Effects of cochlear-synaptopathy inducing moderate noise exposure on auditory-nerve-fiber responses in chinchillas

Abstract

It has been hypothesized that selective loss of low-spontaneous-rate (low-SR) auditory-nerve (AN) fibers following moderate noise exposure may underlie perceptual difficulties some people experience in noisy situations, despite normal audiograms. However, the finding of selective low-SR-fiber loss has not been replicated in an animal model with behavioral thresholds similar to humans. We recently established a behavioral chinchilla model for which neural and behavioral AM-detection thresholds are in line with each other and similar to humans. Here, we report physiological AN-fiber response properties from anesthetized chinchillas exposed to noise that produced cochlear synaptopathy, as confirmed by immunofluorescence histology. Auditory-brainstem responses, distortion-product otoacoustic emissions, and compound action potentials confirmed no significant permanent threshold shift. Stimuli included both simple (pure tones, as studied previously) and complex (broadband noise) sounds. Low-SR fibers were reduced in percentage (but not eliminated) following noise exposure, as shown previously in guinea pigs. Saturated rates to tones were reduced. Similar tuning and temporal coding were observed in broadband-noise responses following noise exposure. Complete characterization of AN-fiber responses to complex sounds in a mammalian behavioral model of noise-induced cochlear synaptopathy will be useful for understanding suprathreshold deficits that may occur due to hidden hearing loss. [Work supported by NIH (R01-DC009838).]