PROJECT SUMMARY Auditory-guided behaviors occur whenever auditory information is used to drive our decisions and actions. The auditory cortex (ACtx) sits at the apex of the ascending auditory pathway but transforming acoustic features into a behavioral outcome necessitates communication with regions outside of this auditory hierarchy. The primary routes for ACtx to propagate auditory information throughout the brain are through intratelencephalic (IT) and extratelencephalic (ET) neurons in cortical layer (L) 5, which are classified based on their distinct and largely non-overlapping projection targets. However, little is known about the functional differences of these distinct excitatory populations within ACtx. Investigating the neural circuitry underlying auditory-guided behavior and how auditory information is used to drive behavior would fill a significant gap in our knowledge. A ubiquitous auditory- guided behavior is categorization: the transformation of acoustic features into discrete perceptual categories that drive a subsequent behavioral outcome. To investigate the role of L5 IT and ET neurons in such a behavior, we designed a novel auditory categorization task for mice and performed two-photon imaging across days of learning. Our preliminary data shows that L5 IT and ET neurons exhibit a divergence in their representation of stimulus category and behavioral choice that is sculpted across learning. This suggests that IT projections are important in initial stages of learning while ET projections are recruited and strengthened throughout learning. Aim 1 will determine the contributions of L5 IT and ET neurons to learning an auditory categorization task and Aim 2 will examine if these neurons differentially encode behavioral choice across learning. Combined, these two Specific Aims will provide initial insight as to how two distinct excitatory cell-types in the largest cortical output layer contribute to auditory-guided behavior. The outcomes of these experiments will expand our understanding of how sensory information is propagated throughout the brain and how cortical circuits enable complex behaviors. Furthermore, this proposal will considerably enhance my personal and professional development as an independent scientist. Successful completion of this proposal will prepare me for my long-term goal of becoming a principal investigator in auditory neuroscience.