Behavioral studies have employed various types of experiment to investigate the links between vision and proprioception. The present study sought electrophysiological evidence of the contribution of visual inputs to visuoproprioceptive integration. Using the brain event-related potential called the lateralized readiness potential, which reflects cortical activity in the primary motor cortex, we showed that viewing the movements of one hand is sufficient to generate cortical motor response preparation related to this hand, even if it is not the one that actually moves. Indeed, we detected LRPs related to movements of a hand seen in a sagittal mirror – indicating the presence of neural activity in the primary motor cortex related to this hand – while the other hand was active. Furthermore, we showed that this cortical activity does not depend on proprioceptive information, since LRPs were recorded even when the apparent position of the hand in the mirror and the real position of the hidden hand were incongruent. The present study also showed that this motor activation only occurs when acute visual information about the hand is available. When the visual information of the hand in the mirror was reduced to small lights, yielding its representation by means of a structure-from-motion process, no motor activity was recorded in the cortical area of the inactive hand. These results indicate that cortical motor activity relies on the viewed image of the hand rather than on its actual position, and that this cortical motor activity can be modulated by visual information. These results give new insights into how the brain integrates visual and proprioceptive information during the execution of voluntary movement.