Re: mechanical cochlear model

["reinifrosch@xxxxxxxxxx" <reinifrosch@xxxxxxxxxx>]

Hello again !

About 8 hours ago, Martin Braun wrote:

>Dick, and others, [...]
>
>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)).
>
>[...]
>
>Martin

In the final section of their (already mentioned) contribution "Measurement
of Basilar Membrane Vibration Using a Scanning Laser Interferometer",
in "Biophysics of the Cochlea" (Titisee Proceedings), Gummer (Ed.), pp. 211-219,  
Ren et al. (2003) have written:

"[...] Preliminary data clearly show the traveling wave in the living cochlea.
The low-level sound-induced traveling wave occurs over a very restricted
longitudinal range (~600 micro-m), [...]"

A part of these data are shown in their Fig. 1: 16-kHz, 40 dB continuous
tone, sensitive gerbil cochlea, "active" peak at 2.6 mm from base, tails
indeed roughly symmetrical, ranging from 2.3 to 2.9 mm. In that range,
a travelling wave is clearly demonstrated by that Fig. 1. The BM velocity
scale is linear, peak at 0.2 mm/s; corresponding BM displacement 2 nm.

I conjecture that the "passive" peak (or shoulder) is more basal by about 
0.4 one-octave distance, i.e., at ~2.1 mm from base, and is lower than the 
active peak by at least 20 dB, i.e., by a velocity or displacement factor of <0.1;
corresponding passive-peak BM-velocity <0.02 mm/s, passive-peak BM
displacement <0.2 nm. So I think that the passive peak (or shoulder), and 
the travelling wave in it, was not observed because the peak height was tiny.  

Reinhart.

Reinhart Frosch,
Dr. phil. nat.,
r. PSI and ETH Zurich,
Sommerhaldenstr. 5B,
CH-5200 Brugg.
Phone: 0041 56 441 77 72.
Mobile: 0041 79 754 30 32.
E-mail: reinifrosch@xxxxxxxxxx .