Changes in audiometric threshold and frequency selectivity correlate with cochlear histopathology in macaque monkeys with permanent noise-induced hearing loss.

Exposure to loud noise causes damage to the inner ear, including but not limited to outer and inner hair cells (OHCs and IHCs) and IHC ribbon synapses. This cochlear damage impairs auditory processing and increases audiometric thresholds (noise-induced hearing loss, NIHL). However, the exact relationship between the perceptual consequences of NIHL and its underlying cochlear pathology are poorly understood. This study used a nonhuman primate model of NIHL to relate changes in frequency selectivity and audiometric thresholds to indices of cochlear histopathology. Three macaques (one Macaca mulatta and two Macaca radiata) were trained to detect tones in quiet and in noises that were spectrally notched around the tone frequency. Audiograms were derived from tone thresholds in quiet; perceptual auditory filters were derived from tone thresholds in notched-noise maskers using the rounded-exponential fit. Data were obtained before and after a four-hour exposure to a 50-Hz noise centered at 2 kHz at 141 or 146 dB SPL. Noise exposure caused permanent audiometric threshold shifts and broadening of auditory filters at and above 2 kHz, with greater changes observed for the 146-dB-exposed monkeys. The normalized bandwidth of the perceptual auditory filters was strongly correlated with audiometric threshold at each tone frequency. While changes in audiometric threshold and perceptual auditory filter widths were primarily determined by the extent of OHC survival, additional variability was explained by including interactions among OHC, IHC, and ribbon synapse survival. This is the first study to provide within-subject comparisons of auditory filter bandwidths in an animal model of NIHL and correlate these NIHL-related perceptual changes with cochlear histopathology. These results expand the foundations for ongoing investigations of the neural correlates of NIHL-related perceptual changes.