Selective enhancement of motor cortical plasticity by observed mirror-matched actions

Watching others learn a motor task can enhance an observer's own later performance when learning the same motor task. This is thought to be due to activation of the action observation (or mirror neuron) network. Here we show that the effectiveness of plasticity induced in human motor cortex (M1) is also significantly influenced by the nature of prior action observation. In separate sessions, 17 participants watched a video showing repeated goal-directed movements (action observation) involving either the right hand (congruent condition) or the same video mirror-reversed to simulate the left hand (incongruent condition). Participants then received pulses of transcranial magnetic stimulation over the hand area of left M1 paired with median nerve stimulation of the right hand (paired associative stimulation; PAS). The resting motor-evoked potential (MEP) in right abductor pollicis brevis (APB) increased significantly 20 minutes after PAS, but only when participants had previously watched the congruent video. In this condition, all participants showed an increase in MEP amplitude at 20 minutes post-PAS. There was no change in MEP amplitude following PAS when participants watched the incongruent video. We conclude that prior action observation is a potent modulator of subsequent PAS-induced neuroplasticity, which may have important therapeutic applications.