Changes in the stereocilia and non-monotonic 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.