Bilateral motor resonance evoked by observation of a one-hand movement : role of the primary motor cortex

In humans, observation of movement performed by others evokes a subliminal motor resonant response, most likely mediated by the mirror neurone system, which reproduces the motor commands needed to execute the observed movement with good spatial and temporal fidelity. Motor properties of the resonant response are here investigated with the ultimate goal of understanding the principles operating in the transformation from observation to internal reproduction of movement. Motor resonance is measured as the modulation of excitability of spinal motoneurones, evoked by the observation of a cyclic flexion-extension of one hand. The first two experiments showed that the observation of a one-hand movement always evokes a bimanual resonant response independent of which hand is observed, and that these bilateral responses are consistently phase-linked. H-reflexes simultaneously recorded in right and left FCR muscles are always modulated “in-phase” with each other. The goal of the third experiment was to define the role of primary motor cortex in the bilateral resonant response. Bilateral H-reflexes were recorded during a temporary inactivation induced by TMS over the left cortical hand motor area of observers. The finding that such cortical depression abolishes the H-reflex modulation of only the right FCR motoneurones, leaving it unchanged on the left side, suggests that both primary motor areas are activated by the premotor cortex, and transmit the resonant activation through crossed corticospinal pathways. The data provide further evidence that the subliminal activation of motor pathways induced by movement observation is organised according to general rules shared with the control of voluntary movement.