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A positive‐reinforcement procedure was used to determine hearing sensitivity and noise‐induced hearing loss in the chinchilla. The animals were reinforced with 45‐mg pellets (Noyes) for depressing and holding a key for 1–6 sec and either releasing it during a 2‐sec tone presentation or not releasing it during a 2‐sec catch trial. Using a tracking procedure, thresholds were determined at 9–15 frequencies during each daily 1‐h session. Following threshold determinations, two animals were exposed to a 123‐dB‐SPL band of noise (710–2800 Hz) for 15 min. Threshold shifts were measured daily after the exposure and the animals were sacrificed at 7 and 28 days post‐exposure, respectively. Their cochleas were examined under the light microscope and the number of hair cells missing was compared with the hearing loss present. Contrary to previous findings, both animals showed a considerable permanent threshold shift that agreed closely with the degree of change seen in the cochlea. The advantages and disadvantages of the new behavioral procedure are discussed.

Old World monkeys were exposed to octave‐band or wide‐band noise at 117–120 dB SPL, 8 hours daily, five days per week for four weeks. The octave bands were centered at 500, 2000, 4000, or 8000 Hz. Thresholds for frequencies 63–40 000 Hz were measured daily before, during, and for one month after exposure, by the staircase method, using operant conditioning procedures with positive reinforcement. After the final threshold measurements the temporal bones were fixed and the inner ears examined by microdissection and phase‐contrast microscopy. Maximum TTS, usually attained on the first day of exposure, ranged from 60 to 100 dB; PTS, from 40 to 60 dB. Both temporary and permanent loss involved at least two or three octaves and for the most part were greatest at, or within the octave above, the center of the exposure band. Hair cell counts showed reasonable correspondence between the location, severity, and extent (5–12 mm) of outer hair cell loss and the degree of PTS, in accordance with the place principle. I...

Absolute thresholds were determined in the chinchilla using a new behavioral procedure. The animal was reinforced with a 45‐mg pellet (Noyes) for depressing and holding a key 1–7 sec and either releasing it during a 2‐sec tone presentation or not releasing it during a 2‐sec catch trial. Using a tracking procedure, thresholds were determined at 9 to 15 frequencies during each daily 1‐hour session. Thresholds are compared to those obtained by conventional shock‐avoidance techniques, and the advantages and disadvantages of the procedure are considered. Following threshold determinations, two of the animals were exposed to a 123‐dB two‐octave band of noise (710–2800 Hz) for 15 minutes. The animals were sacrificed 7 and 28 days post exposure, respectively and their cochleas prepared for examination under the light microscope by the surface preparation technique. The correlation between the amount of hearing loss present and the cochlear pathology is examined, and these results are compared to those obtained by Ward and Duvall [Ann. Otol. 80, 881 (1971)]. [Research supported by NIH research and training grants.]