Intelligence quotient and neuropsychological profiles in patients with schizophrenia and in normal volunteers

Background: The objective of this study was to examine neuropsychological performance at different intelligence quotient (IQ) levels in schizophrenia. Methods: Thirty-six patients with schizophrenia were matched with 36 normal control subjects in two IQ groups: low average (81-94) and average (95-119). Performance level (IQ group main effects) and profile shape (IQ group x function interactions) were compared. Results: Current IQ was lower than estimated premorbid intellectual ability in both patient groups. Patients also displayed poorer neuropsychological function than same-IQ control subjects, suggesting neuropsychological dysfunction beyond their already compromised IQ. Patients had different profile shapes than control subjects, but profile shapes were consistent within patients and control subjects at each IQ level. Patients at both levels had higher verbal and lower performance IQ than control subjects. Abstraction-executive function was one of the lowest neuropsychological scores in both patient groups. Average IQ patients had nonsignificantly better overall neuropsychological performance than low average control subjects, but the effect size (.43) was quite small relative to the IQ difference (effect size = 2.57). Conclusions: Neuropsychological patterns in schizophrenia tend to be consistent at different IQ levels. Even schizophrenia patients with normal current IQs manifest substantial neuropsychological compromise relative to their level of general intellectual ability. The results strengthen the argument that neurocognitive deficits are core deficits of schizophrenic illness. Biol Psychiatry 2001;50:453-462 (C) 2001 Society of Biological Psychiatry.