Movement-Related Cortical Stimulation Can Induce Human Motor Plasticity

Repeated paired associative stimulation combining peripheral nerve stimulation and transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) can produce human motor plasticity. However, previous studies used paired artificial stimuli, so that it is not known whether repetitive natural M1 activity associated with TMS can induce plasticity or not. To test this hypothesis, we developed a movement-related cortical stimulation (MRCS) protocol, in which the left M1 was stimulated by TMS at specific timing with respect to the mean expected reaction time (RT) of voluntary movement during a simple reaction time task using the right abductor pollicis brevis (APB) muscle. Seventeen normal volunteers were subjected to repeated MRCS intervention (0.2 Hz, 240 pairs). Motor function was assessed before and after MRCS. When TMS was given 50 ms before the RT of movement [MRCS(-50)], motor-evoked potential (MEP) amplitude of the right APB, but not other muscles, increased for up to 15 min post-MRCS. The RT of the right APB was also shortened. However, spinal excitability measured by F-wave did not change. When TMS was given 100 ms after the RT [MRCS(+100)], MEP amplitude was decreased. These findings show that this new MRCS protocol can produce timing-dependent motor associative plasticity, which may be clinically useful.