Motor facilitation during action observation: The role of M1 and PMv in grasp predictions

Recent theories propose that movement observation is not a "passive mirror" of ongoing actions but might induce anticipatory activity when predictable movements are observed, e.g., because the action goal is known. Here we investigate this mechanism in a series of 3 experiments, by applying transcranial magnetic stimulation (TMS) to primary motor cortex (M1) while subjects observed either whole hand or precision grasping performed by an actor. We show that corticomotor excitability changes in a grip-specific manner but only once the grip can be decoded based on the observed kinematic cues (Exp. 1). By contrast, presenting informative contextual precues evokes anticipatory modulations in M1 already during the reach phase, i.e., well before the grip type could be observed, a finding in line with a predictive coding account (Exp. 2). Finally, we used paired-pulse (PP) TMS to show that ventral premotor cortex (PMv) facilitates grip-specific representations in M1 but only while grip formation is observed. These findings suggest that PMv and M1 interact temporarily and mainly when motor aspects of hand-object interactions are extracted from visual information. By contrast, no sustained input from PMv to M1 seems to be required to maintain action representations that are anticipated based on contextual information or once the grip is formed (Exp. 3).