Re: Rationale for Critical Bands

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

Martin, your informal observation of bandwidth "as good as unchanged 
in this level range" may also be pretty consistent with peripheral 
models that exhibit a modest bandwidth change over a very large range 
of levels.  Has anyone attempted to test a hypothesis about whether 
the psychophysical effects are constant, as opposed to somewhat level 
dependent?

As you and Brian have noted, speech without interference doesn't show 
much level-dependent effect, but speech in noise does.  But neither 
of these is directly a measure of the critical band; Brian's working 
hypothesis that the reduction in intelligibility of speech in noise 
at higher levels is due to wider critical bands sounds plausible. 
But we'd need a different experiment to test whether the critical 
bands do actually change; the auditory filter fits for tone in noise 
detection seem like they do just that.  If you reject those results, 
can you explain why?  Or you just focus on different aspects of CB to 
which those results are not relevant?

I've been reading up on the concept of critical band in IC, which is 
what you've been talking about.  I have this book on IC, which one 
can read parts of online:
http://books.google.com/books?id=Ift_WTgSKqMC&pg=PA327
The section "Origins of Spectral Tuning and Resolution" in the 
chapter by Ehret and Schreiner starts off with something that I think 
we can all agree on:

"Cochlear filtering represented in the RFs of auditory nerve fibers 
is the basis for spectral tuning in higher centers of the auditory 
pathways."

But then they get into an interpretation of "critical bands" that's 
one I'm not familiar with, but probably is what you are talking 
about, when they say on p.328 "The origin of neural critical 
bandwidths with properties of psychophysical critical bands is 
associated with ICC neural filtering properties" and "The steep 
slopes of the critical-band borders...".  On p.325 they say 
"Perceptual critical-band properties are absent in auditory nerve 
fibers (Ehret 1995)"; so I'll guess I'll have to find that and see 
what they mean.  It's in a book that I can't find a copy of:
Advances in Hearing Research: Proceedings of the 10th International 
Symposium on Hearing : Swabian Conference Centre, Irsee, Bavaria 26 
June-1 July, 1994, Geoffrey A. Manley, G. M. Klump, C. Koppl, H. 
Fastl, H. Oeckinghaus (eds.); anybody have a copy?

They also reference these papers for showing that the "critical-band 
like response" arises first in IC:

Stanley Zerlin
Electrophysiological evidence for the critical band in humans
J. Acoust. Soc. Am. Volume 79, Issue 5, pp. 1612-1616 (May 1986)

Dennis L. Burrows and S. Joseph Barry
Electrophysiological evidence for the critical band in humans: 
Middle-latency responses
J. Acoust. Soc. Am. Volume 88, Issue 1, pp. 180-184 (July 1990)

So I got these to see what they say.  Basically, they're about 
loudness summation, and finding evoked potentials that show a 
loudness-correlated CB effect in their response to tone pairs or 
noise bands.  So I think I see now where these guys are coming from 
-- you can't get loudness summation effects directly in the auditory 
nerve responses, since you need to aggregrate in some way across 
frequencies.  I can well believe that that is part of what goes on in 
IC.  I still don't see any evidence, however, that the question of 
whether the CB inferred from such evoked responses is level dependent 
or not has been addressed by any of these authors.  Rather, they 
appear to assume that it is not level dependent.  Am I missing some 
important piece of the puzzle?

When I said "I don't think there's much actual disagreement" I may 
have been overlooking something; but it's still not clear to me what 
it is.  I know that Langner and Schreiner and Ehret do great work in 
IC, but I don't understand from their papers what exactly they have 
found, or are assuming, about critical bands or their level 
dependence.

It's interesting that the Schreiner and Langner abstract that you 
cite starts with "The perception of sound is based on signal 
processing by a bank of frequency-selective auditory filters, the 
so-called critical bands."  This very direct association of "critical 
bands" as a set of real discrete "auditory filters" puzzles me.  The 
"critical band" is a psychophysical concept, and the auditory filters 
usually are, too, though they can have physiological counterparts in 
some studies.  The relationship to real mechanisms is what they're 
exploring, yet they refer to the auditory filters as if they are 
specific real things.  And this usage seems to be contrary to the 
loudness summation effects that they seem to emphasize in IC, which 
is something that follows the filters.

So I guess it all comes down to being careful to say what we mean by 
critical band or auditory filter when we make claims about it.  I'll 
try to be careful if you will.

Dick