Motor training strengthens corticospinal suppression during movement preparation

Motor training improves the efficiency of trained movements and modifies neural activity at rest and during motor execution. Training may also shape preparatory processes but the neural correlates and the potential behavioral relevance of changes at the level of action preparation remain unclear. In humans, transcranial magnetic stimulation (TMS) studies have shown that movement preparation is accompanied by a suppression of corticospinal (CS) excitability relative to a baseline measure; a phenomenon called preparatory suppression. Here, we trained participants to initiate quick movements in an instructed-delay reaction time (RT) task and investigated resting CS excitability as well as preparatory suppression over the practice blocks. We found that training speeds up motor initiation, with no repercussion on error rates. Moreover, training increased baseline CS excitability and deepened preparatory suppression. Importantly, training-related changes in preparatory suppression were correlated to behavioral improvements: the subjects who showed a stronger expansion of preparatory suppression were also those exhibiting larger gains in RTs. Finally, in line with previous data, we observed a trial-by-trial relationship between the amount of preparatory suppression and the subsequent RT: subjects responded faster on trials with more preparatory suppression. Strikingly though, such a dependency of RTs on preparatory suppression was tuned by the amount of training performed: while it was not evident early on, the link emerged during the later practice blocks. Overall, our data indicate that training induces changes in motor preparatory processes that are linked to an enhanced ability to initiate fast movements.