Re: Reverse traveling wave does not exist (Andrew Bell )


Subject: Re: Reverse traveling wave does not exist
From:    Andrew Bell  <andrew.bell(at)ANU.EDU.AU>
Date:    Fri, 2 Apr 2004 11:12:32 +1000

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 andrew.bell(at)anu.edu.au phone +61 2 6125 9634 fax +61 2 6125 3808 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >>> Robert Withnell <rwithnel(at)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. Andrew. >>> Robert Withnell <rwithnel(at)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 wave 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 direction. Comments by Martin Braun: >He did address the reverse traveling wave issue, and he found there is none. >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 involves 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


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