A Methodological Puzzle

A recent study by Wilson et al. argues for a function that is done by the prefrontal cortex (PFC) but is apparently not done exclusively by any one of its parts. This function is processing of temporally complex events. Temporally complex events are stimuli in which several features that are needed to learn a task are presented sequentially, and are not all available at any one time.

This result is particularly interesting because the authors argue for there being other functions that do seem to be located in different parts of the PFC. This is supported in several ways. One method involves selectively destroying parts of macaque monkey brains, and finding, for each part, a task on which performance is highly impaired by destruction of that part but much less impaired by destruction of the others.

The further function of the whole PFC (i.e., the processing of temporally complex events) is then shown by a task on which performance is only slightly impaired by destruction of each of the parts, but is severely impaired by destruction of the whole PFC.

“Here we have argued that the PFC as a whole has an overarching function that is not localized to any particular subregion, and we have proposed that this role is related to its involvement in the processing of temporally complex events.” (p. 538)

As good studies in science should do, this one raises interesting questions. One that intrigues me is this. How should we go about distinguishing functions? How do we tell whether we have found two independent functions, rather than one function that works by making use of another? Could it be that there is a function that (a) can be performed anywhere in the PFC and (b) is drawn upon in performing each of the functions that also require something further that can be done only in a specific part of the PFC?

I have no answer to offer to this question, but I think I can clarify it by reference to another case. There is a brain part (fusiform gyrus) that is often referred to as “the face processing area”. Work from Eric Cooper’s lab suggests, however, that this is an overly narrow description of what this area does. That’s because its activity seems required whenever we have to make finer discriminations that depend on relative distances and not just on general features of where things are. Faces are alike in having the nose between the eyes and above the mouth, so we have to be able to appreciate different distances between these features in order to recognize a particular person. But we also have to use this ability to distinguish, e.g., different makes and models of cars. A credenza and a dresser are both essentially boxes, so we have to be able to analyze relative proportions to tell the difference.

In short, the suggestion is that the famous “face processing area” would be better thought of as performing the function of making discriminations that depend on relative distances and not just gross placement of parts.

To find out what a part of the brain does, researchers must use some definite task. And then, as responsible scientists, they must relate their descriptions of the functions performed to the tasks they used. Otherwise, they would be merely speculating about how the mind works.

But, somewhat paradoxically, this necessary policy may have a built-in cost. Descriptions that are driven by investigative tasks may turn out to be overly narrow, and that may skew our conception of how each brain part contributes to our whole organization. To avoid this pitfall, we have to keep our minds open. It’s always possible that what seems to be a part that performs a specific function may do something more general than what could legitimately be concluded from any single study.

 [Wilson, C. E., Gaffan, D., Browning, P. G. F. and Baxter, M. G. (2010) “Functional localization within the prefrontal cortex: missing the forest for the trees?”, Trends in Neurosciences 33(12):533-540. Work from Cooper’s lab can be found in, e.g., Brooks, B. E. and Cooper, E. E. (2006) “What Types of Visual Recognition Tasks Are Mediated by the Neural Subsystem that Subserves Face Recognition?”, Journal of Experimental Psychology: Learning, Memory, and Cognition 32(4):684-698.  ]

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