CHANGES IN THE STEREOCILIA AND NONMONOTONIC PATTERN OF THRESHOLD SHIFT AFTER EXPOSURE TO IMPULSE NOISE

The recovery pattern of threshold shift (TS) and 'dynamic' changes in the stereocilia after exposure to high-level impulse noise in guinea pigs were investigated. 33 albino guinea pigs were exposed to 10 impulse noises at the rate of 1/min. The noise had peak level of 166 dB SPL and a duration of 0.1 ms. Thirteen of the exposed animals were used to systematically measure threshold shifts at regular intervals from 0.5 h to 30 days post-exposure by click auditory cortex evoked response (AC-ER). Twelve of the animals who had typical TS at the same intervals were killed for scanning electron microscopic examination. The recovery pattern of threshold shifts was non-monotonic, which was different from that seen with continuous noise. There was an increase in TS after the exposure and the maximum level of TS was found at 8 h post-exposure. Morphological analysis also showed delayed changes in the stereocilia after exposure. The severity of changes in the stereocilia reached a peak at 8 h, at which time complete fusion of hair bundles took place. The tectorial material found on the tips of the stereocilia may be responsible for the sequence of the changes.