Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave

[Martin Braun <nombraun@xxxxxxxxx>]

Dear Erik, A.J., Matt, and others,

Erik Larsen wrote:

> Martin Braun wrote:
> > For many years now, the measurement sensitivity for BM excursions has 
> > been well below 1 nanometer. Now, if BM motion is < 1 nm at 60 dB, how 
> > big then can it be at 0 dB?
> >
> > Please do the maths, and then come back. You can be sure that all readers 
> > of this thread will tremendously enjoy that figure.
> I wouldn't want to insult anybody's intelligence by posting the 
> calculation of that number online. Its not a meaningful thing to do anyway 
> for a cochlea with dysfunctional outer hair cells. Its like asking how 
> long it would take someone to run a marathon with a broken leg.

This is an evasion, and I bet you know. It has been state of the art in 
cochlear experimenting for many years to knock out OHC motility without 
compromising the mechanics of the BM proper.


A.J. Aranyosi wrote:

> In Martin's view, if this passive motion is too small to cause significant 
> OHC bundle deflection, then the cochlea cannot possibly be driven by BM 
> deflection.

> I disagree with this conclusion because it ignores the phenomenon of 
> feedback.  If the OHCs are part of a positive feedback loop, this problem 
> goes away because the feedback insures that the motion of the BM is never 
> as small as it is in the passive case.  Imagine if the BM moves one 
> picometer; this motion deflects hair bundles by a tiny amount, causing 
> OHCs to increase BM motion, leading to more bundle deflection, etc.

At 0 dB the BM on its own would not "move" by one picometer. It would "move" 
by a much smaller non-physical magnitude. Even if it could "move" by one 
picometer, this would not be sufficient to affect the "open" probability of 
an ion channel in a hair cell. For this, as we know, hundreds of picometer 
are needed.

"Feedback" means that there must be an original signal to which something is 
fed back. However, at low hearing levels there definitely is no such 
original signal in the BM.


Matt asked:

> Does anyone else have PHYSIOLOGICALLY BASED models to mention out there?

The "PHYSIOLOGICALLY BASED" electromechanical model of frequency selectivity 
for the turtle hair cell has been fairly complete for quite a number of 
years.

The "PHYSIOLOGICALLY BASED" electromechanical model of frequency selectivity 
for the mammalian outer hair cell (OHC) cannot yet be written down, because 
too many data are still missing. But data collection is in progress, and a 
model will ultimately be written down.

Until this happens I would just like to recommend that all non-physical 
models of BM function are immediately abandoned in order to prevent further 
damage from ongoing research and in order to minimize the waste of human and 
other resources.

Martin

---------------------------------------------------------------------
Martin Braun
Neuroscience of Music
S-671 95 Klässbol
Sweden
web site: http://w1.570.telia.com/~u57011259/index.htm