Long-term effect of acoustic trauma on distortion product otoacoustic emissions in chickens.

Distortion product otoacoustic emissions (DPOAE), 2F1-F2, were measured in chickens before and after exposure to a 1.5-kHz pure tone presented at 120-dB sound-pressure level for 48 h. The low-level component of the DPOAE input/output function was shifted to the right at all frequencies immediately after the exposure with the greatest effect occurring at and above the exposure frequency. The slope of the high-level component of the input/output also increased significantly for test frequencies close to and above the exposure frequency so that the amplitude of the DPOAE was equal to or greater than normal at high stimulus levels. DPOAE obtained at the highest and lowest frequencies were almost normal after 8 weeks of recovery; however, the input/output functions near the exposure frequency showed almost no improvement over the 8-week recovery period. The lack of recovery could conceivably be due to residual damage to hair cells that survived the exposure or to incomplete regeneration of the tectorial membrane.