Taxonomizing the Neural Correlates of Delusion

Abstract

Purpose: Delusions represent a core symptom of schizophrenia and are a major cause of morbidity associated with psychiatric illness, yet the neural correlates of delusions remain relatively under-characterized. Part of the challenge is that delusions are difficult to define, and it is not known whether different types of delusions share underlying etiologies, or where to draw the lines of taxonomy in the first place. The purpose of this paper is to review current evidence for the neurobiological mechanism of delusions, with special focus on persecutory delusions and delusions of reference, which represent two of the more commonly accepted subtypes. A critical discussion of the classification of delusions, the conceptualization of their mechanism, and the impact of these two features on the study of delusions will also be performed. Methods: A review of studies using fMRI to examine the neural correlates of persecutory delusions and delusions of reference was performed. This review was expanded to include papers outlining foundational concepts upon which the assumptions of the fMRI studies were based. Several studies were selected for detailed analysis for the purposes of both providing evidence for a mechanism of delusion and as examples of the challenges of studying delusions. Results: The aberrant salience hypothesis is the prevailing theory for the neural basis for delusions, and the interpretation of fMRI data are informed by this framework and also support it. Persecutory delusions and delusions of reference both appear to share abnormalities in the so-called salience network, and each exhibit abnormalities in specific other networks of the brain that make sense given our understanding of their phenomenology. Though these mechanisms create a sensical picture of each delusion type, much of the data are not perfectly accounted for by prevailing theory. Conclusions: Much remains to be elucidated regarding the neural substrate of delusions. Functional MRI provides a useful though imperfect modality for their study. Given the difficult-to-define nature of delusions, careful consideration must be given to the implications of our assumptions on study design and data interpretation in order for the field to progress.