Enhancing Encoding of a Motor Memory in the Primary Motor Cortex By Cortical Stimulation

Motor training results in encoding of motor memories, a form of use-dependent plasticity. Here we tested the hypothesis that transcranial magnetic stimulation (TMS) synchronously applied to a motor cortex engaged in a motor training task could enhance this plastic process. Healthy volunteers were studied in four sessions: training consisting of performance of directionally specific voluntary thumb movements ( Train alone), training with TMS delivered during the execution of the training movement in a strictly temporal relationship to the motor cortex contralateral ( Train+ TMS synchronouscontra) and ipsilateral ( Train+ TMS synchronousipsi) to the training hand, and training with TMS delivered asynchronous to the training movement to the motor cortex contralateral to the training hand ( Train+ TMS asynchronouscontra). Train alone, Train+ TMS synchronouscontra, and Train+ TMS asynchronouscontra but not Train+ TMS synchronousipsi elicited a clear motor memory. The longevity of the encoded memory was significantly enhanced by Train+ TMS synchronouscontra when compared with Train alone and Train+ TMS asynchronouscontra. Therefore use-dependent encoding of a motor memory can be enhanced by synchronous Hebbian stimulation of the motor cortex that drives the training task and reduced by stimulation of the homologous ipsilateral motor cortex, a result relevant for studies of cognitive and physical rehabilitation.