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INCREASED HEARING SENSITIVITY

Jeffrey G. Sirianni sirianni at uts.cc.utexas.edu
Sat Jun 29 12:33:15 EST 1996


In article <199606272139.QAA21598 at dgabby.mfldclin.edu>, hansenda at DGABBY.MFLDCLIN.EDU says...
>
>}]mCl#Please Confirm[{
>From: hansenda                Thu Jun 27, 1996 -- 04:34:05 PM
>To: audiolog at net.bio.net~@
>GENERAL QUESTION:
>
>WHEN THE COCHLEA IS DAMAGED BY NOISE AND HIGH FREQUENCY HEARING IS DECREASED 
> DOES HEARING SENSITIVITY FOR LOW-MID TONES INCREASE?  CAN THIS OCCUR BECAUSE 
>THE OUTER HAIR CELLS IN THIS AREA REMAIN HEALTHY IN COMPARISON TO THE DAMAGED 
> HAIR CELLS OF THE HIGHER FREQUENCIES?  
>
>DON HANSEN    MARSHFIELD CLINIC
>              AUDIOLOGY SECTION                              

Check out the recent issue of JAAA.  Jim Willott has an interesting article that
states, among other things, that the brain's capability to process sounds for the
frequency region not affected by hearing loss actually increases.  In other words,
the neural centers involved in the processing of high frequency sounds will actually
convert to centers processing mid- to low-frequency sounds.  He does point out that
this may not be so in cases of NIHL, but may be so in cases of presbycusis.

I guess I didn't answer your question... Well I cannot see how this can happen if the
cochlear traveling wave remains unchanged.  But as neurophysiologists have shown, any
one hair cell (or auditory nerve fiber) will respond to any frequency as long as it
is loud enough.  Granted, each hair cell (auditory nerve fiber) responds best to a
sound at a frequency corresponding to the maximal wave displacement in the cochlea
by responding at a very low input level (characteristic frequency).  I guess I would
say that hair cells do not increase in sensitivity when other regions are damaged by
hearing loss, except that a signal may be enhanced in the neural centers.  The
exception to this would be any changes in the efferent olivocochlear bundle signal to
the intact outer hair cells.

Anyone interested in reading about CNS changes caused by age-related hearing loss
ought to check this out.  It is written for an audiological audience (rather than
a neurophysiology audience).

(P.S. Your use of capital letters implies that you are shouting....)

-- 
Jeffrey Sirianni, M.A., CCC-A  @(((<{
Sound Advice / R.G. Delaney, MD
710 Water Street / Suite 404
Kerrville, TX  78028
(210) 896-1433
(210) 896-1440 FAX
sirianni at uts.cc.utexas.edu
jgsaudio at aol.com




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