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Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave
Dear A.J. and others,
One picometer is a small displacement, but it's hardly unphysiologically
small. For a 0 dB SPL tone at 4 kHz, the free-field peak displacement of
air is about 2.5 picometers. Commercial OAE systems have microphones that
can sense sounds at least as low as -20 dB SPL, corresponding to air
displacements of 0.25 picometers. If, as Martin said, "there is no known
physics by which a mechanical signal of this magnitude could be
transported, let alone be detected," then these microphones - and our
ears - would be detecting phantoms.
Microphones are pressure sensors, not displacement sensors. Mechanical
sensors that can detect a displacement of 1 pm are a bad joke. Even single
molecules could not sense a displacement of 1 pm.
As a further example, the company PI has recently announced a positioning
device that has (at least) 50 picometer positioning resolution
The sensor they use to detect this position has an even higher resolution
still; from their graph on that web page (click on the plot in the lower
right), the sensor noise looks to be on the order of a couple of
picometers. So even man-made systems come close to achieving the required
sensitivity. It is hardly a stretch of the imagination to believe that a
micro-scale biological system can perform similarly well.
Also this example is grossly misleading. The sensors for these positioning
devices do NOT operate mechanically. Hair cells, however, are
With regard to whether Brownian motion would preclude OHC amplification of
such small signals, so far I've seen a lot of hand-waving on both sides of
the issue, but few quantitative arguments. Since it's much easier to show
that something is possible than to show that it isn't, the people who wish
to argue that OHCs can't amplify these small signals have a harder job
here. Nonetheless, I would be interested to see someone do this analysis
There is multiple empirical evidence on the question which displacement a
hair bundle can register. The minimum is in the range of hundreds of
picometer. Again, 1 pm is a bad joke.
For any alternate model to become as widely adopted, we would need either
compelling evidence that the traveling wave concept is wrong, .....
The inability of hair cells, or even single molecules, to register a
displacement of 1 pm is fatal evidence against Bekesy's traveling wave
Neuroscience of Music
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