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


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. Implications for models of general and specific heritable dimensions in schizophrenia are discussed.
Badcock
Johanna C
JC
School of Psychiatry and Clinical Neurosciences, University of Western Australia, Perth, Australia. jobad@cyllene.uwa.edu.au
Williams
Rachael J
RJ
Anderson
Mike
M
Jablensky
Assen
A