Extracochlear electrically evoked otoacoustic emissions: A model for in vivo assessment of outer hair cell electromotility

Cochlear outer hair cell (OHC) motion in response to changes in membrane potential (electromotility) has been extensively studied in vitro. Electromotility is thought to actively control the micromechanical properties of the sensory epithelium. In order to understand how OHC electromotility contributes to normal cochlear responses, its role must be assessed in vivo. We have developed a novel animal model for the study of electromotility in vivo. Alternating current is delivered by an electrode to the round window (RW) of gerbil cochlea and the electrically evoked otoacoustic emission (EEOE) is measured from the external ear canal. As much as 45 dB SPL sound could be generated by about 200 mu A RMS extracochlear current delivered to the RW. Except for the fine structure of EEOE transfer function curves, the magnitude of the EEOE has a bandpass appearance ranging from about 4 to 32 kHz and shows a positive linear relationship to the current intensity. The phase has a linear relationship with frequency and shows no significant change with current intensity. Local intracochlear perfusion of 4% paraformaldehyde caused EEOE to decrease by approximately 20 dB. These results indicate that the EEOE is probably generated by OHCs near the electrode location and propagates to the external ear canal. In addition, the force generated by OHCs in vivo is a linear function of the electrical stimulus. The major advantages of our model include: (1) non-invasive procedure and normal cochlea; (2) wide dynamic range of the measurement; (3) simple and easy preparation. With these features this model has potential applications in basic hearing research and in the diagnosis and treatment of otological patients.