Fine alterations of distortion-product otoacoustic emissions after moderate acoustic overexposure in guinea pigs.

Guinea pigs were exposed to a pure tone at 6 kHz and 80 dB SPL for 30 minutes in order to induce mild reversible auditory fatigue over the 4 hours following exposure. Cochlear monitoring aimed to compare the shifts in round-window compound action potentials (CAP) thresholds to those of 2f1-f2 distortion-product otoacoustic emissions (DPOAE, frequency of stimuli f1 and f2). Both responses were evaluated every 1/10th octave between 6 and 12 kHz for CAP thresholds, and from 4 to about 14 kHz for DPOAEs in response to 50- and 70-dB SPL stimuli. The auditory fatigue turned out to be sufficiently mild that DPOAEs remained present, so that their microstructure could be followed up while the stimulus frequency ratio f2/f1 was swept from 1.06 to 1.30 (fixed f2) so as to derive DPOAE level profiles against f2/f1 and group latencies. CAP thresholds decreased by about 10 dB above 7.2 kHz, whereas DPOAE amplitudes decreased at most f2 frequencies from 6.6 kHz to 15.2 kHz, with the range of decrease being slightly narrower at higher stimulus intensities. While the mean DPOAE shift after 1 hour was around 5 dB irrespective of stimulus intensity, it tended to increase slightly after 2 hours despite stable CAP thresholds. DPOAE profiles against f2/f1 were slightly modified by the auditory fatigue, so that the maximum tended to be reached at lower ratio. No significant variation of DPOAE latencies was found after acoustic overstimulation. These experiments show that complex DPOAE changes were induced by auditory fatigue and their relationship to CAP threshold changes does not seem to be straight-forward. Nonetheless, fine DPOAE recordings might be useful to detect early changes in cochlear mechanics.