The spinal substrate of the suppression of action during action observation.

We have previously demonstrated that the forelimb representations of the primary motor and somatosensory cortices, as well as several premotor and parietal areas, are activated by both action-execution and action-observation, indicating that the spectator mentally simulates the observed action. Moreover, several studies demonstrated repeatedly that corticospinal excitability is modulated during action observation, providing evidence of an activation of the observer's motor system. However, evidence for the involvement of the spinal cord in action observation is controversial. The aim of the present study was to explore whether and how action-observation affects the spinal cord. To this end, we analyzed the spinal cord of eight monkeys (Macaca mulatta) trained to either execute reaching-to-grasp movements or observe the experimenter performing the same movements. Observation of grasping induced a bilateral decrease of glucose consumption in the spinal forelimb representation, whereas execution of grasping induced an increase of glucose utilization in the same area, ipsilaterally to the grasping hand. The depression of overall activity in the cervical enlargement of the spinal cord for action-observation may explain the suppression of overt movements, despite the activation of the observer's motor system.