Re: mechanical cochlear model

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

Dear David,

Last night, you have written:

>Many theories have been advanced to explain the details of the peak region
>of the cochlear response but perhaps we can try to agree on some fundamental
>principals.  I would argue that a traveling wave, involving transverse
>motion of the basilar membrane, passively propagates acoustic energy to the
>peak region.  This passive propagation behaves in a manner described by
>Peterson and Bogert (1950).  When this energy reaches the beginning of the
>peak region, some process, usually called the "cochlear amplifier" and
>involving outer hair cells, takes over and shapes the peak response.  We may
>question whether the cochlear amplifier results from somatic motility or
>from hair bundle motility and we may debate questions of how outer hair cell
>forces get coupled back into basilar membrane and inner hair cell responses
>but does anyone question whether the classical traveling wave plays a role
>in at least passive propagation of acoustic energy to the best place?

I for one do not question that. I think that there is a travelling wave both in 
the spatial "active-peak" and "passive-peak" regions. What I mean by these terms 
is illustrated, e.g., in Fig. 14 of the review "Mechanics of the Mammalian Cochlea" 
by Robles and Ruggero (2001), Physiological Reviews 81, 1305-1352. In that Fig. 14,
based on published measurements, by Russell and Nilsen (1997), of BM motion
at various places in a guinea-pig cochlea during 15-kHz tones of many different
sound-pressure levels, the active peak (maximum at SPL < 60 dB) is at 14.45 mm
from apex (~4.35 mm from base), and the passive peak [clearly visible second peak 
at 55 dB < SPL < 100 dB] is at 15.85 mm from apex (~2.95 mm from base). So the 
distance between the two peaks is 1.4 mm, i.e., ~0.5 one-octave distance.

At 55 dB SPL, the BM displacement at the passive peak is seen to be lower than
that at the active peak by a factor of 0.1, i.e., by 20 dB. The passive-peak
BM displacement is seen to be proportional to the sound-pressure of
the stimulus.

In the lower panel of the mentioned Fig. 14, the phase is seen to drop steeply
from 14.9 mm to 14.0 mm from apex, indicating a travelling wave in the spatial
region of the active peak. At 14.9 mm, the phase is -180 degrees and falling
steeply, so a travelling wave is indicated also for the spatial passive-peak region.

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 .