PROJECT SUMMARY Audiologists frequently encounter patients who complain of speech-in-noise perception difficulties despite having normal hearing sensitivity. The neurobiological bases of such deficits are currently debated and remain unknown, which limits development of precision diagnostics and targeted treatment strategies to help these patients. As a consequence, many may suffer untreated and debilitating communication impairments in real world situations, causing social withdrawal, limiting academic achievement, and/or constraining vocational opportunities. Limited evidence suggests that the efferent auditory pathway, which descends from the brain to the inner ears and putatively filters out noise before it enters the nervous system, may be impaired in some of these patients. This research project investigates relationships between inner ear efferent activity and subcortical processing of speech-in-noise during active listening in audiometrically-normal adults with and without clinically-documented speech-in-noise deficits. Specifically, we have developed a method for simultaneously measuring pre-neural (i.e., otoacoustic emissions; OAE) and neural (i.e., frequency following responses) biophysical responses evoked by naturalistic continuous speech. This approach makes it possible, for the first time, to directly study the influence of cochlear efferent effects on neural processing and perception of speech-in-noise. We hypothesize that: 1) “traditional” passive assays of cochlear efferent activity (i.e., OAE inhibition with contralateral noise) underestimate efferent modulation of cochlear mechanics during active listening to speech-in-noise; 2) attentional modulation of cochlear activity predicts attentional modulation of subcortical neural activity; and 3) cochlear and neural attention modulation effects can be used to statistically model speech-in-noise perceptual performance. The outcomes of this research will clarify the role of the efferent system in naturalistic speech-in- noise processing and may elucidate “sites of lesion” responsible for speech-in-noise deficits in audiometrically- normal listeners. Further, our findings may inform development of assistive listening devices and/or brain- computer interfaces that are inspired by typical neurophysiologic auditory function to aid listeners with speech- in-noise deficits.