Re: Traveling waves or resonance?

["Richard F. Lyon" <DickLyon@xxxxxxx>]

Andrew, you are right that von Békésy's theory 
and other traveling wave theories would have been 
known to Gold before 1948, as I've been reminded 
by friends already.  That would be good to 
clarify in your paper.  My point remains that he 
could not have included active traveling wave 
theories in what he was reacting to.


> I am surprised you apparently wish to continue 
> with the broad mechanical tuning idea, and call 
> for broad neural tuning as well. If you say that 
> sharp auditory nerve tuning is somehow 
> imaginary, where does our sharp frequency 
> discrimination come from? There are many sharp 
> tuning tips in auditory nerve investigations, 
> and the standard picture now is that AN tuning 
> reflects BM tuning [e.g., Khanna & Leonard 1982].
You have introduced the word "broad" into the 
discussion, which is not a characterization that 
I would use.  I completely agree that AN tuning 
reflects BM tuning.

When I said that "sharp threshold tuning curves 
are an epiphenomenon" I didn't mean that they're 
"somehow imaginary", but rather that they are "a 
secondary and sometimes unexpected consequence" 
of the threshold-type tuning-curve experiments.

My point about "sharp" versus "not very sharp" is 
the distinction between an iso-response or 
threshold-type tuning curve, which is very sharp, 
and an iso-intensity or revcorr-type curve, which 
is somewhat less sharp; both are correct 
experimental measurements of the same system. 
Both BM and AN experiments show the same things, 
but the iso-intensity curves are less often 
exhibited, so people sometimes mistake the 
iso-response curves are being sort of like 
"transfer functions" and they therefore make 
linear approximations that are way too sharp. 
Resonances are generally way too narrow (too 
sharp and too symmetric) compared to real data or 
to TW models.

I think that your phrase "sharp tuning tips" is a 
clue that you be falling for this confusion.  The 
sharp tips of iso-response or threshold-type 
so-called "tuning" curves should not be literally 
interpreted as "tuning" in the sense of  a linear 
filter approximation, since the points on the 
curve are measured with very different input 
power levels and therefore very different states 
of adaptation of the nonlinear system.


> My position is that the sharp BM tuning comes 
> from resonance of OHCs. You think that there are 
> ways - involving active processes - of making 
> the TW move all the way from base to apex 
> without completely dissipating, but only 
> experiment will decide.
No, that's not what I think.  Rather, the TW 
moves from base to the point of maximum response, 
and not much further.  Only the lowest 
frequencies, which are not substanially 
amplified, make it to the apex.

The TW models have been supported by good 2D and 
3D physical modeling for many years.  Resonance 
models have had a hard time finding either a 
physical basis or a mathematical description that 
fits the data.


> Theories based on analogies with transmission 
> lines may give the right numbers, but we always 
> have to ask how apt the analogies are.
The TW theory is not based on that analogy.  I 
used the analogy just to give you a way to think 
about how active processes can lead to high gain 
and sharp tuning without resonance.


> The combined response of a bank of independent 
> oscillators looks very much like a traveling 
> wave: the question is, are the oscillators 
> independent (resonance) or are they driven 
> sequentially via coupling (traveling wave)?
I agree, that is a much better thing to argue, 
since the answer is likely to be somewhere in 
between.  The traveling wave models can 
acommodate as much local resonance as you want to 
put in, of course.


> Finally, would Gold agree with me? The question 
> is unanswerable. But the fact is that the thrust 
> of his major work was towards a resonance 
> picture. And together with his prescription for 
> positive feedback and active processing, he has 
> been an inspiration to my thinking. In the same 
> way, I owe a lot to Helmholtz, but whether he 
> would agree with me is somewhat academic.
>
> Gold could be wrong. Békésy could be wrong. In 
> fact, we all could be wrong. Science advances as 
> fresh ideas are put forward and scrutinised. But 
> Gold's opinion was "Never judge the strength of 
> foundation by size of building" [Gold 1989]. In 
> the end my resonance model, like any other, must 
> stand or fall on its merits.
I'm with you there.  But if you want your theory 
to go anywhere, you'll need to put more behind it 
in terms of what problems it solves better than 
the TW model, or what experimental evidence 
supports it.  I'm glad you're looking at the 
question.

Dick