Imitation is the most widely used form of learning during development, offering the acquisition of many skills without the time-consuming process of trial-and-error learning. Imitation is also central to the development of fundamental social skills such as reading facial and other body gestures and for understanding the goals, intentions and desires of other people1,2. Despite its central role in both learn-ing and social cognition, until recently imitation had been studied almost exclusively by social scientists (BOX 1), and our knowledge of the neural mechanisms of imitation was limited. The recent discovery of mirror neurons3 — premotor and parietal cells in the macaque brain that fire when the animal performs a goal-directed action and when it sees others perform-ing the same action — has inspired a series of studies on the neural correlates of imitation. Moreover, given the already known behavioural links between imita-tion and other aspects of social cognition4, there is now active research into the relations between mirror neuron areas and social cognitive processes such as understanding the intentions and emotions of other people. Furthermore, the hypothesis that dysfunction of mirror neurons might be one of the core deficits of socially isolating disorders such as autism5 is now under active scrutiny. This article offers, for the first time, a review of recent studies investigating the development of the mirror neuron system (MNS) and its dysfunction in autism, vis-à-vis what we know about the neurophysiology and the connectivity of the MNS in primates and humans (FIG. 1) and in relation to imitation and social cognition.The primate MNSFrontoparietal circuits for sensorimotor integration. So far, mirror neurons have been identified in two cortical areas — the posterior part of the inferior frontal cortex and the anterior part of the inferior parietal lobule3. These two areas are anatomically connected6 and so form an integrated frontoparietal MNS. This system is only one of the many parallel neural systems connecting frontal and parietal areas7. Cells in these frontoparietal neural systems have physiological properties that are highly relevant to sensorimotor integration. The MNS must be considered within the larger context of this massive and parallel frontoparietal network for sensorimotor integration to be properly understood.The posterior sector of the frontal lobe (agranular frontal cortex) is concerned with movement and com-prises various cortical areas8. Multiple representations of body parts are present in these motor areas3,6. This suggests that these motor areas are actively involved in transforming sensory inputs into appropriate actions. In fact, most of the frontal motor areas receive a robust sensory input (visual and somatosensory) from the parietal lobe3,6. Even though each frontal and parietal area is connected with several other areas, a clear pattern emerges. Typically, the examination of only the most robust connections shows that one frontal motor area is connected with one parietal area6. This pattern of con-nectivity supports relatively specialized fronto parietal systems. For example, the frontal area F4 in the ventral premotor cortex and the ventral intraparietal area (VIP) in the intraparietal sulcus form a system for axial and proximal movements including head-orienting Ahmanson-Lovelace Brain Mapping Center, Department of Psychiatry and Biobehavioural Sciences, Neuropsychiatric Institute, Brain Research Institute, David Geffen School of Medicine at the University of California, Los Angeles, Ahmanson-Lovelace Brain Mapping Center, 660 Charles E. Young Drive South, Los Angeles, California 90095, USA. e-mails: [email protected]; [email protected]:10.1038/nrn2024Published online8 November 2006The mirror neuron system and the consequences of its dysfunctionMarco Iacoboni and Mirella DaprettoAbstract | The discovery of premotor and parietal cells known as mirror neurons in the macaque brain that fire not only when the animal is in action, but also when it observes others carrying out the same actions provides a plausible neurophysiological mechanism for a variety of important social behaviours, from imitation to empathy. Recent data also show that dysfunction of the mirror neuron system in humans might be a core deficit in autism, a socially isolating condition. Here, we review the neurophysiology of the mirror neuron system and its role in social cognition and discuss the clinical implications of mirror neuron dysfunction.REVIEWS942 | DECEMBER 2006 | VOLUME 7 www.nature.com/reviews/neuro© 2006 Nature Publishing GroupTheory of mindAwareness that other people have beliefs and desires as we do, but different from our own, and that these beliefs and desires can explain the behaviour of others.and reaching9, for peri-personal space coding10 and for defensive movements11. Cells in this system fire during neck, arm and mouth movements but also respond to tactile stimuli and to visual stimuli that are close to the body12,13. When stimulated, these areas elicit highly coordinated defensive movements14.A second example of a frontoparietal system for sensorimotor integration is provided by a system that controls grasping. This network is composed of frontal area F5 and the parietal area in the anterior part of the intraparietal sulcus (AIP). Cells in this system can be divided into three categories: visual dominant neurons, which fire while the monkey observes a graspable object and during grasping in light conditions, but do not fire during grasping in the dark; visuomotor neurons, which fire more vigorously during grasping in light conditions than grasping in the dark, but also fire at the simple sight of a graspable object; and motor dominant neurons that fire equivalently for grasping in light and dark conditions and do not fire at the sight of a graspable object15,16. Visual dominant neurons are found only in the AIP, whereas area F5 contains more motor dominant neurons and fewer visuomotor neurons than AIP17. Taken together, these physiological properties suggest that the AIP provides multiple object affordance descriptions to F5, where the selection of the grasping action necessary to achieve the intention of the agent ultimately occurs6.Interestingly, the F5-AIP grasping circuit is anatomi-cally adjacent to the frontoparietal MNS. Whereas the F5-AIP circuit for grasping is located deep in the sulci — with F5 neurons grouped in the