Project Summary Advanced social cognition permits cooperative and reciprocal interactions with other group members, making it possible for us and other highly social animals to capitalize on various benefits associated with prosociality. However, investigating the neural bases of cooperative interactions is quite challenging largely due to the fact that standard laboratory animal models of neuroscience do not overtly or reliably exhibit cooperative behaviors, let alone reciprocity based on altruism. Marmosets are cooperatively breeding non-human primates known for their prosociality and socially tolerant characteristics making them a model system with strong potential for use towards understanding complex social behaviors. Marmosets also provide strong promise for generating transgenic primates, and their flat cortices allow easy access to brain regions with high-density electrodes. In this exploratory R21 proposal, we will capitalize on recent advances in video-based continuous tracking of facial features and body orientations using a deep learning network to rigorously characterize cooperative interactions of marmoset dyads. Further, we will study neural activity from the orbitofrontal cortex (OFC) and the dorsolateral prefrontal cortex (dlPFC) – the two prefrontal regions centrally implicated in cooperation based on human functional magnetic resonance imaging studies – using wireless and high-density neural recording, allowing examinations of neural activity during naturalistic social interactions that are not accessible in traditional head- restrained electrophysiology experiments. We will apply these novel approaches in a cooperative behavioral paradigm inspired by marmoset behavioral ethology and primatology. We hypothesize that neurons in the OFC process reinforcing properties of cooperation, whereas neurons in the dlPFC strategically implement cooperation decisions. Overall, our proposal uniquely allows studying neural dynamics with precise, naturalistic, behavioral data, overcoming the difficulty of using naturalistic behaviors in experimental settings and of collecting neural data in observational field studies. The data from this R21 proposal will be critical for developing a larger research program in the future with multiple scopes and causal elements for understanding the neural mechanisms underlying cooperative social interaction.