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

[Andrew Bell <andrew.bell@xxxxxxxxxx>]

Dick:

I'm pleased we now both see the 1 pm for what it is: the fundamental input
signal of the traveling wave theory at 0 dB SPL. You express faith that such
a signal is sufficient to give a 1 nm measurable displacement via a
traveling wave mechanism of some sort. Yes, the evidence is that the OHCs do
the work, but why do you assume it's via a 'traveling wave' mechanism? There
could be other mechanisms, could there not, and if an alternative one
happened to give LARGER input displacements than what the TW offers, it
would seem to make a better candidate for the effective stimulus. 

Calling such an alternative a 'strawman' seems a pejorative term that
prejudges the issue. If AJ's calculation returned a value a thousand times
lower (0.001 pm) would you still have faith that the TW could dig below the
noise threshold and take care of everything? In other words, do you have a
threshold below which you think the traveling wave mechanism would have
insufficient traction?

For me, 1 pm is below my threshold of credibility, but I can rest a bit
easier with a pressure-induced motion of 100 pm. I have recently published a
cochlear model in which outer hair cells produce standing waves in the
subtectorial space (Bell, 2007), and in it that 100 pm figure could be used
as the input to a local, OHC-driven cochlear amplifier. So the TW need not
be the only theoretically possible amplifying mechanism.

Bell, A. (2007). Tuning the cochlea: wave-mediated positive feedback between
cells. Biological Cybernetics 96, 421-438. 



Andrew.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Andrew Bell
Research School of Biological Sciences
The Australian National University
Canberra, ACT 0200, Australia
T: +61 2 6125 5145
F: +61 2 6125 3808
andrew.bell@xxxxxxxxxx
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~



-----Original Message-----
From: AUDITORY - Research in Auditory Perception
[mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Richard F. Lyon
Sent: Thursday, 4 October 2007 5:00 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: [AUDITORY] HC selectivity ... was Re: Physiological models of
cochlea activity - alternatives to the travelling wave


At 2:44 PM +1000 10/4/07, Andrew Bell wrote:
>The point is that we are talking about the input signal to the cochlear 
>amplifier. There has to be a passive signal (the effective stimulus) on 
>which the positive feedback process can work. The BM displacement that 
>is measured in a normal cochlea is _after_ amplification has occurred 
>(remember that AJ's original figure of 1 pm was derived from Ruggero et 
>al. 1997 by looking at their post-mortem data).
>
>So the fundamental question is, how can a normal cochlea detect 1 pm 
>and amplify it a thousand-fold (60 dB) so that we see a 1 nm 
>displacement? I agree with Martin that it can't, and there has to be 
>some other, larger, effective stimulus.

Yes, that is the fundamental question, sort of.  It's not like 
there's some element that detects an "input" of 1 pm and amplifies to 
an "output" of 1 nm; rather, there's a distributed amplifier that 
multiplies up the power of traveling waves.  At the low end of the 
range, everything behaves linearly.  As long as the noises of the 
many hair cells are reasonably uncorrelated, the system will be able 
to work to orders of magnitude below the level that would cause a 
"noticeable" effect in a single hair cell.  Ultimately, the shot 
noise of ion channels, averaged over many OHCs, is what will set the 
sensitivity limits; there's no "threshold" below which amplification 
stops working, the signal just gets down below the noise.

So I reiterate: it's a funny strawman to look at what the motion 
would be in a dead cochlea and say that's too little for the OHCs to 
work; they do work, and the result is that the motion is much greater.

Dick