The efficacy of two protocols for inducing motor cortex plasticity in healthy humans--TMS study.

Stimulation-induced plasticity represents an experimental model of motor cortex reorganization. It can be produced in awaked humans by combining the non-invasive electrical stimulation of somatosensory afferents via mixed peripheral nerves with the transcranial magnetic stimulation (TMS) of the motor cortex. Animal experiments indicate that an application of two converging inputs from various sources in a tightly coupled manner, following the so called Hebbian rule of learning, leads to an increase in motor cortical excitability. The aim of our study was to compare the effects of two plasticity-inducing protocols by quantifying the motor cortex changes using TMS. Plasticity was induced by combining peripheral nerve stimulation with TMS (paired associative stimulation - PAS) and by peripheral motor point stimulation of two adjacent hand muscles (dual associative stimulation - DAS). The protocols were randomly applied in 12 right-handed healthy volunteers. The amplitudes of TMS-induced motor-evoked potentials (MEPs) in the right abductor pollicis brevis muscle were recorded before, immediately after PAS or DAS stimulation, and 10, 20 and 30 min later. Both protocols led to significant and lasting changes in MEP amplitudes, however, a significantly larger increase in MEPs was observed after PAS than DAS. The results indicate that afferent input can differently affect cortical motor circuits and produce variable motor output. Thus, the efficacy of LTP-like mechanisms, presumably involved in Hebbian-like plasticity in humans, varies with the types/origin of the converging inputs. Our findings may be relevant when designing therapeutic interventions for improving motor function after neurological injury or disease.