Electrically-evoked otoacoustic emissions: Direct comparisons with basilar membrane motion

A fundamental question in cochlear physiology is how the otoacoustic emissions measured in the ear canal are related to the motion of the cochlear partition. In this study, otoacoustic emissions were evoked by sinusoidal electrical current injected into the scala media of the gerbil cochlea at the extreme base, and were compared to the electrically-evoked basilar membrane motion in the same animal. It was found that the primary electrically-evoked emission (EEOE) measured at the ear canal has a high frequency cutoff of about 20 kHz, and that the emission has a simple relationship to the basilar membrane (BM) motion at the current injection place: the magnitude of the emission closely follows BM velocity for frequencies between 700 Hz to 10 kHz; the phase of the emission relative to rarefaction at the ear canal, when corrected for emission delay time, matches the BM velocity towards scala vestibuli for a wide frequency range above 700 Hz. The phase relationship between the electrically-evoked BM motion and otoacoustic emission suggests that it is reticular lamina motion and not basilar membrane motion which provides the driving force for the electrically-evoked otoacoustic emission.