Re: mechanical cochlear model

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

I agree with Reinhart Frosch and Alain de Cheveigne and Dave Mountain 
on this.  The cochlear traveling wave may have some open issues, but 
to say that "its function in hearing is not yet universally 
appreciated" is misleading, and I think a bit disingenuous.

It's OK to investigate the open issues, and to use them to motivate 
new ideas that depart from the generally accepted view.  But I think 
it's not a good idea to misrepresent the extent to which the ideas 
are accepted by the mainstream hearing community.  In the case of the 
traveling wave, the acceptance is pretty much universal, and has been 
for quite a long time; the agreement of theory and experiment gets 
better over time, as experimental data get better and as analysis 
techniques get more mature and models converge on physical 
measurements.

With respect to Jont Allen's 2001 remark that "the discrepancy in 
frequency selectivity between basilar membrane and neural responses 
has always been, and still is, the most serious problem for the 
cochlear modelling community" (in his chapter "Nonlinear Cochlear 
Siganl Processing" in Jahn and Santos-Sacchi "Physiology of the 
Ear"), I had encountered that myself recently, and was wondering 
what's behind it.  Jont goes on to say, in italics even, "In my view, 
this discrepancy is one of the most basic unsolved problems of 
cochlear modeling."  and then "Progress on this front has been 
seriously confounded by the uncertainty in, and the interpretation 
of, the experimental data."

It's a great chapter, and I agree with Jont on many of his points and 
attitudes, but I still wonder what he was poking at with that 
paragraph.  My view is a little different:  active hydromechanical 
models are able to yield transfer functions and tuning curves and 
nonlinearities that are pretty much like what we see in both 
mechanical and neural data (FTCs, Weiner kernels, 2TS curves, etc.). 
The problem may be in his intepretation of "frequency selectivity", 
as he sometimes turns FTCs upside-down and compares them with 
transfer functions, as in his Figure 19-12 that he refers back to 
later as an example of a neural/mechanical mismatch.  That seems like 
an odd mistake to make in a chapter on nonlinearity, but it is one 
that is widespread in the hearing literature, not unique to Jont, and 
one that has a history of introducing confusion about sharpness in 
different ways of looking at a system.

Jont also says, a few pages earlier, "In fact, according to 
measurements made over the last 20 years, the response of the basilar 
membrane to a pure tone can change in amplitude by more than 5 orders 
of magnitude per millimeter of distance along the basilar membrane 
(ie, 300 dB/octave is equivalent to 100 dB/mm in the cat cochlea)." 
This strikes me as another misapplication of linear thinking to a 
nonlinear system.  If the FTC requires an increase of 5 orders of 
magnitude to get a given response when the frequency is increased by 
1/3 octave, which I can accept as roughly credible, then that has to 
be understood as a combination of a fairly sharp high-frequency 
rolloff and a fairly strong nonlinear level dependence.  If the 
response compression slope is about 1/3 above CF, then the 
compression explains about 2/3 of the steepness of the 300 dB/octave 
slope, so the estimated change in response with place for a given 
pure tone needs to be cut by a factor of three, to about 1.7 orders 
of magnitude, not 5.  This is the kind of discrepancy that I'd think 
would be included when saying "Progress on this front has been 
seriously confounded by the uncertainty in, and the interpretation 
of, the experimental data," but perhaps Jont is thinking of some 
other problems.

The open problems that Jont points out, of getting good models of 
two-tone suppression and upward spread of masking, that unify 
mechanical and neural data, are indeed important points where 
modelers have more work to do.  I think the situation is actually in 
not such bad shape, though.  I don't see any real basis for thinking 
that a new paradigm is what's needed at this time to make progress, 
and I don't think that's what Jont is saying, either, unless he's 
telling us to stop assuming that the system has active amplification.

Jont, can you fill us in?  Has the problem been updated in your 
opinion since 2001?  Are you referring to interpretations of active 
amplification as part of the problem?

Dick