The effects of interrupted noise exposures on the noise-damaged cochlea

A variety of interrupted noise exposure paradigms will produce a toughening effect in the mammalian auditory system. That is, the threshold shift will gradually become smaller with each successive daily exposure. The ability of the system to be toughened has not been explored in subjects with a pre-existing noise-induced hearing loss. Using the chinchilla as the experimental animal, evoked potential audiometry to obtain thresholds, and surface preparation histology to quantify the sensory cell population, the issue of toughening was examined in the noise-damaged auditory system. Toughening was produced by a 1.0 kHz, narrow-band impact at 115 dB peak SPL for 10 days, 6 h/day, and trauma was produced by a 1.0 kHz, narrow-band impact at 121 dB peak SPL for 5 days, 24 h/day. Four groups of animals were used. Group 1: traumatic exposure followed 30 days later by the toughening exposure. Group 2: toughening exposure followed 30 days later by the traumatic exposure. Group 3: a trauma-only control. Group 4: a toughening-only control. Group 2 that received the toughening exposure 30 days prior to the traumatic exposure showed a 10 to more than 20 dB toughening effect between the 0.5 and 4.0 kHz test frequencies, while Group 1 that received the traumatic exposure followed 30 days later by the toughening exposure showed no toughening. The permanent changes in the evoked response audiograms and sensory cell populations were the same in Groups 1, 2 and 3 that were exposed to the traumatic noise, regardless of whether or not the animals were ever subjected to the toughening noise or whether the toughening noise preceded or followed the traumatic noise.