Auditory nerve fiber activity, behavioral thresholds and cochlear histology following noise induced asymptotic threshold shift

Chinchillas were exposed three to five days to a 95‐dB SPL octave band of noise centered at 0.5 kHz. During the exposure, the behaviorally measured thresholds reached an asymptotic level that was between 40–60 dB SPL from 0.5–16.0 kHz. During a 27–15 h period following the exposure, single auditory nerve fibers and the whole nerve action potential (AP) were studied with clicks and tone bursts. Single neuron thresholds were generally 40–60 dB above normal values and were in general agreement with the behavioral audiogram. Fiber tuning curves were broader than normal with reduced slopes. Some units had a region of increased sensitivity below CF. Click thresholds for the units were generally 40–60 dB above normal levels. The visual detection level for AP was also 40–60 dB above its normal value and consistent with the single neuron data. The cochleagrams obtained 15 h after the exposure generally showed a peak outer hair cell loss (<50%) near the apex with smaller losses (<20%) at more basal locations. [Research supported by NSE.]