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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 (http://www.physikinstrumente.com/en/products/prdetail.php?sortnr=600690). 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 ***mechanical*** sensors.

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 carefully.

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 concept.


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