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