Function and Development of the Mirror Neuron System Project Summary Two fundamental abilities are central to adaptive human functioning: the ability to deploy actions strategically in the service of goals, and the ability to apprehend the goals of social partners in order to produce adaptive social responses. These abilities emerge in infancy and undergo foundational developments across childhood. Evidence from diverse scientific approaches indicates that these capacities may be supported by a common underlying neural network known as the mirror neuron system (MNS). This system is comprised of a network of inter-connected brain regions some of which may contain mirror neurons (MNs) and others that involve feedback loops across brain regions supporting these complex capabilities. The MNS responds both when we perform an action and when we observe someone else perform that action. The discovery of MNs, first made in non-human primates, holds the potential to revolutionize scientific understanding of goal-directed action, social perception and their development. During the initial award period, this Program Project began studies that investigated integrating the complex neural circuitry and functional aspects of the MNS in human infants and children, which arguably are some of the most powerful potential effects of the MNS. Our work highlighted the need for systematic investigations of the neural and functional aspects of the MNS during development, both for understanding typical developmental pathways and for shedding light on developmental disorders in which the development of social cognition and the potential functions of the MNS are disrupted, as in autism spectrum disorder (ASD). As well, it necessitates innovative methodological approaches to measure the networks activated during complex behaviors associated with the MNS. Advances in our work include description of an extended MNS, the result of a complex set of brain networks involved in action execution and observation; findings of significant changes in the MNS across development; and, not surprisingly, that the MNS is linked tightly to the emergence and integration of motor skills not only during infancy but across childhood. In the next five years we will advance an understanding of the neural networks of the MNS and relations to other cognitive systems; explore the potential contributions and limitations of the MNS for the development of social cognition; and investigate the modulation of the MNS via experience including active training. Finally, we will extend our approach to investigate factors that may drive the varied patterns of social deficit seen in autism spectrum disorder (ASD).