The Measure of Madness: Philosophy of Mind, Cognitive Neuroscience, and Delusional Thought

Philip Gerrans’ The Measure of Madness proposes that “delusions arise when default cognitive processing, unsupervised by decontextualized processing, is monopolized by hypersalient information” (p. xix). In unpacking this proposition, Gerrans, a Professor of Philosophy at the University of Adelaide, avoids several topics that have sidetracked the delusion literature, including debates about the nature of belief or rationality and about the involvement of errors of logic or probability. Instead, he argues for an account of delusional thinking that is anchored in cognitive neuroscience but still responsive to subjective phenomenology: “If we describe the cognitive properties of the mechanisms that generate delusion in sufficient detail, then the personal-level aspects of the phenomenon become explicable” (pp. 209–210).

Readers interested in an accessible, general framework for understanding how the brain’s default mode network, executive control network, and dopamine-mediated salience or reward system may interact in delusional thinking should find this new book quite engaging. In early chapters, Gerrans provides a concise introduction to the default mode network. He emphasizes its active role—under the guidance of the executive control network—as a “mental time travel” simulator permitting elaborate problem solving and planning, as well as its resting, unsupervised role as a generator of highly subjective narrative fragments and the substrate for mind-wandering and dreams. Gerrans also introduces the salience system and its management of cognitive resources through mechanisms of reward prediction, error signaling, and corrective adaptation. He argues that unusual ideas arise in everyday life as subjective narrative fragments generated in the default mode network (e.g., the idea that a colleague’s weight loss means that he or she has cancer). For most people, such ideas are eventually subjected to an objective, “decontextualized” reasoning process implemented in the executive control network and related systems to help determine whether they merit further attention. If an idea is inconsistent with additional details (e.g., the colleague has been dieting), stored information, or normative ideas about reality—that is, if most people, on further consideration, would see a particular idea as mistaken or erroneous—it is discarded. For those with brain lesions from strokes, or developmental or degenerative neuropsychiatric conditions, this usual progression is disrupted. The specific content of an unusual thought may reflect a variety of other brain abnormalities—in misidentification syndromes for example, reflecting abnormal interactions between systems involved in identifying familiar others and those representing affective associations to those others. In different clinical contexts, unusual thoughts begin to transform into delusions because they are not adequately vetted, error signaling is absent or irregular, and disproportionate cognitive resources are diverted to them. Salience attribution is also abnormal, and the thoughts come to be strongly felt and imbued with importance, even when they are objectively bizarre, for example, the sense that loved ones have been replaced by identical impostors.

The Measure of Madness is most successful at this level—as a broad, scientifically informed framework. In illustrating how underlying biological and cognitive processes can be linked to personal experience, Gerrans offers a window into some of the most mysterious facets of delusions: their immunity to counterevidence and normative beliefs, their frequently bizarre character, and the sometimes ambivalent hold they have on the deluded, who may present as unshakable in their thinking and yet strangely detached from it.

Some readers, however, will find parts of the book frustrating. Gerrans seeks a mechanistic account of delusion that bridges neural-, cognitive-, and personal-level phenomena and spans fundamentally different diagnostic groups and delusional etiologies, and he attempts this in just over 200 pages. At times, in reaching for this ambitious goal, he oversimplifies. It is clear that Gerrans has digested a great deal of research. However, there are a number of points in which a complex literature is represented by a few selected studies, and tentative, partial hypotheses come across as more settled and comprehensive models. For example, Gerrans describes research suggesting, reasonably, that the face recognition functions of the fusiform gyrus are sometimes involved in misidentification delusions. But his discussion at times forgets that this is one small piece of the puzzle. Of course, the identification of familiar others extends far beyond face recognition, and the current literature falls well short of offering any mechanistic account of it, much less of the various ways it can be disturbed. Another oversimplification is that Gerrans sometimes muddles the important distinction between circumscribed “monothematic” delusions (e.g., Capgras, Fregoli, and Cotard syndromes), which can be associated with discrete lesions in some poststroke patients, and the complex and far-reaching delusional systems (e.g., persecutory, hyper-religious, or grandiose delusions) that may be gradually elaborated over months or years in conditions such as schizophrenia and bipolar disorder. The book might have benefitted from an earlier clarification of this distinction and a more focused contrast between categories in the application of the proposed framework. For example, the salience system appears to be of far greater relevance in delusions associated with neuropsychiatric disorders than in the stroke context. Finally, some may wish that Gerrans had not steered around fascinating questions regarding the kinship of delusional thinking with other unusual but powerfully experienced ideas, such as the characteristic obsessions and compulsions in obsessive-compulsive disorder or the sense of impending disaster in panic episodes.

In the end, The Measure of Madness is a partial success. The book embraces delusional phenomenology and offers a concise, accessible, and well-informed introduction to cognitive and neural systems that are likely to be central to improved understanding of delusions in years to come. At the same time, the work highlights how challenging it is to specify underlying general mechanisms beyond the broadest outlines. Recent scientific progress is encouraging, but the target is complex and heterogeneous and our understanding of mechanisms is still quite limited, providing only the beginnings of a bridge to “the personal-level aspects of the phenomenon.”