Mu Suppression during Anticipation of Predictable Actions Performed by Other Agents

The mirror neuron system (MNS) becomes active during action execution and action observation, which is presumably reflected by reductions in mu (8-13 Hz) activity in the electroencephalogram (EEG) over the sensorimotor cortex. Although the existence of this system is generally accepted, its function is still debated. The aim of the current study was to investigate the role of the MNS in anticipating others’ actions by examining whether the MNS was activated – indexed by mu power suppression - prior to the onset of observed actions when the onset and type of action could be predicted on the basis of environmental cues. Healthy young adults performed and observed cued grasping and placing actions in a card game, while the predictability of the observed actions was manipulated using a fixed set of rules. All actions were performed in a real-life setting. Significant mu suppression, relative to within-trial baseline activity, was found during both action execution and observation. In addition, significant mu suppression was found prior to the onset of executed actions (preparatory motor activity), and, crucially, also prior to observed actions that were predictable. No anticipatory mu reductions were found prior to unpredictable observed actions. These results suggest top-down modulation of MNS activity by conceptual knowledge. This is the first study to demonstrate mu suppression prior to action onset – possibly reflecting MNS anticipatory activity - by explicitly manipulating predictability in a real-life setting.