PROJECT SUMMARY One of the most critical social decisions that animals make is whether to approach or ignore a conspecific based off of previous interactions. For instance, mice will readily investigate novel conspecifics while avoiding familiar conspecifics. Emerging research has established that the ventral hippocampus (vHPC) is necessary for discriminating between novel and familiar conspecifics. However, it remains unclear how novelty recognition information encoded by the vHPC is transformed by a series of downstream brain regions to guide subsequent behaviors such as social approach and investigation. The lateral septum (LS) receives strong projections from the vHPC, and it in turn projects to the nucleus accumbens (NAc), a region that promotes social approach and affiliation. Therefore, the LS is ideally positioned to integrate recognition information from the vHPC and in turn promote investigation through its projection to the NAc. In this proposed project, I will leverage sophisticated in vivo recording techniques, projection-specific optogenetic stimulation, and novel computational approaches to elucidate the role of the vHPC- dLS-NAc pathway in novelty-related social investigation. In Aim 1, I will use cellular resolution calcium imaging to determine how social recognition information and motivated social behaviors are encoded in vHPC-LS and LS-NAc projection populations. In Aim 2, I will optogenetically excite and inhibit LS-NAc neurons to determine if this projection is causally involved in regulating investigation of a novel conspecific. Completion of these aims will answer a fundamental question regarding how sensory information of a social stimulus is transformed into motivated social behavior. Additionally, findings from this study will open avenues for future research that can explore how this pathway might be disrupted in disorders that impact social recognition such as Alzheimer’s Disease.