Compensatory plasticity in the action observation network: virtual lesions of STS enhance anticipatory simulation of seen actions

Observation of snapshots depicting ongoing motor acts increases cortico-spinal motor excitability. Such motor-facilitation indexes the anticipatory simulation of observed (implied) actions and likely reflects computations occurring in the parieto-frontal nodes of a cortical network subserving action perception (action observation network, AON). However, direct evidence for the active role of AON in simulating the future of seen actions is lacking. Using a perturb-and-measure transcranial magnetic stimulation (TMS) approach we show that off-line TMS disruption of regions within (inferior frontal cortex, IFC) and upstream (superior temporal sulcus, STS) the parieto-frontal AON, transiently abolish and enhance the motor-facilitation to observed implied actions, respectively. Our findings highlight the critical role of IFC in anticipatory motor simulation. More importantly, they show that disruption of STS calls into play compensatory motor simulation activity, fundamental for counteracting the noisy visual processing induced by TMS. Thus, short-term plastic changes in the AON allow motor simulation to deal with any gap or ambiguity of ever-changing perceptual worlds. These findings support the active, compensatory and predictive role of fronto-parietal nodes of the AON in the perception and anticipatory simulation of implied actions.