Reverse wave propagation in the cochlea

Abstract

Otoacoustic emissions, sounds generated by the inner ear, are widely used for diagnosing hearing disorders and studying cochlear mechanics. However, it remains unclear how emissions travel from their generation sites to the cochlear base. The prevailing view is that emissions reach the cochlear base via a backward-traveling wave, a slow-propagating transverse wave, along the cochlear partition. A different view is that emissions propagate to the cochlear base via the cochlear fluids as a compressional wave, a fast longitudinal wave. These theories were experimentally tested in this study by measuring basilar membrane (BM) vibrations at the cubic distortion product (DP) frequency from two longitudinal locations with a laser interferometer. Generation sites of DPs were varied by changing frequencies of primary tones while keeping the frequency ratio constant. Here, we show that BM vibration amplitude and phase at the DP frequency are very similar to responses evoked by external tones. Importantly, the BM vibration phase at a basal location leads that at a more apical location, indicating a traveling wave that propagates in the forward direction. These data are in conflict with the backwardtraveling-wave theory but are consistent with the idea that the emission comes out of the cochlea predominantly through compressional waves in the cochlear fluids. basilar membrane vibration ͉ cochlear traveling wave ͉ laser interferometer ͉ otoacoustic emission K emp (1, 2) discovered that the cochlea can generate sounds that are transmitted to the external ear canal through the middle ear. Such otoacoustic emissions (OAEs) are considered a by-product of normal cochlear performance (3), relying on a putative cochlear amplifier (4-6). Because OAEs can provide information on the health status of the cochlea and can be noninvasively measured, they are increasingly used for diagnosing auditory disorders and as a research tool for studying cochlear physiology (3). However, the applications of OAEs have been limited because the generation and transmission mechanisms are not clear.There are two different theories about OAE propagation from the generation site to the cochlear base. Propagation may either be dominated by a traveling wave moving in the backward direction (1, 7-9) or occur through a compressional wave in the cochlear fluids (10-16). Backward-traveling waves are slow and vibrate in the transverse direction, whereas compressional waves are longitudinal and travel at the speed of sound in water.Backward-traveling waves were first postulated in Kemp's original reports (1, 2). Support for this theory comes from indirect measurements, primarily of the delay between the stimulus and the appearance of the emission in the ear canal (17). Although the interpretation of such measurements is not unambiguous, it is commonly believed that the backward delay is the same as the forward delay. Consequently, the emission delay would be twice the forward delay (7, 9, 18). However, a direct proof of the theory requires backward waves to be me...

Scleraxis is Required for Differentiation of the Stapedius and Tensor Tympani Tendons of the Middle Ear

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Electrically evoked reticular lamina and basilar membrane vibrations in mice with alpha tectorin C1509G mutation

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Light-induced basilar membrane vibrations in the sensitive cochlea

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“…The animal use protocol was approved by the Oregon Health & Science University Institutional Animal Care and Use Committee. Laser pulse-and acoustic click-induced basilar membrane vibrations were measured using a heterodyne laser interferometer [5]. The voltage output from the laser interferometer controller was proportional to the vibration velocity of a gold-coated bead, which can reliably track the basilar membrane vibration [3].…”