Mirror neurons (MirNs) within ventral premotor cortex (PMv) and primary motor cortex (M1), including pyramidal tract neurons (PTNs) projecting to the spinal cord, modulate their activity during both the execution and observation of motor acts. However, movement is not produced in the latter condition, and mirror responses cannot be explained by lowlevel muscle activity. Relatively reduced activity in M1 during observation may help to suppress movement. Here, we examined the extent to which activity at different stages of action observation reflects grasp representation and suppression of movement across multiple levels of the mirror system in monkeys and humans. We recorded MirNs in M1 and F5 (rostral PMv), including identified PTNs, in two macaque monkeys as they performed, observed, and withheld reach-to-grasp actions. Time-varying population activity was more distinct between execution and observation in M1 than in F5, and M1 activity in the lead-up to the observation of movement onset shared parallels with movement withholding activity. In separate experiments, modulation of short-latency responses evoked in hand muscles by pyramidal tract stimulation revealed modest grasp-specific facilitation at the spinal level during grasp observation. This contrasted with a relative suppression of excitability prior to observed movement onset or when monkeys simply withheld movement. Additional cortical recording experiments examined how contextual factors, such as observing to imitate, observing while engaged in action, or observation with reduced visual information, modulated mirror activity in M1 and F5. Finally, single-pulse transcranial magnetic stimulation (TMS) in healthy human volunteers was used to examine changes in corticospinal excitability (CSE) during action observation and withholding. Overall, the results reveal distinctions in the profile of mirror activity across premotor and motor areas. While F5 maintains a more abstract representation of grasp independent of the acting agent, a balance of excitation and inhibition in motor cortex and spinal circuitry during action observation may support a flexible dissociation between initiation of grasping actions and representation of observed grasp
