Cochlea -- quality of filtering

Jeffrey Sirianni sirianni at uts.cc.utexas.edu
Fri Sep 22 17:29:00 EST 1995


In article <811761179snz at howl.demon.co.uk>, Tony Woolf <tony at howl.demon.co.uk> says:
>
>In article <43tb8l$jt8 at geraldo.cc.utexas.edu>
>           sirianni at uts.cc.utexas.edu "Jeffrey Sirianni" writes:

>> a sound of this magnitude is presented to the cochlea, many auditory
>> nerve fibers on both sides of the tonal frequency are activated.  So why
>> do we not perceive this as a complex sound?  The brain's 2nd and 3rd order
>> neurons suppress the activity on the sidebands.  This is an oversimplied
>> answer to this question, but I would suggest reading Kim and Molnar (1979)
>> J. Neurophysiol. 42:16-30.  


{SNIP}

>Using some very sophisticated measuring techniques, it's been shown,
>not just that individual hair cell tuning is as sharp as as that of the
>cochlear nerve fibres, but that the basilar membrane vibration also
>shows the identical sharp tuning!  However this is true only if the
>subject is in very good physiological shape, which is why the earlier
>experiments done on cadavers or heavily anaesthetised animals didn't
>show it.
>
{SNIP}

>Tony Woolf  (tony at howl.demon.co.uk)

Fine tuning takes place at low intensity levels.  I hope you did
not misunderstand my explanation.  At low intensities, only those
outer & inner hair cells, plus auditory nerve fibers, with a 
characteristic frequency (CF) of the presented tone will respond.
That is what fine tuning is all about.

At high intensity levels, many of these cells will respond, but those
with a CF of the presented frequency will respond at a larger magnitude.
If you look on a tuning curve, you will notice that there is some response
at frequencies on the "sidebands" of CF.  Look at a study that deals with
population studies of auditory neurons.  You will see that an identical
high intensity stimulus at a constant frequency (say 1000 Hz) will excite
many auditory nerve fibers on both sides of 1000 Hz.  The response for cells
with a CF of 1000 Hz will be greatest and will decline on the sidebands. but
there is a considerable amount of response measured in an increase in driven
rate.

(Sorry to SNIP, but my newsgroup reader likes responses longer than the
quoted portion... A way of curving the "Me Too !" syndrome)

Jeff

Jeff Sirianni     @(((<{
University of Texas at Austin
Communication Sciences and Disorders
CMA, 2nd Floor Clinic
Austin, TX  78712-1089
sirianni at uts.cc.utexas.edu
jgsaudio at aol.com



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