Protection from noise-induced hearing loss by prior exposure to a nontraumatic stimulus: role of the middle ear muscles.

Recent evidence suggests that prior exposure to a moderate-level acoustic stimulus can reduce damage due to later exposure to the same stimulus at high intensity [Canlon et al., Hear. Res. 34, 197-200 (1988)]. To test the role of the middle ear muscles (MEMs) in this phenomenon, Mongolian gerbils were conditioned by exposure to a two-octave band of noise (1414-5656 Hz) at 81 dB SPL for 3 weeks. Either immediately afterward, or following a one week rest period, they were exposed to the same stimulus at 110 dB SPL for one hour. The ABR thresholds of these animals were compared to those seen in animals exposed at 110 dB SPL without conditioning. The MEMs of one ear in each subject were cut, to determine their role in any noise trauma protection effects. In the unoperated ears, conditioning without a recovery period did not alter the effects of the 110 dB stimulus. Conditioning followed by a one week recovery period reduced both temporary (TTS) and permanent (PTS) threshold shift. MEM section had no effect on either TTS or PTS in unconditioned subjects, and did not alter the reduction in TTS or PTS seen with conditioning. It is concluded that the noise trauma resistance provided by acoustic conditioning is not mediated by the MEMs.