Inhibition of movement representation by transcranial magnetic stimulation timed to event-related synchronization

Practice-induced motor memory may be enhanced if TMS is applied to the primary motor cortex (M1) synchronously with movement performance. However, synchronous TMS will facilitate movement memory formation indiscriminately of their similarity to the desired goal. Event-related synchronisation (ERS), an oscillatory EEG phenomenon after termination of a movement may reflect a deactivated cortical state or local cortical inhibition. We tested the hypothesis that post-movement TMS synchronized with ERS is able to disfacilitate training-induced changes in movement representation.

10 healthy volunteers participated in 3 sessions in which they performed brisk, directionally specific thumb movements. In session 1, the latency between movement onset and ERS maximum was established by analysing EEG-signals in the beta-frequency band recorded over contralateral M1. In sessions 2 and 3, subjects had to perform brisk unidirectional thumb movements (0.3Hz, 600 thumb movements, 30min). This direction was congruent to the direction of TMS-evoked movements as established beforehand. Each voluntary training movement was followed by either a subthreshold (80% of resting motor threshold, RMT) TMS pulse (session2, „Pract+ERSstim“) or by application of a SHAM pulse (session3, „Pract+SHAMstim“). In sessions 2 and 3, timing of TMS pulses was set to be synchronous with the ERS maximum, which was determined in session 1 to occur at 1100ms after movement onset. Corticospinal excitability of training agonist and training antagonist muscles was evaluated by corresponding motor evoked potential amplitudes (MEPAgonist and MEPAntagonist) before (pre-training) and after (post-training) movement performance.

Following Pract+SHAMstim, MEP amplitudes were significantly enhanced both in the training agonist and in the training antagonist muscle, and this increase of corticospinal excitability was more pronounced in the training agonist muscle (ratio 1.28±0.55). In contrast, Pract+ERSstim resulted in a significant MEP enhancement in the training antagonist muscle only, but not in the training agonist muscle (ratio 1.01±0.31).

These findings suggest that practice-dependent motor memory formation may be selectively modulated by TMS-pulses delivered after movement termination. Interventional TMS timed to ERS may provide a tool for adaptive movement training that may be useful in rehabilitation.