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