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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 measurement.

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 has it:

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 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?