Effects of Noise Exposure on Cochlear Anatomy, Auditory Physiology, and Hearing-in-Noise in Nonhuman Primates
Mondul, Jane Ann
0000-0003-3256-6984
:
2022-06-15
Abstract
Hearing-in-noise is a complex auditory task that is critical for effective communication in the presence of competing sounds. Damage to the inner ear caused by noise exposure can alter the representation of sounds in the brain and impair hearing-in-noise abilities. Noise exposure can result in loss of outer hair cells and poor hearing sensitivity associated with sensorineural hearing loss (SNHL), which is readily measured in the clinic. Alternatively, the connections between inner hair cells and the auditory nerve can be damaged (cochlear synaptopathy, SYN), leading to deafferentation of the auditory system. SYN may contribute to hearing-in-noise difficulties reported by patients who have clinically normal hearing function (i.e. ‘hidden hearing loss’). Nonhuman primates are a useful animal model of noise-induced hearing loss (NIHL), given their anatomical, perceptual, and phylogenetic similarity to humans, their ability to perform hearing-in-noise tasks, and the ability to examine inner ear tissues. Forming a translational bridge between rodent and human NIHL research, we investigated the consequences of noise-induced sensorineural hearing loss (SNHL) and cochlear synaptopathy (SYN) in macaque monkeys using complementary anatomical, physiological, and perceptual assays. Cochlear histological markers included hair cell counts, ribbon synapse counts and sizes, and efferent innervation density. Auditory brainstem responses, otoacoustic emissions, medial olivocochlear reflexes, and middle ear muscle reflexes were used to probe the function of neuronal mechanisms and pathways that contribute to hearing-in-noise. Hearing abilities were measured using psychoacoustic tone detection paradigms that probed hearing sensitivity, growth of masking, frequency selectivity, forward masking, and overshoot. We identified unique patterns of cochlear damage, physiological changes, and hearing-in-noise deficits accompanying noise-induced SNHL and SYN. Importantly, we provided direct evidence that SYN affects hearing-in-noise abilities and that SYN may contribute to the variable hearing-in-noise abilities accompanying SNHL. This work may help establish biomarkers for SYN, which will enhance site-of-lesion specificity in the differential diagnosis of hearing disorders.