TIME COURSE OF CORTICOSPINAL EXCITABILITY DURING OBSERVATION OF ANATOMICALLY AND SPATIALLY VARIABLE BODY STIMULI

Action observation has generally been reported to modulate corticospinal excitability in the muscles involved in the observed movement. However, previous research suggests this may also be modulated by factors including the timing of stimulus presentation and the positioning of the presented muscles relative to the participant’s own hand. Here we examined the influence of these factors on corticospinal excitability. Participants 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 offset. 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. Corticospinal excitability was lower at early timings (100-400ms) before rising to a plateau at later timings (500-800ms) that 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 cortisospinal excitability. Although there is a modulation of corticospinal excitability induced by action observation with an increase in excitability over time, this is not consistent with facilitation as suggested in previous studies. In fact, we observed lower excitability for action trials than for static ones, which is more consistent with a phenomenon of inhibition at early timings. This suggests the need to rethink the generally accepted view of the influence of action observation on corticospinal excitability.