Training-related modulation of preparatory activity in the motor system

Vassiliadis, Pierre [UCL] Grandjean, Julien [UCL] Derosiere, Gerard [UCL] Duque, Julie [UCL]

The preparation of movements, even the simplest ones, involves drastic changes of neural activity in the motor system well before action initiation (Gao et al., 2018). A recent behavioral study showed that training on a specific motor task can reduce reaction times (RT) for the trained movement, and that this reduction is due to faster motor processing during the preparatory period (Mawase et al., 2018). This finding suggests that motor training reduces RTs by modulating preparatory activity in the motor system. Here, we tested this hypothesis in humans by examining the neural correlates of this training-related reduction in RTs. To do so, we assessed preparatory activity in the motor cortex at different stages of training while subjects performed a RT task. 14 subjects performed 10 blocks of 40 trials of an instructed-delay choice RT task which required them to prepare left of right index finger movements according to a preparatory cue and to withhold their response until an imperative signal was displayed. Subject were told to respond as fast as possible to gain points while avoiding errors (e.g. responding too early or too late or responding with the wrong hand) that would penalize their score. In order to probe corticospinal excitability, double-coil TMS was applied over the right and the left primary motor cortex to elicit near-simultaneous motor-evoked potentials (MEPs) in the left and the right hand, respectively (Vassiliadis et al., 2018). MEPs recorded during the preparatory period (i.e. between the preparatory cue and the imperative signal; MEPPrep) were expressed in percentage of MEPs acquired at rest (i.e. between 2 trials; MEPBaseline). We compared RTs, errors and MEPs from both hands in the early (Trainingearly) and late stages of training (Traininglate). In a second-level analysis, we correlated MEPs to bins of 10 consecutive percentile windows of RT (i.e., 0 to 10th, 10th to 20th . . . 90th to 100th). At the behavioral level, RTs were reduced at Traininglate compared to Trainingearly in both hands despite no significant changes in the number of errors. At the neural level, MEPPrep were systematically suppressed compared to MEPBaseline; a process that has been previously referred to as preparatory inhibition (Duque et al., 2017). Most importantly, this preparatory activity was modulated by the amount of training performed: MEPPrep were more suppressed at Trainninglate than at Trainingearly. Moreover, we found that there was a strong correlation between MEPPrep and the subsequent RT: the smaller the MEPPrep, the faster the RT. Overall, our results suggest that training involves a modulation of preparatory activity in the motor system that subserves RT reduction.