Objective: Hypotheses to explain delusion formation include distorted perceptual processing of meaningful stimuli (e.g. faces), abnormal reasoning, or a combination of both. The study investigated these hypotheses using standardized neuropsychological tests.
Design: A three-patient case-study, compared with a small group (n = 8) of age-matched normal control subjects.
Setting: Hospital in- and outpatients. Age-matched normal controls were from local residential homes.
Patients: Three subjects with late-onset schizophrenia, two currently deluded and one in remission. Both deluded subjects had persecutory beliefs. One had a delusion of misidentification.
Interventions: All subjects were administered standardized neuropsychological tests of facial processing and tests of verbal reasoning.
Main outcome measures: The test scores of the three patients were compared with published normal values and the age-matched control data.
Results: The tests demonstrated impaired matching of unfamiliar faces in deluded subjects, particularly in the subject with delusional misidentification. Increasing the emotional content of logical reasoning problems had a significant effect on the deluded subjects' reasoning but not that of the normal controls.
Conclusion: The findings suggest impaired visual processing plus abnormal reasoning in deluded subjects. However, these impairments are relatively subtle given the severity of psychiatric disorder in the patients studied.