Binaural brainstem and spatial hearing deficits in a guinea pig model of noise-Induced cochlear synaptopathy

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Animal studies have revealed that moderate-level noise exposure can cause a permanent loss of ribbon synapses between inner hair cells and auditory-nerve fibers, but only temporary threshold shifts. Such noise-induced cochlear synaptopathy has been called ‘hidden hearing loss’ because while there is a significant degeneration of ribbon synapses, the resulting hearing dysfunction is effectively hidden from typical clinical assays. Here we used guinea pigs to study the mechanisms leading to hearing deficits resulting from a moderate noise. We measured distortion product otoacoustic emissions and auditory brainstem responses to assay peripheral hearing. We tested spatial hearing ability through the prepulse inhibition (PPI) of the acoustic startle reflex. PPI was used to measure hearing-in-noise ability, or spatial release from masking, as this task approximates the “cocktail party” effect. Finally, we performed immunohistochemistry to confirm synaptopathy. Results show that the noise exposure induces no permanent hearing threshold shift, but despite recovery of normal audibility, both behavioral and electrophysiological binaural hearing deficits persisted. Cochlear synaptopathy was objectively confirmed by visualizing the loss of ribbon synapses in the cochlea. The results demonstrate that cochlear synaptopathy causes deficits in brainstem circuits known to be critical for binaural and spatial hearing. [Work supported by Otolaryngology T32 DC012280.]