| Mirror neurons are unique visuomotor neurons that respond when executing a particular movement (e.g. grasping, placing, or manipulating) and also when passively observing someone else performing that same movement. Importantly, subpopulations of mirror neurons respond in a selective manner to one preferred movement whether executed or observed. It has been proposed that the activity of mirror neurons during observation of actions represents an internal "simulation" of the observed action, which enables us to perceive its goal and intention, thereby forming the foundation for social interactions. Mirror neurons are thought to exist in two cortical areas, the anterior intraparietal sulcus (aIPS) and the ventral premotor (vPM), which have been called the human mirror system. A dysfunction in the responses of this system has been hypothesized to cause an impairment in the ability to understand one another resulting in Autism.;Here we conducted three studies to characterize the responses of the human mirror system and to test the hypothesis that a dysfunction of the system underlies Autism. In two separate studies we used fMRI adaptation and classification techniques to assess the selectivity of several cortical areas, including mirror system areas, for observed and executed hand movements. Movement-selectivity is a critical feature of neurons involved in movement perception, including mirror neurons. For us to understand the meaning of observed movements we must be able to differentiate between them and represent each with a unique neural response. Converging results from both of the studies showed that both mirror system areas responded in a movement-selective manner. We suggest that these responses were generated by overlapping visual, motor, and visuomotor neural populations, perhaps including mirror neurons, that responded selectively to particular movements. These results elucidate the functional characteristics of neural populations in human mirror system areas and highlight their role in movement perception. In a third study we used the adaptation methodology to compare mirror system responses between Autistic individuals and controls. The results showed that, on average, Autistic individuals exhibited indistinguishable movement-selective responses from those of controls, indicative of intact movement perception. However, the responses of several cortical areas (including, but not limited to the mirror system) were more variable in Autistic individuals than in controls. Taken together, these results suggest that Autistic individuals do not have a selective dysfunction in mirror system areas, but rather exhibit generally noisy and unreliable neural responses, perhaps due to an imbalance of excitation and inhibition throughout multiple brain areas. |
