Executive functioning in schizophrenia: Unique and shared variance with measures of fluid intelligence.
OBJECTIVE
Patients with schizophrenia often display deficits on tasks thought to measure "executive" processes. Recently, it has been suggested that reductions in fluid intelligence test performance entirely explain deficits reported for patients with focal frontal lesions on classical executive tasks. For patients with schizophrenia, it is unclear whether deficits on executive tasks are entirely accountable by fluid intelligence and representative of a common general process or best accounted for by distinct contributions to the cognitive profile of schizophrenia.


METHODS
In the current study, 50 patients with schizophrenia and 50 age, sex and premorbid intelligence matched controls were assessed using a broad neuropsychological battery, including tasks considered sensitive to executive abilities, namely the Hayling Sentence Completion Test (HSCT), word fluency, Stroop test, digit-span backwards, and spatial working memory. Fluid intelligence was measured using both the Matrix reasoning subtest from the Weschler Abbreviated Scale of Intelligence (WASI) and a composite score derived from a number of cognitive tests.


RESULTS
Patients with schizophrenia were impaired on all cognitive measures compared with controls, except smell identification and the optimal betting and risk-taking measures from the Cambridge Gambling Task. After introducing fluid intelligence as a covariate, significant differences remained for HSCT suppression errors, and classical executive function tests such as the Stroop test and semantic/phonemic word fluency, regardless of which fluid intelligence measure was included.


CONCLUSIONS
Fluid intelligence does not entirely explain impaired performance on all tests considered as reflecting "executive" processes. For schizophrenia, these measures should remain part of a comprehensive neuropsychological assessment alongside a measure of fluid intelligence.
University of Queensland, Queensland Brain Institute, Australia. Electronic address: a.martin11@uq.edu.au.
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Martin
University of Queensland, Queensland Brain Institute, Australia; University of Queensland, Queensland Centre for Mental Health Research, Australia.
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Mowry
University of Queensland, Centre for Advanced Imaging, Australia.
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Reutens
University of Queensland, School of Psychology, Australia.
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Robinson
D000328:Adult; D003072:Cognition Disorders / Q000175:diagnosis* / Q000503:physiopathology / Q000523:psychology; D056344:Executive Function / Q000502:physiology; D005260:Female; D005625:Frontal Lobe / Q000503:physiopathology; D006801:Humans; D007360:Intelligence; D008297:Male; D008875:Middle Aged; D009483:Neuropsychological Tests / Q000706:statistics & numerical data; D011594:Psychometrics; D012016:Reference Values; D012559:Schizophrenia / Q000175:diagnosis* / Q000503:physiopathology; D012565:Schizophrenic Psychology
Cognition; Fluid intelligence; Frontal lobes; Hayling; Schizophrenia; Stroop; Word fluency