Primary motor cortex crucial for action prediction: A tDCS study.

The neural network underlying action observation - i.e., the action observation network - forms an anticipatory representation of observed actions. Although correlational studies suggest that the motor cortex (M1) might be involved in this anticipatory coding, it is unclear whether M1 is also causally essential for making accurate predictions about observed actions. To test the functional relevance of M1 to action prediction, we used offline monopolar transcranial direct current stimulation (tDCS). In four tDCS groups of healthy participants, we administered 15 min of anodal or cathodal constant currents of 1 or 2 mA over the left M1 before participants performed two tasks requiring them to make predictions about the outcomes of reaching-grasping human actions (Action Prediction - AP) or non-human movements (Non-human Prediction - NP). In each group, participants received sham and active tDCS in two separate sessions. We found that 2 mA cathodal tDCS (c-tDCS2mA ) selectively impaired accuracy in the AP task, but not in the NP task. No change in performance was found following anodal or 1-mA tDCS protocols. Additionally, no change was found following c-tDCS2mA administered over a control site. These findings show task-, polarity-, intensity- and site-specific disruption of AP abilities following c-tDCS2mA over M1. Thus, our study establishes specific tDCS parameters for effective M1 stimulation in AP and highlights the functional relevance of the motor system to making accurate predictions about the outcomes of human actions.