Subject: Reverse traveling wave does not exist From: Andrew Bell <andrew.bell(at)ANU.EDU.AU> Date: Tue, 30 Mar 2004 11:39:18 +1000List members will no doubt be interested to note a recent paper by Tianying Ren [Nature Neuroscience, 21 March 2004] which supplies a convincing demonstration that the reverse traveling wave in the cochlea does not exist. Instead, Ren could find only a fast backward compressional (pressure) wave. The reverse traveling wave is a key entity required by conventional cochlear mechanics to explain acoustic emissions of all kinds, including, in the case under observation, distortion product emissions (DPOAEs). The 2f1-f2 distortion product, for example, originates at a location on the partition through non-linear interaction of two primary tones (e.g., 17000 Hz and 15455 kHz in one gerbil experiment reported) giving a strong vibration at 13910 Hz; this tone is presumed to travel backwards, via a traveling wave of displacement, to the stapes and the ear canal, where it is detected. To Ren's surprise, he found with his scanning laser interferometer that he could detect no displacement of the basilar membrane at 13910 Hz until _after_ the stapes had started vibrating at this frequency. That is, the stapes vibrated some 50 us _before_ the basilar membrane did. There must have been a fast (nearly instantaneous) compressional wave at 13910 Hz which originated at some distance along the partition and excited movement of the stapes; only later did the basilar membrane join in. This paper will cause us to reexamine the basics of how the cochlea works. For example, how do we explain spontaneous emissions in terms of 2-way (forward and backward) travel times in a reverberating cochlea? A factor of 2 is hard to hide. A couple of years back discussion on this list centred around the question of whether the (forward-) traveling wave is itself an epiphenomenon, arising only as a consequence of outer hair cells reacting to a fast pressure wave entering the cochlea. This latest work gives strong support to this notion. If outer hair cells can produce a fast pressure wave, it seems natural to suppose that, by reciprocity, the fast pressure wave is the key stimulus in the cochlea. Andrew. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Andrew Bell Research School of Biological Sciences Institute of Advanced Studies Australian National University Canberra, ACT 0200, Australia andrew.bell(at)anu.edu.au phone +61 2 6125 9634 fax +61 2 6125 3808 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~