Activation of motor pathways during observation and execution of hand movements

Some neural properties of "motor resonance"--the subliminal activation of the motor system when observing actions performed by others--are investigated in humans. Two actions performed with the right hand are observed by experimental subjects: a finalized (transitive) action (reaching for and grasping a ball) and an intransitive action (cyclic up-and-down oscillation of the hand), while the H-reflex and Transcranial Magnetic Stimulation techniques are utilized to test the excitability of the observer's motor pathways to hand and forearm muscles (first dorsal interosseus, flexor digitorum superficialis, flexor carpi radialis). Results indicate that motor resonance: (1) is mainly mediated by the primary motor cortex; (2) involves the same forearm muscles as used in the execution of the observed movement; (3) is also recorded in the homologous muscles of the arm contralateral to the one observed; and (4) is evoked by both transitive and intransitive movements of the human hand, but not by similar movements of inanimate objects. The similarities and discrepancies between the resonant response in humans and the properties of monkey "mirror neurons" are discussed.