Neural Sufficiency, Reductionism, and Cognitive Neuropsychiatry

Kanaan and McGuire (2011) elegantly describe three challenges facing the use of functional magnetic resonance imaging (fMRI) to uncover cognitive mechanisms. They show how these challenges ramify in the case of identifying the mechanisms responsible for psychiatric disorders. I want to raise another difficulty for fMRI that also seems to ramify in similar cases. This is that there are good reasons for doubting one of the assumptions on which many fMRI studies are based: that neural mechanisms are always and everywhere sufficient for cognition. I suggest that, in the case of the mechanisms underlying psychiatric disorders, this assumption should be doubted. I do not dispute that a malfunctioning neural mechanism is likely to be a necessary component of a psychiatric disorder—as Kanaan and McGuire say, the experimental evidence from cognitive neuropsychiatry gives us excellent reasons to think that this is so. My question is whether a story only in terms of these neural mechanisms is sufficient to explain the mechanism of a psychiatric disorder. Is the reduction, projected by cognitive neuropsychiatry, of psychiatric disorders to disorders in neural functioning even in principle possible? Drawing on recent concerns about the location of mental states, I argue that such a reduction is likely to fail. Even if the considerable problems raised by Kanaan and McGuire for fMRI could be addressed, we have no reason to think that the mechanisms involved in psychiatric disorders are entirely neural, and that fMRI, or even a perfect science-fiction brain scanner, would be capable of uncovering them. Psychiatric disorders, like numerous other cognitive processes, are liable to cross the brain-world boundary in such a promiscuous way as to be resistant to neural reduction.

As Kanaan and McGuire argue, part of the appeal of fMRI to psychiatry is that it offers the tantalizing prospect of putting psychiatry on a firm biomedical foundation: namely, mechanistically explaining psychiatric disorders in terms of functional brain disorders. In the ideal case, this would involve finding a neural mechanism responsible for, sufficient for, or characteristic of, a given psychiatric disorder. A psychiatric patient could, say, be diagnosed with one or more malfunctioning cognitive mechanisms, which ultimately would be identified with a neural malfunction. The proposed identification would run in two steps. Mental disorders would first be characterized within the framework of cognitive psychology: in terms of malfunctioning cognitive, perceptual, behavioral, systems. Then those systems would be identified with underlying brain structures and functions responsible for their action via imaging studies such as fMRI. Thus, fMRI would link our existing cognitive and behavioral characterization of mental disorders (patients that act, feel, or think in dysfunctional ways) with underlying neural mechanisms. If such a mechanistic reduction could be achieved, it would have the potential to dramatically increase our understanding of the nature of mental disorders. Psychiatric disorders would be understood as medical disorders, continuous with other bodily disorders studied in the biomedical sciences.

Kanaan and McGuire raise three challenges for fMRI as a way of uncovering neural mechanisms underpinning psychiatric disorders. These concern problems with task choice, statistical power, and interpretation of fMRI results. Task choice is the problem that, because of the nature of psychiatric disorders, it is hard to get a meaningful or sharp contrast between 'ill' and 'normal' behavior from patients in an fMRI scanner. Statistical power is the problem that psychiatric disorders tend to involve fMRI measurements with a low signal-to-noise ratio, and small or heterogeneous groups of patients, which produces results of dubious statistical significance. The interpretation problem is that scans of psychiatric patients are open to a variety of interpretations, but unlike fMRI scans of normal subjects, the assumptions relied on to select the correct interpretation may not hold true in psychiatric cases. Given these three problems, fMRI is far from a straightforward tool for identifying the neural mechanisms involved in a psychiatric disorder.

At least as we would wish it, the logic of fMRI is that it allows one to see the regions of the brain sufficient for a given cognitive function. The lesson from Kanaan and McGuire is that in practice...