The Definitions and Implications of Cognitive "Normality" in Schizophrenia
Parlar, Melissa Emily
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Cognitive dysfunction is widely considered to be a core feature of schizophrenia, yet subpopulations that are cognitively normal have been repeatedly identified. There is no agreed upon definition of cognitive normality, with prevalence rates ranging from 0% to 55%, and the clinical and functional implications are unclear. Interpretation of cognitive normality is further complicated by the possibility that normal-range patients have declined relative to premorbid ability. The purpose of this dissertation was to provide a deeper understanding of cognitive normality and its clinical and functional implications. Study 1 assessed the prevalence and clinical and functional validity of two definitions of cognitive normality: a narrow (i.e., IQ) and a broad (i.e., MCCB) definition. Participants included 99 patients with schizophrenia or schizoaffective disorder and 80 healthy controls. Prevalence rates of cognitive normality ranged from 14% to 48%. Regardless of the criterion applied, cognitively normal patients were functionally disadvantaged relative to cognitively normal controls. They demonstrated no advantage in functionality or clinical symptom severity relative to cognitively impaired patients. Study 2 evaluated a cognitive decrement algorithm (Keefe, Eesley, & Poe, 2005) and its clinical and functional validity among 156 patients and 74 controls. Patients were classified as having average range cognition with a decrement (37.1%), average-range without a decrement (9.3%), and above-average range without a decrement (12.1%). The combination of above average-range cognition and lack of a cognitive decrement conferred a functional advantage among patients, but they remained functionally disadvantaged relative to controls. Study 3 assessed the utility of the functionally relevant Breakfast Task (Craik & Bialystok, 2006) in predicting disability relative to standard measures of cognition and symptom severity among 30 patients and 37 controls. Breakfast Task performance increased the ability to predict disability beyond that provided by cognitive and symptom measures. Results suggest that the prevalence of cognitive normality (9.3% to 48%) is a byproduct of the definition used. There was insufficient evidence supporting the functional or clinical validity of any of the definitions of cognitive normality, suggesting that the role of cognition in functioning has been largely over-emphasized in the literature. Ecologically valid tools, however, may hold promise for disability assessment.