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More on "How does a hearing aid help?"

Stephen Black sblack at UBISHOPS.CA
Thu May 15 15:36:06 EST 1997

I want to apologize for taking so long to reply to Jerry Larson's
interesting and detailed response. Also, I want to mention (although I
expect no one remembers) that the version Jerry responded to had (as he
guessed) some of Ron Blue's insights mixed in. If a post contains the
words "wavelets" or "opponent-process", that's Ron, never me. 

I appreciate the correction of the terminology from "nerve deafness" 
(which I realized was a misnomer but had no substitute) to the appropriate
"sensorineural hearing loss" (SHL).  The basis of my question was that a
hearing aid can help in SHL but I couldn't understand why. This is because
the most likely cause of SHL is the loss of hair cells, the neural 
elements which transduce the signal from vibration to neural activity. 
Either the hair cells work or they don't, and if they don't, no amount of 
amplification is going to bring them back to life. So what does the 
hearing aid do?

Although Jerry provided useful information, he didn't touch on the
underlying mechanism. A clue may be the observation he cited that
frequencies are often not entirely missing in SHL, but can only be
detected at higher sound level. But what is the physiological basis of
this phenomenon? 

I know that there are three rows of outer hair cells so it might be argued
that (say) only a single row is damaged at a particular location, and the
others have to work harder. However, current theory is that it is the
inner hair cells which generate the nerve impulse, and there is only a
single row of these. 

My conclusion is that probably no one really knows why a hearing aid helps
in SHL. This is analogous (or possibly even directly relevant) to the
puzzle of tinnitus. Tinnitus (an often-distressing noise heard within the
ear) is caused by hair cell damage, but why the absence of input should
give rise to the perception of sound is unknown (although there are
speculations, of course). I find it interesting that it seems to parallel
the equally unusual phenomenon of phantom limb pain (the sufferer feels
pain in a body part that's no longer there, having been amputated). 

Not having access to hearing aid newsgroups, I guess my question stops
here.  I have tried to locate an advanced textbook, but haven't found one
that discusses the question (probably, as I conclude, because they don't
know).  Fortunately, this is not an urgent question, merely intellectual


Stephen Black, Ph.D.                      tel: (819) 822-9600 ext 2470
Department of Psychology                  fax: (819) 822-9661
Bishop's University                    e-mail: sblack at ubishops.ca
Lennoxville, Quebec               
J1M 1Z7                    Bishop's Department of Psychology web page at:                                                      
Canada                        http://www.ubishops.ca/ccc/div/soc/psy

              Wish us a happy Victoria/Queen's Birth/Dollard Day June 19

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