THE ROLE OF MIDDLE-EAR MUSCLES IN THE DEVELOPMENT OF RESISTANCE TO NOISE-INDUCED HEARING-LOSS

The role of middle ear muscles (MEMs) in the development of increased resistance to noise induced hearing loss (NIHL) was studied using mona ural chinchillas. Animals with severed MEMs as well as those with inta ct MEMs were exposed to an octave band noise (OBN) centered at 0.5 kHz at 95 dB for six hours/day for ten consecutive days. Results indicate d that animals with severed MEMs showed greater initial threshold shif ts (TS) than the animals with intact MEMs. Both the groups showed a de crease in TS over the ten days of exposure. The subjects were given fi ve days of recovery and then re-exposed to the same noise at 106 dB fo r 48 h. Permanent threshold shifts (PTS) in each group was compared ag ainst those in a control group exposed to the noise only at the higher level. Interestingly, both the 'conditioned' groups incurred substant ially less PTS than the control group exposed only to the higher level .