Human and robotic action observation elicit automatic imitation

Recent behavioural and neuroimaging studies found that observation of biological action, but not of robotic action, elicits imitation and activates the'mirror neuron system'in the premotor cortex (Kilner, Paulignan, and Blakemore, 2003; Castiello, Lusher, Mari, Edwards, and Humphreys, 2002; Meltzoff, 1995; Tai, Scherfler, Brooks, Sawamoto, and Castiello, 2004). This implies that the actions of other people and of mechanical devices are processed in categorically different ways. However, if the mirror system develops through learning (Heyes, 2001), generalisation should result in some activation when observing robotic action. We asked subjects to perform a prespecified action on presentation of a human hand or a robotic device in the final posture of the same action or the opposite action (Heyes, Bird, Johnson, and Haggard, 2004; Stürmer, Ascherschleben, and Prinz, 2000). Both the human and the robotic stimuli elicited automatic imitation: the prespecified action was initiated faster when it was cued by the same action than when it was cued by the opposite action. However, even when the human and robotic stimuli were of comparable size, colour and brightness, the human hand had a stronger effect on performance. These results point to the shape of the human hand as a source of features distinguishing human from robotic action. They also suggest, as one would expect if the mirror neuron system develops through learning, that to varying degrees both human and robotic action can be'simulated'by the premotor cortex (Gallese and Goldman, 1998).