Subject: Re: HC selectivity ... was Re: Physiological models of cochlea activity - alternatives to the travelling wave From: Martin Braun <nombraun@xxxxxxxx> Date: Thu, 4 Oct 2007 18:42:04 +0200 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>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