PROJECT SUMMARY Practice can improve the detection or discrimination of auditory stimuli. This process, called perceptual learning, facilitates language acquisition and the development of musical skills, and improves speech understanding in the hearing-impaired. Previous studies have highlighted the importance of the auditory cortex in auditory perceptual learning, but we lack a complete understanding of the neural circuit mechanisms involved. Corticofugal projections from the auditory cortex to subcortical auditory regions comprise a massive descending pathway. This pathway integrates non-sensory inputs, which are essential to perceptual learning, with sensory information and sharpens neural responses to relevant stimuli in subcortical auditory regions. Inactivation of this corticofugal pathway also impairs learning in sensory tasks with difficult, near-threshold stimuli. Taken together, the existing evidence suggests that corticofugal projections are well-positioned to play an important role in perceptual learning. The core hypothesis of this proposal is that perceptual training strengthens corticofugal modulation of activity in the ascending auditory pathway, leading to gradual enhancements in signal detection that underlie perceptual learning. Aim 1 will use in vivo electrophysiology in freely-moving, behaving animals to determine whether there are learning-related changes in neural activity in subcortical auditory regions. Aim 2 will investigate whether corticofugal projections are necessary for the acquisition and/or maintenance of training-induced improvements in perceptual sensitivity. Aim 3 will use projection-specific fiber photometry recordings to assess if the activity of corticofugal neurons correlates with behavioral improvements during perceptual learning. The findings from the proposed experiments will shed light on the neural circuits underlying auditory perceptual learning and may ultimately inform strategies to improve auditory perception in clinical populations.