Changes in motor cortical excitability induced by paired associative stimulation

Objective: Changes in afferent input have been shown to be capable of inducing reorganisations of motor cortex in humans. Using TCMS we examined the efficacy of a new associative afferent stimulation paradigm in inducing motor cortical reorganisation in humans. Methods: Using TCMS, stimulus response curves were constructed before and following a 1 h period of associative stimulation of two muscles motor points. The effect of an asynchronous peripheral stimulation paradigm was investigated in a separate series of control experiments. Results: One hour of associative stimulation of two muscles motor points resulted in a significant increase in the excitability of the corticospinal projection to those stimulated muscles. The increase in excitability peaked 1 h following the stimulation period. This increase in excitability did not generalise to either adjacent or more remote muscles. The control stimulation paradigm produced no significant change in corticospinal excitability. Conclusions: These results confirm the importance of associative input for the induction of plasticity in the human motor cortex. Significance: The findings reported here further elucidate the role of afferent input in motor cortical reorganisation. These findings have implications for our understanding of the mechanisms of motor learning and may also be relevant to the design of new afferent stimulation therapies.