PROJECT SUMMARY Living organisms interact with complex environments that are inherently multisensory. Within these environments, the brain must integrate information from multiple sensory modalities, including the auditory and olfactory systems, for perception and behavior. However, despite the ubiquity and importance of multisensory integration, there is a critical gap in our understanding of how the brain integrates auditory and olfactory stimuli. This proposal will fill this knowledge gap by revealing the mechanisms underlying auditory-olfactory integration in the auditory cortex. Previous studies have reported that neurons in the auditory cortex project to the olfactory (piriform) cortex; however, preliminary results in this proposal identify a novel pathway in which the piriform cortex directly projects to the auditory cortex. These experiments will test the hypothesis that the cross-modal integration of olfactory sensory information with auditory information is facilitated by neuronal connections from the piriform cortex to the auditory cortex. To this end, these experiments will use innovative approaches to characterize and interrogate newly discovered brain circuitry. Specifically, cutting-edge intersectional viral tracing strategies will label projections from the piriform cortex to auditory cortex, and identify auditory cortical neurons in an auditory-olfactory feedback loop. To functionally interrogate this circuitry, we will develop a state-of-the-art experimental system for delivering combinations of auditory and olfactory stimuli. Using in vivo electrophysiology and optogenetics we will test whether and how piriform-to-auditory cortex projections mediate cross-modal auditory-olfactory integration. This work, part of a collaborative effort by the Geffen and Gottfried laboratories, will lay the groundwork for a new frontier in multisensory neuroscience research, and provide an ideal postdoctoral training environment.