PROJECT SUMMARY Romantic bonds are a fundamental part of the human experience, positively contributing to our health and well-being by decreasing risk for physical and mental illness. A key feature of pair bonds - and social relationships more generally - is that they are highly reciprocal, evolving as a function of ongoing feedback from the other individual. Yet the interdependent nature of neural activity within pair bonding remains understudied, due in part to the dearth of laboratory-amenable mammals that form selective attachments with their mating partners. One promising metric for explaining aspects of interaction and its progression in dynamic social contexts is interbrain synchronization, or correlation, in which the neural dynamics of two individuals become more similar to each other as they interact. Interbrain correlation scales with relational variables; it is stronger in romantic partners than strangers during social interaction. I have found that interbrain correlation is also evident in prairie voles and that pair bonded voles show stronger correlations when interacting with their partner than with a stranger. In this proposal, I will leverage modern optical tools to detect and functionally manipulate interbrain correlation in prairie voles. In my first aim, I will perform within-animal comparisons to determine how interbrain correlation varies across discrete behaviors and as a function of pair bonding. I will use fiber-photometry to conduct simultaneous real-time calcium recordings in the medial prefrontal cortex of interacting animals, and use supervised and unsupervised machine learning to conduct thorough behavioral annotations of the interaction. Using a variety of analytical approaches, including generalized linear models, I will delineate the relationship between pair bonding, affiliative behavior and interbrain correlation. In my second aim, I will use synchronized real-time optogenetic stimulation in the mPFC to test whether induced interbrain correlation can facilitate pair bond formation. Together these results will characterize the occurrence of interbrain correlation during distinct behaviors and as a function of bond state and test the function of interbrain correlation in pair bond formation. Such work is instrumental for determining whether therapeutic approaches targeting interbrain correlation will be beneficial for treating disorders that impair social attachment like autism spectrum disorders.