Timing and specificity of early changes in motor excitability during movement observation.

Motor resonance is typically considered a modulation of motor activity during action observation, which mirrors the pattern of muscle activation occurring during execution of the observed action. There is preliminary evidence that motor activity accompanying action observation comprises two stages: an initial non-specific modulation, followed by a later change in excitability that is specific to the muscles involved in the action. We built on this previous work, with the aims of (1) clarifying the time-course of muscle specificity during action observation and (2) establishing whether modulation at each time point reflects significant changes (facilitation or suppression) from baseline. Motor-evoked potentials were elicited in two muscles-the first dorsal interosseous (FDI) and abductor digiti minimi (ADM)-while participants watched stimuli showing index or little finger abduction. Our results provide partial support for previous findings of muscle-specific modulation after, but not before, 200 ms. Interestingly, however, this was shown only by facilitation of the FDI during index finger observation; there was no significant facilitation of the ADM when little finger movement was observed. Furthermore, we found no facilitation of corticospinal excitability (specific or non-specific) at time points earlier than 200 ms (100-150 ms). We discuss these results in the context of previous work showing a non-specific increase in excitability at 90 ms during action observation. Together, these findings suggest a fast and short-lived facilitation of corticospinal excitability that occurs at around 60-90 ms and ends by 100-150 ms.