Abstract
It is commonly believed that the cochlea emits sounds through backward-traveling waves. In the present experiment using a scanning-laser interferometer, I detected forward-traveling but not backward-traveling waves and found that the stapes vibrates earlier than the basilar membrane. These results contradict the current theory and show that the ear emits sounds through the cochlear fluids as compression waves rather than along the basilar membrane as backward-traveling waves.
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Acknowledgements
I thank A.L. Nuttall, P. Gillespie and K. Grosh for comments on an earlier version of the manuscript, E.V. Porsov for writing software, and S. Matthews for technical help. Supported by the National Institute on Deafness and other Communication Disorders (NIDCD), and the National Center for Rehabilitative Auditory Research (NCRAR), Portland Veteran's Administration Medical Center.
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Supplementary information
Supplementary Fig. 1.
Diagram of the widely accepted theory of the propagation of the cubic distortion product otoacoustic emission in the cochlea3,4,5. (a) Waveforms and envelopes of basilar membrane (BM) vibration at f1, f2, and 2f1 - f2; (b) corresponding phase curves. The acoustic energy at distortion product (DP) frequency of 2f1-f1 is generated near "DP origin" and propagates in both directions along the BM. The backward traveling wave vibrates the stapes and appears as the emissions in the ear canal. The forward-traveling wave is partially reflected near DP characteristic frequency place (DP CF) and forms a second backward traveling wave. The phase of the backward traveling waves shows a positive relationship with the distance from the base. (JPG 45 kb)
Supplementary Fig. 2.
Cochlear fluids-based mechanisms for the transmission of the otoacoustic emission in the cochlea. (a) Waveforms and envelopes of BM vibration at f1, f2, and 2f1 - f2; (b) corresponding phase curves. The acoustic energy generated near "DP origin" propagates through the cochlear fluids to the whole cochlea at the speed of sound in water. It vibrates the stapes, resulting in emissions in the ear canal, and launches a forward traveling wave at 2f1 - f2. Thus, in panel b, the backward propagation of the emission shows no significant phase delay. (JPG 44 kb)
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Ren, T. Reverse propagation of sound in the gerbil cochlea. Nat Neurosci 7, 333–334 (2004). https://doi.org/10.1038/nn1216
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DOI: https://doi.org/10.1038/nn1216