Physiological and histological changes associated with the reduction in threshold shift during interrupted noise exposure

The compound action potential (AP) was recorded from one group of chinchillas exposed to interrupted noise (95 dB SPL, octave band centered at 500 Hz, 3 h on, 9 h off) for 15 days. A second group of chinchillas was exposed to the same interrupted noise for 1, 2 or 15 days and their cochleas were analyzed by scanning electron microscopy (SEM). During the first few days of the exposure, the AP threshold was elevated approximately 40 dB at the low-to-mid frequencies; however, the threshold shifts decreased with increasing exposure duration so that the threshold shift was only about 10 dB after the 15th day of exposure. The amplitude of the AP also recovered with exposure time. In contrast to the improvement in AP threshold, the number of missing hair cells increased and the condition of the stereocilia on inner and outer hair cells deteriorated between the first and 15th day of the exposure.