General and specific cognitive deficits in schizophrenia: Goliath defeats David?

Abstract

Background: Our earlier work suggested that the cognitive performance impairment in individuals with schizophrenia relative to healthy control subjects was generalized, cutting across narrower cognitive ability dimensions. Current analyses sought to extend these findings.

Methods: Seventeen neuropsychological variables, available for 148 schizophrenia subjects and 157 control subjects, were subjected to structural equation modeling. Analyses incorporated a hierarchical model, grouping the variables into six familiar cognitive domains and linking these to a higher-order, general cognitive ability factor. We added diagnosis to the model as a grouping factor and estimated loadings from diagnosis to the general cognitive factor and, separately, to the domain factors.

Results: The fit of the final model was good (e.g., Non-Normed Fit Index [NNFI] = .988). Approximately 63.6% of the diagnosis-related variance in cognitive performance was mediated through the general factor, with smaller direct effects on verbal memory (13.8%) and processing speed (9.1%).

Conclusions: The schizophrenia cognitive deficit is largely generalized across performance domains, with small, direct effects of diagnostic group confined to selected domains. This generalized deficit sometimes has been seen as a function of the psychometric limitations of traditional cognitive test batteries. Alternatively, it may be a fundamental manifestation of schizophrenia, with similarly general neurobiological underpinnings.

Figure 1

Illustration of the hypothesized model and structural equation modeling methodology. The part of the figure indicated by the bracket labeled “a” illustrates the hierarchical measurement model. Performance on observed variables (boxes) is assumed to be driven by domain-specific latent factors. These factors are determined, in turn, by the common cognitive ability factor. The whole figure (indicated by bracket “b”) illustrates the overall structural model for current analyses. These analyses test whether the effects of diagnostic grouping (i.e., schizophrenia vs. healthy control status; represented by the unshaded box marked “Diagnosis”) on the observed cognitive variables is mediated through the common factor or whether diagnosis affects observed variables through direct effects on domain-specific factors.

Figure 2

The final model with standardized estimates for all significant parameters. Parenthesized values are the common factor-domain factor coefficients multiplied by the diagnosis-common factor coefficient (i.e., .58). The parenthesized values are thus scaled to be directly comparable with the direct diagnosis coefficients for verbal memory and processing speed. CVLT, California Verbal Learning Test; WCST, Wisconsin Card Sorting Test.