Electrically evoked otoacoustic emissions from the chicken ear

Abstract

The outer hair cell electromotile response is believed to underlie the sharp tuning and exquisite sensitivity of the mammalian inner ear, and contribute to the production of electrically evoked otoacoustic emissions (EEOAEs) and sound-evoked otoacoustic emissions (OAEs). Avian ears are also sharply tuned, extremely sensitive and generate spontaneous and sound-evoked OAEs, but avian hair cells do not exhibit somatic electromotility. However, stereocilia bundle movements have been observed in avian and amphibian hair cells suggesting that EEOAEs might arise from electrically evoked bundle movements. Here, we demonstrate for the first time that AC current applied to the round window of the chicken evokes EEOAE of up to 18 dB SPL. The EEOAE produces a bandpass response with maximum amplitude in the 1000–3000 Hz range; the response drops off rapidly above 4000 Hz and below 500 Hz. The impulse response to current pulses is characterized by a large peak sometimes followed by a damped oscillation with a frequency around 2000 Hz. EEOAEs decreased significantly after anoxia and paraformaldehyde damage of the cochlea. Kanamycin-induced hair cell loss also caused a significant reduction in EEOAE and distortion product OAE; these emissions showed only a small recovery at long recovery times, when most hair cells should have regenerated. These results suggest that the EEOAE has a biological origin in the cochlea, which could presumably involve electrically evoked stereocilia bundle movements.