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During the last two decades, functional brain imaging has emerged at the forefront of the neuroscientific study of the interface between mind and brain. In their thoughtful and original paper “Freud's Theory of Mind and Functional Imaging Experiments,” Shulman and Reiser critique some of the implicit assumptions that have been made in designing functional imaging experiments of mental activity.
Specifically, they argue that Freud's topographical model (1900) could provide a conceptual framework of mental function that might account for the very high “baseline” metabolic activity of the brain. This activity is often subtracted out, and therefore ignored, in experiments that compare regional cerebral activity during the execution of a task, or during sensory stimulation, with “resting” brain activity. The authors propose that the “resting” activity—that is, the brain activity of the awake subject when she or he is not presented with any specific stimulus or task—is not background noise but rather “represents an aggregate of internal and subjective activities that must be accepted, for the present, as unexplained but measurable.” Despite its relevance, I am somewhat concerned that Drs. Shulman and Reiser's critique may have implicated the whole field of functional brain imaging in making naïve reductionistic assumptions about the negligibility of “baseline” cerebral activity. It is my impression that such assumptions are much less common today than they were perhaps a decade ago.
The authors' main argument is that even in regions that were found by the subtraction method to be specifically involved in a particular mental task, the high “baseline” activity implies that the brain devotes the overwhelming majority of its energy to support neuronal firing that is not narrowly related to that task, but is nevertheless crucial for its execution. Indeed, this “baseline” activity, which is usually subtracted out, is often 50 to 100 times greater than the remaining activation tha