Altered visual feedback modulates cortical excitability in a mirror-box-like paradigm

Watching self-generated unilateral hand movements reflected in a mirror–oriented along the midsagittal plane–enhances the excitability of the primary motor cortex (M1) ipsilateral to the moving hand of the observer. Mechanisms detecting sensory–motor conflicts generated by the mirror reflection of such movements might mediate this effect; if so, cortical excitability should be modulated by the magnitude of sensory–motor conflict. To this end, we explored the modulatory effects of an altered visual feedback on M1 excitability in a mirror-box-like paradigm, by increasing or decreasing the speed of the observed movement. Healthy subjects performed movements with their left index finger while watching a video of a hand superimposed to their right static hand, which was hidden from view. The hand observed in the video executed the same movement as the observer’s left hand, but at slower, same, or faster paces. Motor evoked potentials (MEPs) induced by transcranial magnetic stimulation were measured from the first dorsal interosseous and the abductor digiti minimi of the participant’s hidden resting hand. The excitability of the M1 ipsilateral to the moving hand was systematically modulated by the speed of the observed hand movement: the slower the observed movement, the greater the MEP amplitude from both muscles. This evidence shows that the magnitude of the visual–motor conflicts can be used to adjust the activity of the observer’s motor system. Hence, an appropriate alteration of the visual feedback, here the reduction in the movement speed, may be useful to increase its modulatory effect on motor cortical excitability.