Subject: Re: subtraction and distributed function [was Re: The Auditory Continuity Illusion/Temporal Induction] From: Al Bregman <bregman(at)HEBB.PSYCH.MCGILL.CA> Date: Tue, 13 Dec 2005 19:47:28 -0500Dear Chris, Thank you for your thoughtful reply. It is clear that using imaging data is a lot more complicated than people think. You wrote: "The question, as you suggest, should be "how" distributed functions take place, and is unlikely to be answered by any one method, but rather through the development of computational models on the basis of descriptive data about the brain and behavior." The approach that you suggest (fitting models to data) could be a productive one, assuming that the models are based on assumptions about actual types of processing rather than being of the "hip bone is connected to the ankle bone" type (North American readers will know the song). Best wishes, Al ---------------------------------------------------- Albert S. Bregman, Emeritus Professor Psychology Dept., McGill University 1205 Docteur Penfield Avenue Montreal, Quebec Canada H3A 1B1 Voice & Fax: +1 (514) 484-2592 Aug 15 - Sept 15 annually: Voice & Fax +1 (207) 729-0986 ---------------------------------------------------- ----- Original Message ----- From: "Chris Stecker" <cstecker(at)UMICH.EDU> To: <AUDITORY(at)LISTS.MCGILL.CA> Sent: Monday, December 12, 2005 2:57 PM Subject: subtraction and distributed function [was Re: The Auditory Continuity Illusion/Temporal Induction] > Hello Al and list, > > I'd just like to interject that the difficulty (folly?) of localizing > distributed functions in the brain is not lost on (all) fMRI > researchers. I've tried to focus attention on this issue when describing > to others the direction of my own work in auditory fMRI. There are > really two kinds of approaches through which we could imagine using the > subtraction technique. The first depends on assuming modularity of > function in the brain. If that assumption holds, then two carefully > designed conditions---one that includes the process of interest, and one > identical to the first except for the lack of that process---can be used > via subtraction to identify the module in question. (This is of course > not a new idea and has been used to look at chronometry, etc. for quite > some time now.) Many of us, however, doubt the applicability of strict > modularity, and instead recognize that---even with perfectly designed > conditions---many things are likely to change in related modules (if one > expects modularity to hold weakly) or throughout an extremely > distributed system that subserves the process of interest along with > many others (if one does not). The second approach is to use subtraction > as a tool to examine the effects of specific manipulations upon the > "activations" observed in the brain, and interpret those effects mainly > in a descriptive sense. For example, we might look at tone-evoked > activations at different sound levels. We might use subtraction to > isolate sound-related activity ("sound" - "silence") which we then > compare across levels, or we might directly compare activations produced > at different levels ("intense" - "soft"). Either way, we try to describe > the sensitivity to tone level in different regions of the brain (which > might in turn tell us a lot about the kinds of computations each region > is likely to be involved in) rather than to localize the "module that > processes sound level" or the "module that processes intense sound." [It > might also be worth pointing out that "subtraction" in this case can be > replaced by correlation or another statistical technique for assessing > sensitivity to manipulation of the independent variable across > potentially many levels; I'm not sure how to interpret such data via the > modularity assumption, but it makes good sense for the descriptive > approach.] > > Personally, I think the evidence for distributed function (throughout > the auditory cortex at least) is pretty good, and strongly prefer the > second, descriptive, approach to interpreting fMRI data. So I agree with > your post, especially regarding concerns about designing appropriate > conditions for subtraction and about the interpretation of subtraction > results via an implicit assumption of modularity. But I want list > members to realize that the method of subtraction can be employed > without that assumption, even though they are often encountered > together. The question, as you suggest, should be "how" distributed > functions take place, and is unlikely to be answered by any one method, > but rather through the development of computational models on the basis > of descriptive data about the brain and behavior. > > -Chris Stecker > >