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Reverse traveling wave does not exist

List 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

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 Bell
Research School of Biological Sciences
Institute of Advanced Studies
Australian National University
Canberra, ACT 0200, Australia
phone +61 2 6125 9634
fax +61 2 6125 3808