Backward propagation of otoacoustic emission in the cochlea

Otoacoustic emissions have been commonly believed to be generated in the cochlea and emitted through backward-traveling waves. A recent study (Ren, 2004), however showed that there is no detectable backward traveling wave and that the stapes vibrates earlier than the basilar membrane (BM) at the emission frequency. These findings indicate that a cochlear-fluid compression wave is responsible for backward propagation of the emissions. This study contradicts with a widely accepted view that the delay of the otoacoustic emissions is approximately two times the forward traveling wave delay. In this study, the emission was measured in the ear canal, at the stapes, and at different locations on the BM. It was found that the slope of the phase-frequency curve measured from an apical location is always steeper than that measured from basal locations. Derived from the distance between two measured locations and their phase difference, the propagation velocity demonstrates that the BM vibration at the emission frequency propagates from base to apex through the observed region. Moreover, the emission group delay measured at the stapes is less than twice the traveling wave delay.