The human mirror neuron system
Marco Iacoboni
University of California, Los Angeles
Mirror neurons are cells in the frontal and parietal cortex of the monkey brain that fire during action execution and also during the observation of the same or similar actions performed by other individuals. They are thought to be important brain cells for understanding other minds. In humans, neural mechanisms with mirroring properties have been identified with brain imaging, Transcranial Magnetic Stimulation and electrophysiological studies. In my talk I will first review these human studies and then discuss some new findings in humans that may help filling the gap between monkey and human studies on the mirror neuron system. In humans, several studies have suggested that the mirror neuron system is important for a variety of aspects of social behavior, from action recognition and intention understanding, to imitation and empathy. Indeed, brain imaging studies in healthy volunteers and patients with autism show correlations between activity in mirror neuron areas and behavioral measures of empathy and sociality, suggesting that the human mirror neuron system may well be considered a bio-marker of sociality. Recently, we had the opportunity to record the activity of individual neurons in the medial frontal and medial temporal cortex of the human brain, from a series of 21 epileptic patients that had implanted depth electrodes for medical reasons. The patients executed or observed hand grasping actions and emotional facial expressions. We found mirror neurons in both medial frontal and medial temporal cortex, thus demonstrating that these cells have a wider anatomical distribution than previously thought. A subset of mirror neurons exhibited excitation during action execution, but inhibition during action observation. This class of mirror neurons may support the functions of inhibiting unwanted imitation and differentiating between actions performed by self and others.