Functional imaging of face and hand imitation: towards a motor theory of empathy.
Empathy requires the ability to map the feelings of others onto our own nervous system. Until recently, there was no plausible mechanism to explain how such a mapping might occur. The discovery of mirror neurons, however, suggests that the nervous system is capable of mapping the observed actions of others onto the premotor cortex of the self, at least for reaching and grasping movements. Is there a mirroring system for emotive actions, such as facial expression? Subjects (N = 15; all right-handed; eight men, seven women) watched movies of facial expressions (smile or frown) and hand movements (move index or middle finger) while brain activity was imaged using functional magnetic resonance imaging (fMRI). Subjects watched the movies under three different conditions: passive viewing, active imitation, and an active motor control. Subjects also performed a verb generation task to functionally identify language-processing areas. We found evidence for a common cortical imitation circuit for both face and hand imitation, consisting of Broca's area, bilateral dorsal and ventral premotor areas, right superior temporal gyrus (STG), supplementary motor area, posterior temporo-occipital cortex, and cerebellar areas. For faces, passive viewing led to significant activation in the right ventral premotor area, whereas imitation produced bilateral activation. This result is consistent with evidence for right hemisphere (RH) dominance for emotional processing, and suggests that there may be a right hemisphere mirroring system that could provide a neural substrate for empathy.