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Re: mechanical cochlear model

The cochlear traveling wave is still alive and well.  We may debate the details of how the cochlear amplifier works but all the fully developed computational models start with the known physical properties of the cochlear fluids and the basilar membrane and some basic Newtonian physics.  These models all support the classical traveling wave theory.  Here are some of my comments w/r to some of the arguments against the traveling wave:

1) The peak of the traveling wave is much broader than one would expect for a simple resonant system.  The gerbil basilar membrane is only ~12 mm long so a peak extending over 0.5 mm (8%) is pretty broad for a cochlea that is tuned from 300-60,000 Hz.

2) The jury is still out on the basilar membrane stiffness gradient.  Emadi et al (2004) Hear. Res. found a larger gradient than what Ram Naidu and I found.

3)  Even if the Naidu and Mountain stiffness data are correct, the cochlear frequency range could be the result of different modes of vibration in different regions of the cochlea.

4) There is some very good evidence that there is a third window in the cochlea from H. Nakajima et al that was presented at this year's ARO meeting.

On Sat, Mar 6, 2010 at 10:00 PM, Andrew Bell <andrew.bell@xxxxxxxxxx> wrote:
In addition to Martin's 2 pieces of evidence against the traveling wave
model, we can add:

1. The peak of the traveling wave is unrealistically sharp.

  A) In a gerbil, Ren found that the peak occurred over a region extending
less than 0.5 mm at 16 kHz (Ren 2002, PNAS 99, 17101).

  B) Russell & Nilsen saw a peak only 0.15-1 mm wide at 15 kHz in a guinea
pig (R&N 1997, PNAS 94, 2660).

  C) Lonsbury-Martin & Martin found histologically a gap only 60-70 um wide
due to damaging pure tone levels applied to monkeys' ears (L-M & M 1987,
JASA 81, 1507).

I quote Jont Allen's remark from 2001 that "the discrepancy in frequency
selectivity between basilar membrane and neural responses has always been,
and still is, the most serious problem for the cochlear modelling
community." Jont, do you still feel that way?

2. The variation is stiffness is inadequate to tune the cochlea from 20 to
20000 Hz. Three decades of frequency calls for a million times variation in
stiffness (more than between foam rubber and tungsten), and this is in
contrast to measurements of 2 or 3 orders at most. See Naidu & Mountain
1998, Hear Res 124, 124. Bekesy found the value to be about a hundred-fold
(p. 476 of Exp in Hearing).

3. Some workers (eg, Stenfelt) find that the spiral lamina is as flexible as
the basilar membrane, removing another avenue by which tonotopic tuning can
be achieved - because its width is about constant. (Stenfelt 2003, Hear Res
181, 131).

4. Cases are reported where a person has been found to possess holes in the
basilar membrane - and the holes don't appear to affect hearing. In some
birds, there is a naturally occurring shunt called the ductis brevis, which
connects the upper and lower chambers at the basal end. Yet these birds hear
perfectly well nonetheless.

5. People lacking a middle ear can still hear (and yet pressure on the oval
and round windows should in these cases be equal).

Together, all these anomalies cast doubt on the adequacy of the traveling
wave model. I think the TW model, as currently framed, cannot work at low
sound pressure levels, and have formulated a resonance model in which the
effective stimulus is the fast pressure wave (not the pressure difference
across the membrane) and where the OHCs are pressure sensors (they are
compressible). I've mentioned various publications previously, but the most
comprehensive one is my thesis. It discusses the above anomalies, and
others, in some detail - the link is

The traveling wave can drive some passive processes, but in terms of
efficiency we need an active system, and I think that here we need living
pressure sensors (outer hair cells) forming resonant elements.


Andrew Bell
Research School of Biology (RSB)
College of Medicine, Biology and Environment
The Australian National University
Canberra, ACT 0200, Australia

> -----Original Message-----
> From: AUDITORY - Research in Auditory Perception
> [mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Martin Braun
> Sent: Sunday, 7 March 2010 7:29 AM
> To: AUDITORY@xxxxxxxxxxxxxxx
> Subject: Re: [AUDITORY] mechanical cochlear model
> While the cochlear traveling wave has appeared in numerous
> empirical reports
> on real physical models and real biological animals, it's function in
> hearing is not yet universally appreciated. Some people still
> think that it
> provides the well known frequency selectivity that we observe in the
> auditory nerve. This view, however, has been proved wrong by
> multiple direct
> experimental evidence. Just consider two bodies of evidence:
> 1) Hearing sensitivity is not affected, when endolymphatic
> hydrops presses
> the basilar membrane flat upon the bony cochlear wall of the
> scala timpani:
> http://www.neuroscience-of-music.se/Nageris.htm
> http://www.neuroscience-of-music.se/Xenellis.htm
> 2) It is a well established observation for more than 50
> years that closure
> of the round window does not affect hearing sensitivity. This
> means that a
> pressure difference across the basilar membrane and a
> resulting traveling
> wave cannot be a necessary condition of hair cell excitation.
> Recently,
> Perez et al. (2009) reported that closure of the round window
> not only
> leaves hearing sensitivity unchanged but increases cochlear
> vulnerability at
> high sound levels. This second new observation is a further
> compelling
> indication as to the real function of the cochlear traveling wave.
> http://www.neuroscience-of-music.se/Sohmer.htm
> Martin
> ---------------------------------------------------------------------
> Martin Braun
> Neuroscience of Music
> S-671 95 Klässbol
> Sweden
> email: nombraun@xxxxxxxxx
> web site: http://www.neuroscience-of-music.se/index.htm


David C. Mountain, Ph.D.
Professor of Biomedical Engineering

Boston University
44 Cummington St.
Boston, MA 02215

Email:   dcm@xxxxxx
Website: http://www.bu.edu/hrc/research/laboratories/auditory-biophysics/
Phone:   (617) 353-4343
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Office:  ERB 413