Discharge patterns in the cochlear nucleus of the chinchilla following noise induced asymptotic threshold shift.

Chinchillas were exposed to an 86 dB SPL octave band of noise centered at 4.0 kHz for 3.5--5 days. The noise elevated the hearing thresholds between 4.0 and 16.0 kHz to between 60 and 75 dB SPL. Measurements from single neurons in the cochlear nucleus revealed abnormalities in the response properties of neurons with characteristic frequencies (CF) above 2.0 kHz. Units above 2.0 kHz had elevated thresholds (between 50 and 90 dB SPL) and broad tuning curves due to a greater loss in sensitivity near CF than at lower frequencies. The tuning curve Q10dB values for high frequency neurons were generally less than 3.0 and approached the Q10dB values for basilar membrane displacement. Spontaneous activity rates in units above 2.0 kHz were also low. In a few units, the threshold for single tone inhibition was significantly lower than that for excitation; the best inhibitory frequencies were always below 2.0 kHz. Two-tone inhibition was present in both low and high threshold neurons, but its strength was not assessed. Cochleagrams obtained 12 hours postexposure revealed discrete hair cell lesions in the basal third of the cochlea. The locations of the lesions were consistent with the frequencies of maximum hearing loss. The behavioral thresholds and the thresholds at CF of the most sensitive units were within 10--15 dB of each other. The results indicate that intense sounds reduce the sensitivity, frequency selectivity and spontaneous activity of units in the cochlear nucleus. The findings are similar to those obtained in auditory nerve fibers with ototoxic drugs and hypoxia.