Neural firing patterns in the chinchilla cochlear nucleus after noise‐induced asymptotic threshold shift

Chinchillas were exposed to an octave band of noise (CF = 4.0 kHz, 86 dB SPL) for three to five days. At the cessation of the noise, unit activity was sampled in the dorsal cochlear nucleus and ventral cochlear nucleus over a period of 1–12 h post‐exposure. In previous experiments [J. Mills, J. Speech Hear. Res. 16, 426–438 (1973)], the same exposure produced 40 dB of asymptotic threshold shift at 2–8 kHz measured behaviorally. Single units having CF's below 1.0 kHz had normal tuning curves while those above 1.0 kHz had thresholds that were elevated, on the average, by 40 dB and, in some instances, by as much as 70 dB. Tuning curves from the high‐threshold units were abnormally broad due to the shallow slope on the low‐frequency side of the tuning curves. In some units only inhibitory areas were present, while in others, the threshold for the low‐frequency inhibitory areas was considerably lower than for the excitatory area. Spontaneous activity from the high‐threshold units appeared to be normal or lower than normal. The single unit results will be related to the postexposure behavioral audiogram and cochleagram.