Speed of processing and individual differences in IQ in schizophrenia: General or specific cognitive deficits?

Abstract

Introduction. The aim of this study was to re‐examine the role of cognitive processing speed in the vulnerability to schizophrenia, using an inspection time (IT) task that does not require a speeded manual response. Since IT consistently correlates with intelligence, the relationship between IT and general intelligence was also explored. Correspondence should be addressed to Dr J. C. Badcock, Centre for Clinical Research in Neuropsychiatry, Private Mail Bag No 1, Claremont, Western Australia 6010; e‐mail: jobad@cyllene.uwa.edu.au. The research was supported by grants from the National Health and Medical Research Council and infrastructure funding from the Department of Health of Western Australia (CPP Grant 9937102 NHMRC, HP Grant 960579 NHMRC). We would, first and foremost, like to thank the patients, community volunteers, and their families who participated in this research. The assistance of nursing and medical staff at Graylands Hospital and related clinics, and their continuing support of our research, are gratefully acknowledged. Methods. Patients with schizophrenia were compared with unaffected siblings of schizophrenia patients and healthy controls on a visual IT task. Both speed and variability of IT performance were examined. Current intellectual ability (IQ) was estimated with the Shipley Institute of Living Scale (SILS), which yields separate assessments of verbal and abstraction abilities. Results. Schizophrenia patients produced ITs that were significantly slower and more variable than either unaffected siblings or healthy controls. These results are not related to demographic or clinical variables. A significant negative correlation was obtained between IT and IQ; specifically, longer IT values were associated with abstraction scores only. Conclusions. These findings confirm that basic cognitive processing efficiency is impaired in patients with schizophrenia. In contrast, ITs in unaffected siblings did not differ from healthy controls, in line with their better IQ test scores. Implica tions for models of general and specific heritable dimensions in schizophrenia are discussed.