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

["A.J. Aranyosi" <aja@xxxxxxx>]

I think the point Martin is trying to make is a bit more complicated 
than that, if I'm interpreting it correctly.  I suspect most of us would 
agree that the amplitude of BM displacement at 0 dB SPL for a "dead" or 
compromised cochlea is not functionally significant.  However, Martin's 
claim seems to be that in a cochlea with normal sensitivity, this 
passive BM displacement is what drives OHC amplification because the 
OHCs are dependent sources (i.e., their hair bundles must be displaced 
in order for them to amplify).  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.  When you try to reason it through step-by-step like this it still 
sounds like the OHCs have to detect the passive vibration, which 
technically is true.  But the key to feedback is that an input that 
causes an insignificant (and probably undetectable) response on a single 
pass through the feedback system can grow rapidly as it passes 
repeatedly through this loop.

Having said all of this, there are problems with positive feedback 
models as well (for example, they are extremely sensitive to the amount 
of gain, which is usually not well controlled in biological systems).  I 
wholeheartedly agree with Martin's statement that we need more 
measurements before we can truly separate the good from the bad models.

Erik Larsen wrote:
> 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.
>
> Specifically for the issue of motion sensitivity, you appear to assume 
> that the cochlea somehow behaves as a laser doppler vibrometer: you 
> take the limitations of this piece of equipment to somehow imply that 
> the cochlea would have the same limitations under similar 
> circumstances. If you could describe to the readers of this thread how 
> you arrived at that conclusion, I'm sure we will tremendously enjoy 
> reading about it.
>
> Martin Braun wrote:
> > Erik Larsen wrote:
> >
> > > Perhaps Martin meant that nobody has been able to measure BM motion 
> > > below 60 dB with dysfunctional outer hair cells? But that wouldn't 
> > > be a proof there is no motion. In fact, it would be difficult to 
> > > understand why there wouldn't be any motion below 60 dB, however small.
> >
> > 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.
> >
> > Martin
> >
> > ---------------------------------------------------------------------
> > Martin Braun
> > Neuroscience of Music
> > S-671 95 Klässbol
> > Sweden
> > web site: http://w1.570.telia.com/~u57011259/index.htm