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This is: Bayes for Schizophrenics: Reasoning in Delusional Disorders , published by Scott Alexander on the LessWrong.
Related to: The Apologist and the Revolutionary, Dreams with Damaged Priors
Several years ago, I posted about V.S. Ramachandran's 1996 theory explaining anosognosia through an "apologist" and a "revolutionary".
Anosognosia, a condition in which extremely sick patients mysteriously deny their sickness, occurs during right-sided brain injury but not left-sided brain injury. It can be extraordinarily strange: for example, in one case, a woman whose left arm was paralyzed insisted she could move her left arm just fine, and when her doctor pointed out her immobile arm, she claimed that was her daughter's arm even though it was obviously attached to her own shoulder. Anosognosia can be temporarily alleviated by squirting cold water into the patient's left ear canal, after which the patient suddenly realizes her condition but later loses awareness again and reverts back to the bizarre excuses and confabulations.
Ramachandran suggested that the left brain is an "apologist", trying to justify existing theories, and the right brain is a "revolutionary" which changes existing theories when conditions warrant. If the right brain is damaged, patients are unable to change their beliefs; so when a patient's arm works fine until a right-brain stroke, the patient cannot discard the hypothesis that their arm is functional, and can only use the left brain to try to fit the facts to their belief.
In the almost twenty years since Ramachandran's theory was published, new research has kept some of the general outline while changing many of the specifics in the hopes of explaining a wider range of delusions in neurological and psychiatric patients. The newer model acknowledges the left-brain/right-brain divide, but adds some new twists based on the Mind Projection Fallacy and the brain as a Bayesian reasoner.
INTRODUCTION TO DELUSIONS
Strange as anosognosia is, it's only one of several types of delusions, which are broadly categorized into polythematic and monothematic. Patients with polythematic delusions have multiple unconnected odd ideas: for example, the famous schizophrenic game theorist John Nash believed that he was defending the Earth from alien attack, that he was the Emperor of Antarctica, and that he was the left foot of God. A patient with a monothematic delusion, on the other hand, usually only has one odd idea. Monothematic delusions vary less than polythematic ones: there are a few that are relatively common across multiple patients. For example:
In the Capgras delusion, the patient, usually a victim of brain injury but sometimes a schizophrenic, believes that one or more people close to her has been replaced by an identical imposter. For example, one male patient expressed the worry that his wife was actually someone else, who had somehow contrived to exactly copy his wife's appearance and mannerisms. This delusion sounds harmlessly hilarious, but it can get very ugly: in at least one case, a patient got so upset with the deceit that he murdered the hypothesized imposter - actually his wife.
The Fregoli delusion is the opposite: here the patient thinks that random strangers she meets are actually her friends and family members in disguise. Sometimes everyone may be the same person, who must be as masterful at quickly changing costumes as the famous Italian actor Fregoli (inspiring the condition's name).
In the Cotard delusion, the patient believes she is dead. Cotard patients will neglect personal hygiene, social relationships, and planning for the future - as the dead have no need to worry about such things. Occasionally they will be able to describe in detail the "decomposition" they believe they are undergoing.
Patients with all these types of delusions1 - as well...
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