Neural and behavioral binaural hearing impairment and its recovery following noise-induced cochlear synaptopathy

Abstract

Noise-induced cochlear synaptopathy refers to a loss of synapses in the inner ear but no change in audiometric thresholds. Animal studies have focused primarily on peripheral hearing measures to diagnose synapse loss, while suggesting binaural listening deficits such as speech-reception-in-noise result from this disorder, but have not accounted for the possible regeneration of synapses. To address this, we measured binaural physiological and behavioral function in adult guinea pigs with noise-induced synaptopathy. Physiological measurements extended to two months post noise exposure. While common audiological assessments showed temporary threshold loss, reduced evoked potential amplitudes indicative of synaptopathy and measurable binaural electrophysiological hearing deficits post exposure, all measures recovered by two months including binaural behavior. Suspected regeneration of synaptic ribbons occurred by two months post exposure and cochlear histology revealed no synaptic loss two months post exposure. We show that the same noise exposure protocol that caused synaptic loss in prior studies causes physiological binaural processing deficits in the brainstem and that the regeneration of synapses corresponds with recovered binaural processing including behavior. Results suggest that functional recovery of ribbon synapses post a single moderate noise exposure is sufficient to restore binaural hearing abilities.