The human premotor cortex is 'mirror' only for biological actions

Previous work has shown that both human adults and children attend to grasping actions performed by another person but not necessarily to those made by a mechanical device [1-3]. According to recent neurophysiological data, the monkey premotor cortex contains "mirror" neurons that discharge both when the monkey performs specific manual grasping actions and when it observes another individual performing the same or similar actions [4-7]. However, when a human model uses tools to perform grasping actions, the mirror neurons are not activated [4-6]. A similar "mirror" system has been described in humans [8-15], but whether or not it is also tuned specifically to biological actions has never been tested. Here we show that when subjects observed manual grasping actions performed by a human model a significant neural response was elicited in the left premotor cortex. This activation was not evident for the observation of grasping actions performed by a robot model commanded by an experimenter. This result indicates for the first time that in humans the mirror system is biologically tuned. This system appears to be the neural substrate for biological preference during action coding.