Direct mapping rather than motor prediction subserves modulation of corticospinal excitability during observation of actions in real time

Abstract

Motor facilitation refersto the specific increment in corticospinal excitability (CSE)elicited by the observation of actions performed by others. To date,the precise nature of the mechanism at the basis of this phenomenonis unknown. One possibility is that motor facilitation is driven by apredictive process reminiscent of the role of forward models in motorcontrol. Alternatively, motor facilitation may result from a model-freemechanism by which the basic elements of the observed action aredirectly mapped onto their cortical representations. Our study wasdesigned to discern these alternatives. To this aim, we recorded thetime course of CSE for the first dorsal interosseous (FDI) and theabductor digiti minimi (ADM) during observation of three graspingactions in real time, two of which strongly diverged in kinematicsfrom their natural (invariant) form. Although artificially slow movementsused in most action observation studies might enhance theobserver?s discrimination performance, the use of videos in real timeis crucial to maintain the time course of CSE within the physiologicalrange of daily actions. CSE was measured at 4 time points within a240-ms window that best captured the kinematic divergence from theinvariant form. Our results show that CSE of the FDI, not the ADM,closely follows the functional role of the muscle despite the mismatchbetween the natural and the divergent kinematics. We propose thatmotor facilitation during observation of actions performed in real timereflects the model-free coding of perceived movement following adirect mapping mechanism.