Subject: subtraction and distributed function [was Re: The Auditory Continuity Illusion/Temporal Induction] From: Chris Stecker <cstecker(at)UMICH.EDU> Date: Mon, 12 Dec 2005 11:57:20 -0800Hello 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