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

[Martin Braun <nombraun@xxxxxxxxx>]

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

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