The role of the motor system in action imagination and comprehension

Covert actions encompass cognitive processes such as motor imagery, action observation, and action language. These processes ignite motor simulations within the mind and engage neural circuits underlying sensorimotor functions, offering promising potential for optimizing motor performance and rehabilitation strategies. Yet, despite broad consensus on the motor system's involvement, its precise role - whether a dynamic embodiment mechanism or a mere epiphenomenon - remains a matter of intense debate. In this review, we present a comprehensive analysis of motor system engagement across motor imagery, action observation, and action language. First, we examine correlational evidence derived from functional magnetic resonance imaging and transcranial magnetic stimulation studies in healthy individuals, including those with impaired mental simulation abilities. This evidence helps elucidate whether, and to what extent, these cognitive processes recruit the motor system, particularly the primary motor cortex. We then turn to causal evidence, which allows us to infer the functional role of the motor system in these processes. Specifically, we review findings from studies investigating behavioral changes, patients with lesions, and artificial lesions in healthy participants created using neuromodulation techniques. By integrating these perspectives, we seek to clarify this ongoing debate by illuminating the functional role of the motor system in motor imagery, action observation, and action language.