Hierarchical structure of the cognitive processes in schizophrenia: the fundamental role of processing speed

Objective: Decreased processing speed (PS) is a key feature of schizophrenia with respect to cognition, functional outcome and clinical symptoms. Our objective was to test whether PS slowing mediates other neuropsychological deficits among patients with chronic schizophrenia.

Method: One hundred patients with schizophrenia and 53 healthy adults completed a series of neuropsychological measures that assess six cognitive domains. In addition to PS these included attention, verbal memory, visual memory, working memory, and executive functioning. Confirmatory factor analysis (CFA) was used to evaluate the fit of the 6-factor model. The cognitive performances of both groups were compared before and after controlling for the effect of PS, but also after controlling for the effect of each cognitive factor at a time. Finally, the PS-related variance was removed and the effect of the other cognitive factors was tested again.

Results: CFA supported the hypothesized 6-factor cognitive structure. As expected, the patients and controls differed on all cognitive measures. However, after controlling for the effects of PS, group differences on the other five cognitive factors decreased substantially. Controlling for other factors produced smaller attenuation of group differences, and these effects were also partially accounted for by decreased PS.

Conclusions: PS deficits account for most of the differences in cognition between patients with schizophrenia and healthy controls. PS slowing appears to be a core feature of schizophrenia, one that underlies impairments of working memory, executive functioning, and other abilities.