The effects of sound overexposure on the spectral response patterns of nucleus magnocellularis in the neonatal chick.

Spectral response plots from single cells in the chick nucleus magnocellularis were obtained following a 48 h exposure to a 0.9 kHz pure tone at 120 dB sound pressure level and after a recovery period of 12 days. Immediately after removal from the exposure, a variety of changes in the spectral response patterns of nucleus magnocellularis cells were noted. In particular, at recovery day 0, there was a significant decrease in spontaneous rate; a substantial loss of threshold sensitivity and frequency selectivity; and alterations in both the slope and dynamic range of the rate-intensity function. Interestingly, the maximum discharge rate appeared unaffected by the intense sound. Twelve days after removal from overstimulation, auditory function appeared to be operating at normal levels of efficacy as spontaneous rate, threshold sensitivity, frequency selectivity, and the dynamic range of the rate-intensity function were statistically identical to similar measures in nonexposed control birds. The loss and restoration of auditory function is correlated to structural damage and repair of the avian basilar papilla.