Successful social interaction depends on not only the ability to identify with others but also the ability to distinguish between aspects of self and others. Although there is considerable knowledge of a shared neural substrate between self-action and others' action, it remains unknown where and how in the brain the action of others is uniquely represented. Exploring such agent-specific neural codes is important because one's action and intention can differ between individuals. Moreover, the assignment of social agency breaks down in a range of mental disorders. Here, using two monkeys monitoring each other's action for adaptive behavioral planning, we show that the medial frontal cortex (MFC) contains a group of neurons that selectively encode others' action. These neurons, observed in both dominant and submissive monkeys, were significantly more prevalent in the dorsomedial convexity region of the MFC including the pre-supplementary motor area than in the cingulate sulcus region of the MFC including the rostral cingulate motor area. Further tests revealed that the difference in neuronal activity was not due to gaze direction or muscular activity. We suggest that the MFC is involved in self-other differentiation in the domain of motor action and provides a fundamental neural signal for social learning.
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