Effects on guinea pig cochlea from exposure to moderately intense broad-band noise.

Guinea pigs, which were either anesthetized (A) or conscious (U), were exposed to four 2 h sessions of broad-band noise of 96 dB SPL. Cochlear microphonics and N1 thresholds were measured prior to killing from 1 to 13 days later. The cochleas were examined by SEM and by section. The U series suffered less N1 threshold loss and recovered within 10-13 days, while the large initial loss in the A series did not completely reverse within the period of study. Initially, the IHC stereocilia in the basal half of the cochlea showed marked bending, the affected area being somewhat more extensive in the A group. These hairs gradually recovered, although not progressively. On the other hand, the disturbance to OHC stereocilia, which appeared to be less pronounced early on and was more apically centered, developed over time into marked permanent damage. The outermost row consistently showed the greatest effect with hairs becoming elongated or fused, and occasionally lost altogether. Susceptibility to noise varied between individuals of both groups. The recovery of N1 threshold was concomitant with the recovery of the erectness of the IHC hairs.