We propose to explore the role of the medial olivocochlear (MOC) efferent system in enhancing and “sharpening” the encoding of complex sounds. We will use extracellular recordings in awake animals of inferior colliculus (IC) and ventral nucleus of the trapezoid body, where MOC neurons are located. We also propose psychophysical studies in humans, click-evoked otoacoustic-emission measurements, and a new computational model that includes efferent pathways. In previous grant cycles, we discovered a novel neural- coding strategy based on relatively slow fluctuations in the responses of auditory-nerve (AN) fibers, referred to as neural fluctuations (NFs). For listeners with normal hearing, contrasts in NF profiles along the tonotopic axis provide a robust code for spectral peaks and valleys across a wide range of sound levels and frequencies, as well as in noise, but NF contrasts are adversely affected by sensorineural hearing loss. NF contrasts are shaped by peripheral tuning and nonlinearities, which are in turn strongly affected by cochlear gain, motivating our focus on the MOC system in this application. The MOC has traditionally been described as the hub of a short reflex loop, with wide-dynamic-range inputs from the cochlear nucleus and an output that projects directly to cochlear outer hair cells to modulate cochlear gain. An assumed role of the efferent system is to increase the dynamic range of neurons, reducing rate saturation. It has not been possible to test this assumption using AN recordings, but cochlear nucleus recordings in unanesthetized animals reveal that responses are saturated at moderate levels and dynamic ranges are not increased, bringing into question this presumed function of the efferent system. We propose a new concept: a purpose of the efferent system is to maintain and enhance, or sharpen, NF contrasts. A less-studied but major input to the MOC descends from the IC. Because nearly all IC neurons are sensitive to slow fluctuations on their inputs, NF contrasts are represented by the profile of rates across the population of IC neurons. We hypothesize that IC input to the MOC provides the NF-driven signal that is required by a control system that maintains and enhances NF contrasts. The IC-rate profile, combined with the wide-dynamic-range input from the cochlear nucleus, provides the efferent system with the information required to control NF contrasts across a wide range of acoustic conditions. Exciting preliminary results support the hypothesis that this system actively sharpens the representation of complex sounds. Our new computational model that includes efferent pathways provides testable predictions for IC and MOC physiological responses and for psychophysical performance in human listeners, with varying degrees of sensorineural hearing loss. Understanding nonintuitive effects of different degrees of sensorineural hearing loss on NF profiles, in the context of dynamic MOC feedback, provides new insight into the pe...