Suppression of left ventral premotor cortex impairs action prediction: transcranial direct current stimulation studies

Influential theoretical models suggest that the human motor system is designed to act as an anticipation device and that humans predict others’ forthcoming actions by using their own motor system as an internal forward model. However to date evidence for a causative role of the motor system in predicting the outcome of observed actions is lacking. Here we used transcranial direct current stimulation (tDCS) to test the role of ventral premotor cortex (vPMc) in predicting the end-state of an observed action. In the Action-Prediction (AP) task, participants observed the initial phase of a right-hand reaching-grasping action. The final phase of the action was masked and subjects had to guess which of two objects were going to be grasped by the hand. In a difficulty-matched control task, subjects observed similarly interrupted movements of a non-biological (NB) stimulus approaching one of two targets. Participants performed both tasks in two separate sessions (1 week interval) that were carried out after 15 minutes of inhibitory (cathodal) real- or sham-tDCS over the left-vPMc (experiment 1) or the right-vPMc (experiment 2). Relative to sham stimulation, suppression of left-vPMc but not of right-vPMc brought about a selective reduction of accuracy in the AP-task, but not in the NB-task. These findings indicate that left-vPMc is necessary for extracting the future end-state of human actions based on the observation of the initial phases of the movement and suggest a left frontal lateralization in the predictive coding of others’ right-hand actions.