title: The puzzle of processing speed, memory, and executive function impairments in schizophrenia: fitting the pieces together.
abstract: BACKGROUND
  Substantial impairment in performance on the digit-symbol substitution task in patients with schizophrenia is well established, which has been widely interpreted as denoting a specific impairment in processing speed. However, other higher order cognitive functions might be more critical to performance on this task. To date, this idea has not been rigorously investigated in patients with schizophrenia.
  
  
  METHODS
  Neuropsychological measures of processing speed, memory, and executive functioning were completed by 125 patients with schizophrenia and 272 control subjects. We implemented a series of confirmatory factor and structural regression modeling to build an integrated model of processing speed, memory, and executive function with which to deconstruct the digit-symbol substitution task and characterize discrepancies between patients with schizophrenia and control subjects.
  
  
  RESULTS
  The overall structure of the processing speed, memory, and executive function model was the same across groups (χ(2) = 208.86, p > .05), but the contribution of the specific cognitive domains to coding task performance differed significantly. When completing the task, control subjects relied on executive function and, indirectly, on working memory ability, whereas patients with schizophrenia used an alternative set of cognitive operations whereby they relied on the same processes required to complete verbal fluency tasks.
  
  
  CONCLUSIONS
  Successful coding task performance relies predominantly on executive function, rather than processing speed or memory. Patients with schizophrenia perform poorly on this task because of an apparent lack of appropriate executive function input; they rely instead on an alternative cognitive pathway.
authors: Department of Psychiatry, Yale University School of Medicine, New Haven; Olin Neuropsychiatric Research Center, Institute of Living, Hartford Hospital, Hartford, Connecticut. Electronic address: emma.knowles@yale.edu.
authors: Emma E M
authors: EE
authors: Knowles
authors: Department of Psychiatry, Sheba Medical Centre, Ramat-Gan, Israel.
authors: Mark
authors: M
authors: Weiser
authors: Department of Psychosis Studies, Institute of Psychiatry, King's College London, London, United Kingdom.
authors: Anthony S
authors: AS
authors: David
authors: Department of Psychiatry, Yale University School of Medicine, New Haven; Olin Neuropsychiatric Research Center, Institute of Living, Hartford Hospital, Hartford, Connecticut.
authors: David C
authors: DC
authors: Glahn
authors: Department of Psychiatry, Sheba Medical Centre, Ramat-Gan, Israel.
authors: Michael
authors: M
authors: Davidson
authors: Departments of Psychiatry and Preventive Medicine and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, New York.
authors: Abraham
authors: A
authors: Reichenberg
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D003072:Cognition Disorders / Q000209:etiology*; D056344:Executive Function / Q000502:physiology*; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D008569:Memory Disorders / Q000209:etiology*; D008875:Middle Aged; D009483:Neuropsychological Tests; D011569:Psychiatric Status Rating Scales; D011596:Psychomotor Disorders / Q000209:etiology*; D012559:Schizophrenia / Q000150:complications*; D012565:Schizophrenic Psychology; D055815:Young Adult
keywords: Digit-symbol substitution; Information processing; Neuropsychology; Processing speed; Schizophrenia; Structural equation modeling