Revising the Standard Definition of Hallucination: Delirium and Schizophrenia.

Background: According to the standard definition, a hallucination is 1) a perceptual experience occurring in the absence of a relevant perceptual object, 2) it has the sense of reality of a veridical perception, and 3) it is unwilled and not under voluntary control of the hallucinator. This definition is supposed to encompass all hallucinations, across mental disorders and experiential modalities.

Summary: In this article, we examine the standard definition's validity by comparing visual hallucinations in delirium and auditory verbal hallucinations (AVHs) in schizophrenia, focusing especially on the definition's second criterion, i.e., the sense of reality criterion. Our analysis reveals stark differences between these two disorders, their concomitant experiential structures, and the experience of hallucinations. While hallucinated objects are experienced as real in both disorders, they are typically not experienced as real in the same sense of the term. In delirium, hallucinatory objects are experienced as real in the way perceptual objects are, and they often are indistinguishable from such objects. In schizophrenia, by contrast, hallucinatory objects are often not experienced as real in the way perceptual objects are, and they often are distinguishable from such objects.

Key message: An appreciation of the different kinds of sense of reality of hallucinations is relevant for developing more precise psychopathological concepts, clinical care, and empirical research. Based on our findings, we offer a revision of the standard definition of hallucination, making it valid for the variety of hallucinations that are found across mental disorders.