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Re: mechanical cochlear model

Dick, and others,

....... You are correct in saying that there is good "evidence for the view that the basilar membrane traveling wave peaks about half an octave basalwards of the neural CF", for high enough levels.

Good we agree on that.

Only at very low levels does the wave, or the neural response, peak near the CF place.

At low levels (below ca 60 dB SPL) a traveling wave (TW) has never been observed, despite 20 years of laser interferometric measurements of basilar membrane (BM) motion in vivo down into the subnanometer range. To the contrary, the labs of Ian Russell in England and Tianying Ren in Oregon have indpendently and by different methods reported a wealth of data that show a point excitation of the BM at low levels, without any indication whatsoever that also a TW might be involved. The main findings are: 1) Pure tones move a very short longitudinal section of the BM, with the adjacent sections on either end remaining totally unmoved. 2) The excited section extends symmetrical around the place of characteristic frequency (CF).

These findings are not compatible with a low-level TW. However, they are compatible with local resonance (e.g. via tuned outer hair cells (OHC))

As an example of evidence that's hard to explain, we have the blocked-round-window experiment of Perez 2009. It needs to be investigated further, [.........] Martin, since I don't have the Perez paper handy, can you tell us what the statistical support was for their observation that "In the four control ears, there was no change in ABR threshold 24 hours after the round window was occluded."? Would a hypothesis of a few dB of threshold rise be equally compatible with their data? That's what the "third physiological window" idea would suggest.

No, it would not. The threshold statistics in dB pe SPL (mean & SD) for the four control ears are:
Initial: 56.3 +/- 2.5
Post-block: 58.8 +/- 4.8
24 h later: 57.5 +/- 5.0


Martin Braun
Neuroscience of Music
S-671 95 Klässbol
email: nombraun@xxxxxxxxx
web site: http://www.neuroscience-of-music.se/index.htm