Mechanisms of hearing question

Didier A. Depireux didier at
Mon Dec 14 12:10:51 EST 1998

TONYJEFFS (tonyjeffs at wrote:
: >
: > but why do you invoke higher centers in this "tuning"? 
: Because the common theory is that the Inner hair cells only send messages   to
: the brain, while the OHCs mostly (90%) receive messages from the brain, and I

If you really want to know all the details, they can be found in one of the
7 "Green Books", edited by Art Popper and published by Springer-Verlag. One
of the green books (vol 8, I think) is entitled 'The cochlea'.

There is a big difference between the retina and the cochlea, in that the
local inhibitory circuit that's found in the retina itself doesn't appear
in the auditory pathway until the cochlear nucleus. In the auditory nerve,
about 5 % of the auditory nerve fibers course from the CNS to the cochlea,
whereas the rest goes the other way. The descending fibers all target outer
hair cells (and nothing else), whereas the ascending fibers connect only to
inner hair cells. In other words, the only possible lateral inhibition
would be from descending fibers, although I should point out that if this
were the mechanism for lateral inhibition, anatomy tells us it would have
to be through the superior olivary complex.

The truth is that no one knows what the descending pathway does, really.
Cats can detect sounds at a -15 dB level (re SPL), at which power level the
sound vibrations in the cochlea are about 1/10th of the average brownian
motion. What this means in all likelihood is that OHCs are very unstable
oscillators, that amplify very weak vibrations. Look at Bialek's article in
Physics Letters A (1987 or so) to see an interesting analysis of the fact
that a passive cochlea cannot account for our perception.

I should probably stop here! I don't want to go on and on...


Didier A Depireux                              didier at
Neural Systems Lab       
Institute for Systems Research          Phone: 301-405-6557 (off)
University of Maryland                                -6596 (lab)
College Park MD 20742 USA                     Fax: 1-301-314-9920

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