stimulation paradigm, which paired loud auditory stimuli with transcranial magnetic stimulation over the motor cortex in healthy participants and observed enhanced motor output after stimulus pairing ended. Direct measurements in monkeys undergoing a similar protocol implicate corticoreticular connections as the most likely substrate for the plastic changes. Using a wearable device capable of pairing auditory clicks with electrical stimulation over a muscle for extended periods, we observed changes in electrophysiological measurements consistent with the induction of subcortical plasticity in the biceps. However, both in healthy volunteers and in people with spinal cord injury, there was no evidence of this effect in the triceps, suggesting elbow flexors and extensors differ in their ability to undergo plastic changes.