Cognitive impairment is a core feature of schizophrenia which is associated with the functional disabilities that have proven resistant to treatment. Similar to our previous findings (9
), with a smaller, but overlapping sample, this study reveals complex relationships between cognitive abilities and functional outcomes. By modeling different behaviors (work skills, interpersonal, community activities), the same cognitive skill may relate to performance directly or through its relationship with living skills and/or social competence, which similarly have specific relationships depending on the outcome domain. Certain cognitive abilities appear important for the acquisition of social or living skills, while others may also be important for deployment of these skills in real time in the real world. Verbal memory and executive functions appear important for functional competence, but not necessarily for social skills. Processing speed, as well as attention and working memory are related to both social and functional competence. Moreover, processing speed predicts work skill, interpersonal functions, and community behaviors independently of its relationship with everyday living skills. Attention/working memory and executive skills also have direct effects on work and interpersonal behavior, respectively. The strength and independence with which each NP domain contributes to outcome depend on which of the outcome domains is assessed.
These direct and mediating effects have real world implications. For example, it is interesting to note that executive functioning’s effect on community activities and work skills is mediated by living skills, as assessed by the UPSA, but executive functions directly relate to real world interpersonal behavior. Executive functions encompass reasoning, problem solving, and mental flexibility; thus, it logically follows that they would be important for guiding behavior during dynamic interpersonal interactions.
In contrast to recent work using confirmatory factor analyses that suggested schizophrenia is characterized by a unitary cognitive impairment (28
), we found evidence for distinct cognitive domains using principal axis factoring, similar to other work using confirmatory path analysis (30
) and empirical literature review (31
). The divergence between these findings may reflect differences in samples, NP assessment methods, or other issues. It is important to keep in mind that NP instruments produce variables that have overlapping features; our discussion of distinct cognitive domains does not presume, for example, that attention and processing speed are not critical to performance on tasks of verbal memory or executive skills. More importantly, the present results suggest that it is critical to examine individual cognitive skills because these domains relate to real world functional behaviors in different ways and thus might represent separable treatment targets.
Our findings extend the argument for considering processing speed impairments as a core cognitive deficit in schizophrenia. From a theoretical perspective, processing speed and its cognitive components underlie performance on other cognitive domains and is likely to be essential learning and executive functions. A recent meta-analytic study by Dickinson et al (32
) found that the digit symbol task, a measure of processing speed, represented the greatest deficit among cognitive abilities in individuals with schizophrenia and in their unaffected relatives, making it a candidate endophenotype. The WAIS technical manual (33
) describes the processing speed index as the most sensitive in schizophrenia patients. In the CATIE baseline cognition analyses (28
), processing speed, indexed by the digit symbol task, accounted for substantial variance (~65%) in overall cognitive performance and was the best single predictor of the total score. In the present study, the processing speed factor, which included the digit symbol task, consistently predicted social competence and living skills and was the only factor to have a direct effect on all three real world behaviors. This finding is not simply a measurement artifact, as the severity of impairment on the processing speed measures was comparable to that observed on variables in the other NP domains. Thus, processing speed is a core cognitive feature, underlies performance on other cognitive domains, and is essential for skill acquisition and for the deployment of those skills in everyday functional performance. Taken together, these findings support the study of remediating processing speed in schizophrenia patients as a means for improving skill acquisition and outcomes.
An explosion of pharmacological and behavioral treatments have been proposed in the past 10 years to treat cognitive dysfunction in schizophrenia, culminating in the MATRICS initiative, which sought to bring together academia, the pharmacological industry, and the Food and Drug Administration in an effort to streamline approval for treatments of cognitive dysfunction. The impetus for this work is the well-replicated finding of substantial zero-order correlations between cognition and functional outcomes, which provided a target, in cognitive dysfunction, for promoting functional recovery in schizophrenia. The present findings, making use of path analysis to model intercorrelations between and within symptom, cognitive and functional domains, suggest that cognitive remediation might be viewed as an initial step in promoting functional recovery. Cognition was consistently associated with competence in living skills and social functioning, but its relationship with actual performance in the real world was largely mediated by these latter abilities. Although cognition is correlated with functional and social abilities, skill acquisition in these domains might not directly mirror cognitive gains, as negative symptoms also affect social skills and the cognitive factors leave much of the variance in the two competence measures unexplained. Further, both traditional symptoms of schizophrenia and mood disturbance affect real world performance outcomes independent of the influences of these competence measures. To reduce disability and foster functional recovery, it will be important to view cognitive remediation and enhancement as a platform upon which skills could be more easily taught and later generalized when facing dynamic social and occupational demands. Since negative and depressive symptoms might be rate limiting factors even with cognitive and functional skill attainment, it is critical to assess and treat these symptom domains. An encouraging recent study (34
) suggests that even modest gains in cognitive performance with second generation antipsychotic treatment account for significant improvements in performance-based social skills, but also that social competence was more responsive to treatment than cognitive performance. It will be interesting to see whether longer-term trials incorporating broad efforts to remediate cognitive dysfunction, cultivate and encourage deployment of skills, and reduce negative and depressive symptoms lead to reduction in disability.
Addressing the limitations of the present study would provide several avenues for future research. These baseline findings are part of a longitudinal study, which will be better able to address the issue of causality in examining the interrelationships between cognition, symptoms, skills, and outcomes. Cross-validation of these findings will be important, particularly because we did not make specific a priori hypotheses regarding the interrelationships. We did not use three domains of the SLOF due to ceiling effects. This sample of stabilized community living patients did not display deficits in the areas of physical functioning, self-care, or socially acceptable behavior. Older, more chronically psychotic patients tend to have poorer basic self-care, while acutely psychotic patients might be examined to find predictors of socially unacceptable behavior. Indeed, the heterogeneity in illness presentation and in degree of disability in schizophrenia might suggest that different patterns of interrelationships would be found in other samples. It will also be interesting to develop novel methods for examining real world behaviors to assess whether they are interrelated or hierarchical. Since the rates of employment in schizophrenia in general and this older sample in specific are very low, we examined skills that are important for vocational success, rather than occupational status or performance during work as an outcome. Future studies, particularly with younger samples, can make use of instruments, such as the COMPASS system (35
) that assess work readiness and performance. Prediction of actual vocational performance is increasingly important as multiple investigators have reported behavioral treatments that improve vocational success (36
) Several external factors, not the subject of this study, are also likely to affect models of functional outcome, particularly at the post-competence level. For example, social stigma, lack of social support, and financial resources might be barriers between skill competence and real world performance (39
). Recent reports of the mediating effects of social cognition (40
) and personality (41
) are clearly important to evaluate in subsequent studies.