Phenomenological and neurocognitive perspectives on polythematic and monothematic delusions
Authors: Max Coltheart
Journal: World Psychiatry (2015)

Sass and Byrom 1 end their paper by advising us to doubt the wisdom of viewing delusion as a unitary phenomenon. I share their view. We must make distinctions here. And as far as the phenomenology of delusion is concerned, the most important distinction is between polythematic and monothematic delusional conditions 2, 3 – critical here because the various phenomenological features of delusion that Sass and Byrom discuss are characteristic of polythematic delusion, but not of monothematic delusion. In polythematic delusional conditions, the patient expresses delusional beliefs about a wide variety of unrelated topics. For example, amongst the beliefs expressed by P. Schreber were that “nerves” and “rays” were taking over his soul, that he changed into a woman, and that he was omnipotent, omniscient and even omnipresent 4. And amongst the beliefs expressed by the Nobel Laureate J. Nash, diagnosed with schizophrenia, were that he would become Emperor of Antarctica, that he was the left foot of God on Earth, and that his name was really Johann von Nassau 5. In contrast, in monothematic delusional conditions, the patient expresses only a single delusional belief concerning a single topic. Numerous different monothematic delusions have been described in the literature. Eight of these monothematic delusions 6 are: Capgras delusion (“one of my closest relatives has been replaced by an impostor”) 7-10; Cotard delusion (“I am dead”) 11, 12; Frégoli delusion (“I am being followed around by people who are known to me but who are unrecognizable because they are in disguise”) 13-16; mirrored-self misidentification (“the person I see in the mirror is not me, but some stranger who looks like me”) 17; reduplicative paramnesia for persons (a stroke patient affirmed both that her husband had died and had been cremated four years earlier (true) and that he was currently a patient on the ward in the same hospital that she was in (not true)) 17; somatoparaphrenia (the patient denies ownership of a limb insisting that this limb actually belongs to someone else, such as a relative or the clinical examiner) 18; delusion of alien control (“someone else is able to control my actions; I am a puppet and someone else is pulling the strings”) 19; delusion of thought insertion (“thoughts are put into my mind like ‘Kill God’; it's just like my mind is working but it isn't; they come from this chap Chris; they're his thoughts”) 20. Sass and Byrom summarize the phenomenological perspective on delusion as follows: “Phenomenological accounts of delusional mood/atmosphere discuss loosening of perceptual context, promiscuous linkages between de-contextualized elements, a sense of being at the center of things, and alienating hyperreflexivity. Such a perspective helps to clarify how paranoid, metaphysical, influence-related, bodily, and solipsistic delusions – together with altered experience of self, intersubjectivity, and felt reality – might develop on the basis of overall structural or ontological changes associated with delusional mood and its aftermath”. Examples of all of this are seen in patients with polythematic delusional conditions, especially in what they say when describing their experiences, such as Sass and Byrom's quotations from Schreber. But scrutiny of the (now quite extensive) literature on monothematic delusions such as the eight delusions listed above reveals nothing like this. Such patients do not report anything like, to quote from Sass and Byrom, “feelings of strangeness and tension and of suggestive yet ineffable meaning” or that “something is in the offing” or “a sense of threat and associated anxiety” or “an abnormal, sometimes excruciating sense of meaningfulness” or an experience of the world as “somehow false, inauthentic, or insinuating”. These are all part of the phenomenology of polythematic delusion, but they are not part of the phenomenology of monothematic delusion. As for neurocognitive hypotheses about delusion, we don't know much about how to explain polythematic delusion, but the two-factor theory 21-23 seems to offer a good explanation of the various monothematic delusions. I will illustrate this for the case of Capgras delusion. Seeing a familiar face normally generates a much larger response of the autonomic system than seeing an unfamiliar face. But not in patients with Capgras delusion: their autonomic responses are no stronger to familiar than unfamiliar faces 24-26. The two-factor theory of monothematic delusion uses this finding thus: “Suppose that as we go about everyday life we use an internal model of the world 27, 28 to continuously predict what we will experience next. These predictions will normally be fulfilled, but occasionally not: occasionally something not predicted by the internal model occurs. That event indicates that there is something wrong with the database of beliefs that the model uses to predict what will happen next in the world. So the database needs to be fixed (by modifying existing beliefs or adopting new ones) so that it becomes compatible with the unexpected event. Adopting the belief ‘This is not my wife’ will make the Capgras patient's belief database compatible with the lack of arousal response to the sight of the wife's face” (29; my emphasis). As the emphasized phrase indicates, this theory proposes that what triggers the search for a new belief (which will become the Capgras delusion) is prediction error: that is Factor 1. But this is not sufficient to bring about Capgras delusion, because patients with frontal ventromedial damage show the same autonomic symptom as Capgras patients, yet are not delusional 30. The idea prompted by the autonomic abnormality (“That's not my wife”) ought to be rejected; it instead becomes adopted as a belief because of the presence of Factor 2, which is defective ability to evaluate beliefs, perhaps associated with right dorsolateral prefrontal cortex abnormality 22. Analogous two-factor explanations have been offered for the other monothematic delusions 6, 21-23.