Training voluntary motor suppression with real-time feedback of motor evoked potentials

Abstract

Training people to suppress motor representations voluntarily could improve response control. We evaluated a novel training procedure of real-time feedback of motor evoked potentials (MEPs) generated by transcranial magnetic stimulation (TMS) over motor cortex. On each trial, a cue instructed participants to use a mental strategy to suppress a particular finger representation without overt movement. A single pulse of TMS was delivered over motor cortex, and an MEP-derived measure of hand motor excitability was delivered visually to the participant within 500 ms. In experiment 1, we showed that participants learned to reduce the excitability of a particular finger beneath baseline (selective motor suppression) within 30 min of practice. In experiment 2, we performed a double-blind study with 2 training groups (1 with veridical feedback and 1 with matched sham feedback) to show that selective motor suppression depends on the veridical feedback itself. Experiment 3 further demonstrated the importance of veridical feedback by showing that selective motor suppression did not arise from mere mental imagery, even when incentivized with reward. Thus participants can use real-time feedback of TMS-induced MEPs to discover an effective mental strategy for selective motor suppression. This high-temporal-resolution, trial-by-trial-feedback training method could be used to help people better control response tendencies and may serve as a potential therapy for motor disorders such as Tourette's and dystonia.

Keywords: inhibitory control; mental imagery; transcranial magnetic stimulation.

Copyright © 2015 the American Physiological Society.

Figures

Fig. 1.

Feedback-driven task. A: Null trials at the start of each block. B: Suppression trials followed Null trials. Motor evoked potential (MEP) amplitudes compared with baseline were fed back visually on each trial. A red bar corresponded to the muscle cued for suppression. Participants saw “Good Job!” if 1) the cued MEP was reduced compared with baseline (i.e., the red bar was pointing downward), and 2) the cued MEP was reduced compared with the uncued MEP (i.e., the red bar was more negative than the blue bar). Otherwise, participants were told to “Try Again.” C: placement of the first dorsal interosseous (FDI, index) and abductor digiti minimi (ADM, pinky) electrodes. D: qualitative shape of MEP in both FDI (index) and ADM (pinky). Lightning bolt indicates time of stimulation with associated artifact. MEP amplitudes were measured in microvolts from peak to trough.

Fig. 2.

MEP results. A: experiment 1. There was a main effect of Cue, particularly seen in the later blocks. B: experiment 2 with real vs. sham feedback groups. The real feedback group suppressed the cued finger (as for experiment 1), whereas the sham feedback paradoxically activated both cued and uncued fingers. C: experiment 3. Participants showed no significant suppression of the cued finger. “Percent Modulation” represents the percentage change of the mean MEP amplitude on Suppression trials compared with that on Null trials: (Suppression cue MEP − Null MEP)/Null MEP × 100%. Asterisks indicate a significant change from the zero baseline (P < 0.05). Error bars represent 1 standard error of the mean.