Timecourse of corticospinal excitability for observed action: evidence of early suppression followed by return to baseline without facilitation

IntroductionAction observation modulates corticospinal excitability, with most previous studies indicating an increase in excitability in the muscles involved in the observed movement. In addition, previous work suggests that modulation of corticospinal excitability could be specific to the timing of the stimulation, muscle, and direction of movement. Here we examined the influence of these factors on corticospinal excitability. MethodParticipants observed stimuli presenting a static hand, followed by an image of the endpoint of an index/little finger abduction movement. Transcranial magnetic stimulation was delivered at time points from 100-800ms after movement onset. Stimuli were presented in various orientations to study possible effects of anatomical positioning and movement direction, compared relative to the control condition of a static hand. ResultsCorticospinal excitability was lower at early timings (100-400ms), before rising to a plateau at later timings (500-800ms) which did not differ from the static hand condition. This facilitation was muscle-specific, with higher excitability for the muscle involved in the observed movement. By contrast, the relative direction of movement did not influence corticospinal excitability. DiscussionThese results replicate the time-dependent modulation of corticospinal excitability induced by action observation; however, we argue that simply interpreting such effects as an increase in excitability may be overly simplistic. In line with previous studies, we argue that the choice of control condition used during action observation studies may be critical to the overall direction of effects.