Abstract
Four Old World monkeys were exposed 24 h a day for 90 days to a 90 dB SPL octave band of noise centered at 2 kHz. Monaural pure-tone thresholds were determined prior to, during, and after exposure by means of the tracking method, using operant conditioning with positive reinforcement. Maximum TTS was observed at 2.8 kHz. For three animals, the TTS function showed an initial plateau of 19 to 21 dB after 7 to 12 h of exposure; the TTS function in the fourth animal reached an initial plateau of approximately 60 dB by the fourth day of exposure. Further shifts from this plateau were observed in the TTS functions for three of the animals. Following the period of exposure, threshold in two of the animals recovered to within 5 dB of baseline after 72 h. The remaining two animals showed PTSs of 15 and 25 dB. Microscopic examination of the organ of Corti indicated some scattered hair cell loss in the ears of all animals. Details of the histopathology and hair cell counts are presented. Implications of these results to noise-induced threshold shift in humans and chinchillas are discussed. [Work supported by grants from NINDS and NIOSH.]
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