DPOAE modifications induced by pure tone overstimulation in guinea pigs.

Distortion Product Otoacoustic Emissions (DPOAEs) shows a vulnerability to acoustic overstimulation that is easily detectable by changes in amplitude, frequency distribution and time course. The frequency-specific changes in DPOAE provide more information than the more general changes in click evoked otoacoustic emissions. In this work five anesthetized guinea pigs were examined for changes DPOAE after exposure to pure tones. The noise was a 110 dB SPL pure tones for 45 minutes and the fatiguing tone centered on the geometric mean (GM) of primaries or 2/3 of GM. The most measurable effects were obtained in the latter conditions. The main DP level reduction lasts about one hour after exposure, but the complete recovery is observable only after 24 hours. The frequency distribution of fatiguing effects on the DP- audiogram shows a remarkable fine tuning and a pattern like a low-pass filter. After four hours, in one guinea pig, it is observable an enhancement of the DP amplitude, compared to the pre-exposure level; in another guinea pig, a second overstimulation produced a more extensive and time-lasting effects than the first exposure.