Effect of low-frequency "conditioning" on hearing loss from high-frequency exposure.

Recent research has revealed that repeated exposures to a low-frequency noise results in a progressive reduction in threshold shifts (TS). This reduction in TS is not restricted to the exposure frequency, but can be observed at frequencies up to 3 or 4 octaves higher. Such "conditioning" exposures have also been observed to protect the auditory system against hearing loss from exposures to the same noise at higher levels. The aim of this study was to determine if "conditioning" using low-frequency exposures protects the auditory system against hearing loss from a high-frequency exposure. Monaural chinchillas were exposed to a 0.5-kHz octave band noise (OBN) at 95 dB SPL for 6 h a day. The animals were allowed to recover for 5 days, following which they were exposed to a 4-kHz OBN at 100 dB SPL for 48 h. Hearing thresholds determined using evoked potential recordings, indicated significantly greater permanent threshold shifts in this group of animals when compared to a control group exposed only to the 4-kHz OBN. These results were confirmed by histological examination which revealed greater hair cell loss in the experimental group.