Schizophrenia Bulletin Advance Access originally published online on August 16, 2007
Schizophrenia Bulletin 2008 34(3):423-434; doi:10.1093/schbul/sbm092
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Published by Oxford University Press 2007.
Less Unique Variance Than Meets the Eye: Overlap Among Traditional Neuropsychological Dimensions in Schizophrenia
2 VISN 5 Mental Illness Research, Education and Clinical Center, Baltimore, MD
3 Department of Psychiatry, University of Maryland School of Medicine, Baltimore, MD
1 To whom correspondence should be addressed; VISN 5 Mental Illness Research, Education and Clinical Center, 10 North Greene Street, Suite 6a (BT/MIRECC), Baltimore, MD 21201; tel: 410-605-7817, fax: 410-605-7739, e-mail: Dwight.Dickinson{at}va.gov.
The magnitude of the overlap among dimensions of neuropsychological test performance in schizophrenia has been the subject of perennial controversy. This issue has taken on renewed importance with the recent focus on cognition as a treatment target in schizophrenia. A substantial body of factor analytic literature indicates that dimensions are separable in schizophrenia. However, this literature is generally uninformative as to whether the separable dimensions are independent, weakly correlated, or strongly correlated. Factor analyses have often used methods (ie, principal components analysis with orthogonal rotation) that preclude this determination, and correlations among factor-based domain composites and underlying measures have been reported infrequently in these studies. Current meta-analyses of reported "between-dimension" correlations for individual neuropsychological measures and for cognitive domain composite variables indicate that cognition variables in schizophrenia are correlated, on average, at a "medium" level of r = 0.37 for individual measures from different cognitive dimensions and r = 0.45 for domain composites. Because these are mean bivariate correlations, the multiple correlation of an individual measure with all the other measures in a cognitive battery is likely to be higher. Measure reliabilities of 0.80 or less also imply greater commonality among traditional neuropsychological measures. In short, there are underappreciated constraints on the amount of reliable cognitive performance variance in traditional neuropsychological test batteries that is free to vary independently. The ability of such batteries to reveal cognitive domain–specific treatment effects in schizophrenia may be much more limited than is generally assumed.
Keywords: schizophrenia / cognition / correlation / neuropsychological assessment / clinical trials
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