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Re: Rationale for Critical Bands
A follow-up, in case anyone still cares...
I got a copy of Ehret and Romand's book "The Central Auditory
System", with Ehret's chapter "The Auditory Midbrain, a 'Shunting
Yard' of Acoustical Information Processing", to see what he says
about critical bands in 1997. It's a fascinating chapter, with lots
of good stuff.
On p.277, "In accordance with psychophysical terminology, these
bandwidths of spectral integration or spectral resolution are called
critical bands." That is, he adopts the term critical bands, for
something that is not psychophysical, and is not measured with an
approach that much resembles the paradigms for the psychophysical CB
and "the tuning curves for complex sound analysis of most neurons
(62%) in the ICC of the cat remain rather constant in width when the
intensity of the sound is changed." This seems to be the association
of CB as level independent, but the case is not made that the CB and
this curve, whatever it means, are in some sense the same phenomenon.
It he meant to claim that, he probably would have said so.
On p.278, "Whether neurons in more peripheral auditory nuclei have
tuning curves for complex sound analysis with the features mentioned
above remains to be determined." That is, he has backed away from
the earlier conclusion that such curves (whatever they are, that he
associates with CB) arise first in ICC.
Then he refers to the "receptive field size (the critical bandwidth)
of ICC neurons in the spectral domain." Again, mixing the
physiological and psychophysical terminology, as if they're the same,
but not really making a claim that they are.
It's quite possible that these neurons do extract some kind of
spectral feature in a fairly level-independent way, and that may be
closely related to some kinds of critical band phenomena. But for
other phenomena, such as detection of tones in notched-noise maskers,
and other psychophysical tests, the bandwidth must vary a fair bit
with level to fit the data, and the filter shape is nothing like
rectangular, which is what he seems to be suggesting the shapes of
the ICC tuning curves show.
In my opinion, applying ideas like spectral domain, that are so
intimately tied to the dynamics of linear systems, to the nonlinear
neural processing several stages beyond the periphery is bound to be
ambiguous and unsatisfying for understanding the function. In the
periphery, where the nonlinearities can be understood and accounted
for well enough, linear systems concepts such analysis by sinusoids
and noise bands can be applied with reasonable success, if care is
taken in not extrapolating to different levels and such. But in the
ICC, observations about the tonotopy and response curves really don't
have any directly interpretable relationship to things like critical
bandwidths. That opinion probably explains why I always associated
critical bands with peripheral filtering -- that is, after all, where
the frequency response, and its level dependence, is shaped, as Ehret
said in the bit I quoted before. I believe that's the prevailing
interpretation: the CB reflects cochlear filtering, and is
moderately level dependent.
ps. I got no replies to my query for finding the most relevant source
from Ehret, at the send of this snippet, so I'm still hoping someone
At 11:43 AM -0700 6/15/10, I wrote:
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:
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
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?