Changes in Perception of Loudness and Absolute Threshold Associated with Hair Cell Loss in the Patas Monkey.

Abstract

This work tested the adequacy of the patas monkey (Erythrocebus patas) treated with dihydrostreptomycin as a model of loudness pathology in sensorineural hearing loss. It further sought to compare measures of loudness, hearing loss and cochlear pathology in an effort to identify orderly relations among these three categories of data, and to test the hypothesis that hair cell loss underlies recruitment of loudness. Three patas monkeys were behaviorally trained to report the presence of pure tones. The speed with which the animal made the response was used as an index of loudness. Hearing loss was produced by administration of the ototoxic aminoglycoside dihydrostreptomycin, to which the patas monkey is remarkably sensitive. Loudness and threshold functions were monitored until hearing loss stabilized. Surface preparations of the organ of Corti were examined post-mortem, and cytocochleograms were plotted. Progressive changes in loudness functions accompanied the development of hearing loss. Perception of low-level tones changed first and most, the magnitude of the overall change in loudness being directly related to frequency and amount of threshold shift. Complete recruitment was observed in early stages of the loss, when threshold shift was small to moderate, or when outer hair cells were selectively missing. Partial recruitment was observed in later stages, when threshold shift was moderate to large, and some inner hair cells were also missing. The results suggest that a normal rate of loudness growth requires the functional integrity of outer and inner hair cells, and that complete recruitment requires the integrity of the inner hair cells. At least some of the differences in recruitment reflect differences in severity rather than in kind of auditory pathology.