The observation of a cyclic movement of one hand induces bimanual motor resonance

Recent studies have shown that during attentive observation of movements performed by others, the observers’ motor pathways are modulated subliminally (motor resonance), reproducing with high temporal and spatial specificity the motor commands needed to execute the observed movement. Here we show that the observation of the rhythmic movement of one hand of a different subject induces in the observer a parallel rhythmic modulation of excitability in forearm spinal motoneurons driving muscles of both hands. Observers were sitting in a relaxed position, with feet on the floor and forearms prone on armchair rests. H-reflexes were elicited simultaneously in the right and left forearm muscle Flexor Carpi Radialis (FCR) of observers, while they were watching the 1Hz oscillation of one hand (the left) performed by a different subject, sitting in front of them. Different moments of the observed movement cycle were tested by eliciting H-reflex responses in the observers’ FCR muscles at various delays timed from a triggering position of the moving hand. In all subjects a bilateral sinusoidal modulation with identical period as that of the observed movement was recorded in the FCR muscles. Moreover, right and left FCR modulations also had the same phase difference with respect to the observed movement, showing a phase advance of 64° and 54° respectively, so that the two maintained a parallel time course. This bilateral modulation could result from a parallel activation of motor pathways of the two hands, evoked independently by observing the oscillating hand, or it could reveal neural links normally involved in the execution of coordinated bimanual tasks.