Uncertainty in contextual and kinematic cues jointly modulate motor resonance in primary motor cortex

Contextual information accompanying others actions modulates \"motor resonance\", i.e. neural activity within motor areas that is elicited by movement observation. One possibility is that we weight and combine such information in a Bayesian manner according to their relative uncertainty. Therefore, contextual information becomes particularly useful when others actions are ambiguous. It is unclear, however, whether this uncertainty modulates the neural activity in primary motor cortex (M1) during movement observation. Here we applied single-pulse transcranial magnetic stimulation (TMS) while subjects watched different grasping actions. We operationalized motor resonance as grip specific modulation of corticomotor excitability measured in the index (FDI) versus the little finger abductor (ADM). We experimentally modulated either the availability of kinematic information (Exp. 1) or the reliability of contextual cues (Exp. 2). Our results indicate that even in the absence of movement kinematics, reliable contextual information is enough to trigger significant muscle-specific corticomotor excitability changes in M1 (p<.0001) which are strongest when both kinematics and contextual information are available (p<.005). These findings suggest that bottom-up mechanisms that activate motor representations as a function of the observed kinematics, and top-down mechanisms which activate motor representations associated with arbitrary cues converge in M1 in a statistically optimal manner.