Transmission of Oto-Acoustic Emissions Within the Cochlea

Abstract

Oto-acoustic emissions (OAEs) are low intensity sounds which can be recorded in the external ear canal with a sensitive microphone. They are initiated by the activated motility of the outer hair cells which provide mechanical feedback (the cochlear amplifier) to the basilar membrane, enhancing its displacement. Therefore it has been thought that the OAEs are propagated toward the base as a backward mechanical traveling wave along the basilar membrane. Such a wave would be accompanied by pressure differences across the cochlear partition in the closed cochlear system, filled with incompressible fluid. In order to test this OAE propagation mechanism, holes were made in several places in the bony wall of the inner ear, reducing such possible pressure differences. In experiments in which it was possible to avoid damage to the organ of Corti, there was no change in detection thresholds of distortion product OAEs. This result provides further support for the suggestion that oto-acoustic emissions are not propagated as mechanical vibrations backward along the basilar membrane. Instead it is more likely that they are transmitted through the cochlear fluids to the stapes footplate as alternating condensation/ rarefaction fluid pressures.