Elsevier

Biological Psychiatry

Volume 50, Issue 6, 15 September 2001, Pages 453-462
Biological Psychiatry

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

https://doi.org/10.1016/S0006-3223(01)01099-XGet rights and content

Abstract

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 × 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.

Introduction

Intelligence quotient (IQ) as a measure of general intellectual ability has been extensively studied in schizophrenia. Low IQ is associated with increased risk for schizophrenia (Aylward et al 1984; Crow et al 1995, Davidson et al 1999, Erlenmeyer-Kimling et al 1991, Jones et al 1994, Kremen et al 1998, Russell et al 1997). Comparisons of estimated premorbid ability with current IQ are consistent with a postmorbid decline in general intellectual functioning in schizophrenia, even in individuals with average IQs Dalby and Williams 1986, Goldman et al 1999, Kremen et al 1996, Kremen et al 2000a.

High IQ can be a protective factor in that it is likely to be associated with better overall level of functioning and with better outcomes in schizophrenia (Seidman et al 1992). Some evidence indicates that paranoid subtype patients tend to have higher IQs than do nonparanoid patients Magaro and Page 1983, Seidman 1983, Seidman et al 1992; however, a recent review concluded that there was no difference (Zalewski et al 1998). In addition, higher verbal than performance IQ is the predominant pattern in schizophrenia (Heaton and Drexler 1987). In the general population, it is common for there to be higher verbal than performance IQ at the higher IQ levels, and the opposite pattern at low IQ levels (Matarazzo and Herman 1984).

Despite the importance of IQ as an index of general intellectual ability, little is known about how patterns of neuropsychological function may differ at different levels of IQ in schizophrenia. Although IQ may be a protective factor, individuals of high intellectual ability can still develop schizophrenia. In a large patient sample, a subset of which comprise the present sample, we found that estimated IQ was positively correlated with Wisconsin Card Sorting Test performance, particularly the perseveration factor (Koren et al 1998). Weickert et al (2000) observed that a subgroup of chronic schizophrenia patients had average current and estimated premorbid IQ but still manifested some cognitive deficits, primarily in executive and attentional function; however, preliminary results by Vinogradov et al (2000) suggested preserved aspects of executive-attentional and motor function in high-IQ schizophrenia patients.

Identifying commonalities across different IQ bands might elucidate neuropsychological abnormalities that are prototypical of schizophrenia. In particular, if certain deficits are present even in schizophrenia patients with average or above-average IQs, it would strengthen the argument that those types of neuropsychological dysfunction are universal features of schizophrenic illness (i.e., that they are core deficits). In the present study, our goal was to specifically examine neuropsychological differences in schizophrenia patients as a function of IQ via creation of IQ subgroups. Our group has focused on subgrouping patients with schizophrenia as a way of elucidating the extensive heterogeneity of neuropsychological performance (Seidman 1990). Different approaches provide different ways of thinking about the data and may provide different insights into schizophrenic illness. We have utilized classification of individual neuropsychological profile types Kremen et al 2000a, Kremen et al 2000b as well as comparisons of groups subdivided according to paranoid-nonparanoid subtype (Kremen et al 1994), age (Fucetola et al 2000), and gender (Goldstein et al 1998). Subgrouping by IQ represents another strategy that provides a way of examining neuropsychological function as it relates to different levels of competence.

We grouped schizophrenia patients and control subjects into two IQ levels and examined their neuropsychological profiles to address the following issues: 1) Do IQ-matched patients and control subjects differ in level of neuropsychological performance? 2) Are the profile shapes (i.e., pattern of neuropsychological strengths and weaknesses) the same or different at different levels of IQ, and if so, how do the patterns compare for patients and control subjects? 3) Do IQ-matched patients and control subjects differ in terms of verbal versus performance IQ?

Section snippets

Methods and materials

Methods are summarized below and have been described in detail elsewhere Faraone et al 1995, Kremen et al 1995, Kremen et al 2000a, Kremen et al 2000b.

Demographic characteristics

There were no significant patient-control differences in age, education, or ethnicity at each IQ level, although low average patients were nearly significantly older than low average control subjects (p < .06). Despite some dissimilarities across groups, gender ratios were not significantly different. Parental education tended to be higher in schizophrenia patients at both IQ levels, although the difference was significant in the low average IQ group only [t(29) = −2.06, p < .05]. It should be

Neurocognitive performance in patients versus control subjects

In almost all schizophrenia-control studies of neuropsychological function, IQ is lower in patients given efforts to equate groups on premorbid ability (cf. Kremen et al 1996). In contrast, we equated patient and control IQs in the present study to address a different set of issues. At each IQ level, patients had significantly poorer overall neuropsychological performance compared with control subjects. Results were similar for average IQ patients in the study of Weickert et al (2000).

Conclusion

Regardless of IQ level, patients had significantly poorer overall neuropsychological performance than their control counterparts. Patients at each level had different neuropsychological profile shapes than their control counterparts, but profile shapes were consistent within the patient and control groups, respectively. At each IQ level, patients tended have higher verbal IQ and lower performance IQ than control subjects.

Abstraction-executive function was among the lowest scores in both patient

Acknowledgements

Preparation of this article was supported in part by National Institute of Mental Health Grants MH43518-01 (MERIT Award) and MH46318, the Veteran’s Affairs Medical Research and Health Services Research and Development Programs, and a Distinguished Investigator Award from NARSAD to Dr. Tsuang, NARSAD grants to Drs. Kremen and Seidman, and a Stanley Foundation grant to Dr. Seidman.

An earlier version of this article was presented at the annual meeting of the Society for Research in

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