PROJECT SUMMARY In our daily life, even in the face of multiple sound sources, our brain binds together frequency components that belong to the same source and recognizes individual sound objects. In humans, the integration of spectral components into single sound perception relies on the precise synchrony of their onset timings, and this grouping plays a critical role in our perception of language. In our ongoing parent grant (R01 DC017516), we have identified the mouse secondary auditory cortex, A2, as a locus of multi-frequency integration, whose synchrony-dependent sound responses mirror perceptual binding in humans. However, the circuit mechanisms underlying how A2 neurons integrate information across parallel auditory channels to achieve coherent perception remain unknown. Secondary sensory cortices are generally considered to receive pre-processed inputs from the primary cortices. Interestingly, recent studies across sensory modalities suggest that secondary cortices may also receive sensory inputs directly from the periphery, bypassing primary cortical areas. In the auditory system, both a recent paper and our preliminary data showed that A2 receives direct inputs from the primary auditory thalamus (ventral division of the medial geniculate nucleus: MGv). These results require reconsideration of the sensory pathways that underlie the specialized functions of higher-order cortical areas. In this proposed project, we will delineate the MGv-A2 ascending pathway using anatomical and electrophysiological techniques to understand its contribution to the integrative function of A2 circuits. Our specific goals aim to 1) identify the inferior colliculus subdivision upstream to the MGv-A2 pathway using tracer injections and 2) determine the contribution of MGv inputs to A2 sound representations using optogenetics during in vivo electrophysiological recordings. These aims will expand the scope of the parent study, which focuses on the cortical circuits within A2 that integrate multi-frequency sounds. Dissection of the ascending pathways that reach A2 will provide insights into the mechanisms that shape A2 sound representations and provide a step towards understanding the “feature binding” circuits that enable verbal communication. Through the proposed Post-baccalaureate Supplement, Ms. Michellee Garcia will work closely with her mentor, Dr. Kato, to receive training in cutting-edge neuroscience techniques, auditory systems neuroscience, and writing/communication skills. Ms. Garcia’s long-term goal is to join a neuroscience Ph.D. program and continue her career in academia as an independent scientist. This supplement project will help her career development by preparing her to pursue neuroscience research as a graduate student and beyond.