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Re: Reverse traveling wave does not exist

Distortion product emissions from the cochlea are a tricky subject. It seems
that Robert Withnell has not understood Ren's experiments, for he makes the
following statement:

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

>>> Robert Withnell <rwithnel@INDIANA.EDU> 3:06:06 am Friday, 2 April 2004
>Regardless of whether the reverse propagation of energy
>involves a reverse traveling wave or a compressional wave, there is the
>little issue of causality if the stapes vibration precedes the generation
>of the source energy.

No, that cannot be, of course, yet by calling the causality issue "little",
Withnell implies that Ren has overlooked that most fundamental of questions.
Reading Ren's paper, which I again invite list members to do, it is clear
that Ren makes no such preposterous claim. He has carefully considered the
issue and his results should not be quickly dismissed.

The source energy, of course, is in the primaries, which interact on the
partition and create a distortion product. What happens next? Ren sees that
there is a vibration of the stapes and a forward traveling wave. The
important thing he finds is that the stapes vibration occurs before the
forward traveling wave (and, naturally, after the primaries have entered the
cochlea). He sees no evidence of a reverse traveling wave. Therefore, what
caused the vibration of the stapes?  Answer: a fast pressure wave (which was
undetectable by his motion-sensitive apparatus). How is the pressure wave
generated? Ren leaves this question open but it would have to be by
synchronous volume expansions and contractions of a small group of outer
hair cells at the 2f1-f2 frequency.

Perhaps Robert Withnell has not read the "Supplementary Note" that
accompanies the Nature Neuroscience paper. Here, Ren gives a full discussion
of his results. He points to Dallos' statement that "there is absolutely no
experimental evidence that shows that there is a backward travelling wave."
[Biophysics of the Cochlea, 2003, p. 584.]  He also discusses work by
Narayan et al. (1998) and concludes that "(t)his unambiguous finding in
different species of experimental animals by two independent laboratories
clearly demonstrates that the stapes vibration at the emission frequency and
the consequent resulting otoacoustic emission in the ear canal are not
mediated by the hypothetical backward traveling wave."

Would Nature Neuroscience's reviewers accept a paper that made a basic
logical howler? I suggest we give it a fair reading.


>>> Robert Withnell <rwithnel@INDIANA.EDU> 3:06:06 am Friday, 2 April 2004
Andrew Bell continues to question the exiting notion that energy
propagation in the cochlea involves an inertially-mediated fluid flow
coupled in to the basilar membrane as a traveling wave. He cites a recent
paper by Tianying Ren in Nature Neuroscience as evidence that the reverse
propagation of energy does not involve such a traveling wave. As observed
by David Mountain, the experimental design of Ren does not exclude a
reverse traveling wave. For measurement on the basilar membrane to
elucidate a reverse traveling wave requires that the reverse traveling
be larger in magnitude than any forward traveling wave at the same
frequency. This is most likely at a point basal to f2 - at a stimulus
frequency ratio that results in a 2f1-f2 DP generated from the nonlinear
interaction of f1 and f2 that is larger in the reverse than the forward

Comments by Martin Braun:
>He did address the reverse traveling wave issue, and he found there is
>He simultaneous measured stapes vibration and found that, for DPs, it
>preceded that of BM vibration.

It is possible that Ren's phase-gradient data for stapes vibration
a wrapping error. Regardless of whether the reverse propagation of energy
involves a reverse traveling wave or a compressional wave, there is the
little issue of causality if the stapes vibration precedes the generation
of the source energy.

Robert Withnell