Evidence for a sensorimotor prediction process in action observation

The mirror neurons network in the human brain is activated both during the observation of action and the execution of the same action, facilitating thus the transformation of visual information into motor representations, to understand the actions and intentions of others. How this transformation takes place, however, is still under debate. One prevailing theory, direct matching, assumes a direct correspondence between the visual information of the actors movement and the activation of the motor representations in the observers motor cortex that would produce the same movement. Alternatively, the predictive coding theory postulates that, during action observation, motor predictions (e.g., position, velocity) are generated and compared to the visual information of the actors movement. Here, we experimentally interrogate these two hypotheses during a locomotion task. The motor prediction process was assessed by measuring the timing of imagined movements: the participants had to imagine walking, forward or backward, for 9 m (linear path). Action observation was assessed by measuring time estimation in an inference locomotor task (the same 9 m linear path): after perceiving an actor walking forward or backward for 3 m, the vision of the observer was occluded and he/she had to estimate when the actor would reach the end of the 9 m path. We manipulated the timing processes during the two tasks by creating sensory illusions via peripheral mechanical muscle vibration on leg muscles, which has provided consistent results in the literature (acceleration of forward and deceleration of backward locomotion). We found that sensory illusions specifically affected the timing processes of both locomotion inference and mental locomotion, suggesting the involvement of sensorimotor predictions, common to both tasks. These findings seem to support the predictive coding hypothesis.