The delusional brain

Delusions are pathological beliefs which persist despite clear evidence that they are actually false. They can vary widely in content, but are always characterized by the absolute certainty with which they are held. Such beliefs reflect an abnormality of thought processes; they are often bizarre and completely unrelated to conventional cultural or religious belief systems, or to the level of intelligence of the person suffering from them.

The delusions experienced by psychiatric patients are sometimes categorized according to their theme. For example, schizophrenics often suffer from delusions of control (the belief that an external force is controlling their thoughts or actions), delusions of grandeur (the belief that they are a famous rock star or historical figure) or delusions of persecution (the belief that they are being followed, attacked or conspired against).  

Although often associated with psychiatric disorders, delusions can also occur as a symptom of neurodegenerative disorders, and improved diagnostic methods have led to an increase in the identification of brain damage in patients who suffer from them. To date, however, there has not been an all-encompassing theory of how the brain generates delusions. Now though, Orrin Devinsky, a professor of neurology, neurosurgery and psychiatry at New York University, proposes that delusions are generated by a combination of right hemisphere damage and left hemisphere hyperactivity.

In a review published in the journal Neurology, Devinsky examines the neuropathologies underlying two delusional syndromes with the aim of identifying anatomical abnormalities that are common to all four. Specifically, he looks at Capgras syndrome, the delusional belief that close friends or relations are imposters or have identical body doubles with different identities and reduplicative paramnesia (or Capgras for places), in which one believes that a familiar place exists in two locations simultaneously.

These syndromes are related to, and often co-exist with, confabulation  (the pathological production of false memories) and anosognosia, a condition in which one fails to recognize, or is unaware of, a neurological deficit such as blindness or paralysis. They also share common mechanisms and pathologies. However, whereas confabulating patients can be convinced that their memories are false, deluded patients hold on to their beliefs firmly.  

Devinksy looked at numerous case studies of individuals with these syndromes and, when possible, pinpointed the site of brain damage in each. His analysis showed that the four conditions do indeed share common pathological features. In 69 patients with replicative paramnesia, for example, 52% had incurred damage to the right frontal lobe (as a result of stroke or Alzheimer’s Disease), 41% had damage to both, and 7% had damage to the left. Likewise, the case studies of patients with Capgras syndrome showed that they had damage primarily to the right frontal lobe.

The ubiquity of frontal lobe damage in the cases studies supports the hypothesis that these delusions involve impairments in executive function, working memory, decision-making and the abilities to make accurate predictions and to estimate and sequence time. One consequence of damage to the right frontal lobe would therefore be an impairment in the patients’ ability to monitor the accuracy of their own cognitive processes.

According to Devinsky’s hypothesis, this leads to increased left hemisphere activity – the left hemisphere compensates for the lack of inappropriate inputs from the right, “filling in” the gaps and conjuring a creative and extravagant narrative which leads to false explanations of the patient’s experiences. Damage to the right hemisphere may prevent the patient from recognizing his or her cognitive errors, and therefore from changing their false beliefs.


Devinsky, O. (2009). Delusional misidentifications and duplications: Right brain lesions, left brain delusions. Neurology 72: 80-87. DOI: 10.1212/01.wnl.0000338625.47892.74

8 thoughts on “The delusional brain

  1. You might be interest in this:
    The Cognitive Neuropsychology of Delusion
    by Robyn Langdon & Max Coltheart
    at Macquarie Centre for Cognitive Science, Macquarie University:
    After reviewing factors implicated in the generation of delusional beliefs, we conclude that whilst a perceptual aberration coupled with a particular type of attributional bias may be necessary to explain the specific thematic content of a bizarre delusion, neither of these factors, whether in isolation or in combination, is sufficient to explain the presence of delusional beliefs. In contrast to bias models (theories which explain delusion formation in terms of extremes of normal reasoning biases), we advocate a deficit model of delusion formation – that is, delusions arise when the normal cognitive system which people use to generate, evaluate, and then adopt beliefs is damaged. Mere bias we think inadequate to explain bizarre delusions which defy commonsense and persist despite overwhelming rational counter-argument. In particular, we propose that two deficits must be present in the normal cognitive system to explain bizarre delusions: (1) there must be some damage to sensory and/or attentional-orient-ing mechanisms which causes an aberrant perception – this explains the bizarre content of the causal hypothesis generated to explain what is happening; and (2) there must also be a failure of normal belief evaluation – this explains why a hypothesis, implausible in the light of general commonsense, is adopted as belief. This latter deficit occurs, we suggest, when an individual is incapable of suspending the natural favoured status of direct first-person evidence in order to critically evaluate hypotheses, given equal priority whether based on direct or indirect sources of information. In contrast, delusions with ‘ordinary’ content may arise when a single deficit of normal belief evaluation occurs in the context of an extreme (but normal) attentional bias, thus causing failure to critically evaluate hypotheses based on misperceptions and misintrepretations of ambiguous (but ordinary) first-person experience.

  2. There´s something odd with the definiton of delusion porvided by standard manuals (DSM-IV):
    A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everybody else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary
    The first lines of it are correct: “A false belief based on incorrect inference about external reality” but when the definiton says: “…that is firmly sustained despite what almost everybody else believes…” is strange to my ears.
    Because it could be possible that everybody believes that something is false (while you hold it to be true), when in fact its true. I mean that the definition of delusion seems to be based on an “epistemology of the majority”. Something cannot be justified by what the majority hold to be true.
    In relation to how our brains generates delusions, I think delusion and normal perception rely on a continuum. Our brains are encapsulated in a bony skull and for that reason they are always guessing what happens outside (in the external world, reality) constantly fabricating its own realities.
    The senses cannot produce a faithful neural copy of the external world (memory and vision are reconstructive processes) and for this reason the exact border between delusions and healthy perceptions is blurred.

  3. Anibal,
    it’s simply a fact that we have to presuppose that what we experience is indeed experience of some objectively existing world in which I am the function of my body – but since we can never have second order knowledge about anything concerning external reality (and most things concerning our own inner workings), there is always the problem that any reasonable definition of delusion will have to make use of a notion of ‘truth’ – or at least of ‘overwhelming evidence to the contrary’. That means we make normative assertions here – and there’s no way around that.
    Without a working definition of evidence, in fact without a reasonable epistemology, the normative criteria for delusion have no firm foundation. Once we are ready to accept that the external world is a necessary but unprovable presupposition of ours we can go on and devise a working, reasonable epistemology and thus provide a basis for our normative claims.

  4. I really like this article and the insightful posts.
    Still though, regardless of any epistemology devised, we will continue to believe that our method of discovery is giving us the essence of absolute truth. All along viewing evidence from our purely subjective consciousness, even if we have collectively agreed on the foundation of said truth.
    However, without this cohesion there would be no collective reality.

  5. Hi – commenting from the perspective of a parent who is seeking answers. Have a 21 year old son diagnosed with schizophrenia. His abnormal belief is that he will eventually be able to fly. This is not about throwing himself off bridges, but being up all night meditating to generate the power to physically lift off. The impact on his life is significant as this is his key focus and significantly reduces his functionality for daily living as well as driving him away from anyone (including his treatment team)who suggests this belief is inappropriate.
    While he takes medication reluctantly, this appaers to have had little or no impact on the level of conviction he has for this belief.
    Any suggestions about how to break through such a strong delusional belief and help him to recognize this?

  6. Blair G: When you’re in the middle of a delusion, it’s impossible to recognize. My blog at has a post on Delusions that might help you to understand what he might be going through. I wish there was something I could do, some magic bullet, that could help you help your son and ease your pain.😦 What I can recommend is talking to your son’s psychiatrist and getting a different medication because whatever he’s on isn’t working. I also recommend running your finger around inside his mouth after he says he has taken the medication to make sure he actually swallowed it. Because if he doesn’t believe that he’s delusional, maybe he doesn’t really believe he needs medication either. They kind of go together, unfortunately😦

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