Efforts to identify differential or core cognitive deficits in schizophrenia have been made for several decades, with limited success. Part of the difficulty in establishing a cognitive profile in schizophrenia is the considerable inter-patient heterogeneity in the level of cognitive impairment. Thus, it may be useful to examine the presence of relative cognitive weaknesses on an intra-person level. In the present study we examined the rates of significant intra-person differences between crystallized verbal ability versus five other cognitive abilities among 127 persons with schizophrenia or schizoaffective disorder and 127 demographically matched normal comparison (NC) subjects. We found that the rates of significant discrepancies above the NC group base-rates was significantly greater in reference to those discrepancies involving visual memory relative to those associated with auditory memory, working memory, processing speed, and perceptual organization. The findings conflict with prior suggestions that working memory or auditory episodic memory are differential or core deficits in schizophrenia, and highlight the importance of considering visual memory in characterizing the cognitive effects of this condition.
Much neuropsychological research on schizophrenia over the last three decades has been motivated by the hope that discovery of specific neurocognitive profiles may elucidate the underlying neuropathology of the disorder or foster development of effective rehabilitation programs (Palmer et al., 2009). The answer to one basic question remains unclear: are there differential or “core” cognitive deficits in schizophrenia? A variety of cognitive domains have been proposed as the core deficit in schizophrenia: attention (Elvevag and Goldberg, 2000, Barr, 2001 and Hilti et al., 2008), working memory (Goldman-Rakic, 1994, Elvevag and Goldberg, 2000, Silver et al., 2003, Mitropoulou et al., 2005, Lee and Park, 2005 and Forbes et al., 2009), processing speed (Rodriguez-Sanchez et al., 2007), episodic memory (Saykin et al., 1991, Palmer et al., 1997, Aleman et al., 1999, Gur et al., 2000 and Whyte et al., 2005), and executive function (Zec, 1995 and Wobrock et al., 2008).
Meta-analyses indicate episodic memory tests have the largest or near largest effect sizes in schizophrenia patients versus normal comparison (NC) subjects (Heinrichs and Zakzanis, 1998, Fioravanti et al., 2005 and Mesholam-Gately et al., 2009), supporting the century-long focus on temporal/hippocampal and frontal–subcortical regions as prime suspects in the neurogenesis of schizophrenia (Kraepelin, 1971/1919/1913, , 1971/1919/1913, 1971/1919/1913 and Goldstein, 1939). However, it is difficult to draw definitive conclusions due to heterogeneity in effect sizes between studies (Fioravanti et al., 2005). Part of the difficulty establishing differential cognitive impairment in schizophrenia rests in the marked inter-patient heterogeneity in level of cognitive impairment. On average, schizophrenia is associated with mild-to-moderate cognitive impairment, but 20–30% of patients have normal range neurocognition (Palmer et al., 2009). Thus, it may be useful to examine the presence of relative cognitive weaknesses on an intra-person level.
The present study examined within-person cognitive differences among 127 patients with schizophrenia and 127 NC subjects. Cognition was measured with the Index scores from the 6-factor model for Wechsler Adult Intelligence Scale—Third Edition (WAIS-III)/Wechsler Memory Scale—Third Edition (WMS-III) (The Psychological Corporation, 1997 and Tulsky et al., 2003).
Crystallized knowledge is relatively unaffected by schizophrenia and many other neurocognitive disorders (Allen et al., 1998, Dickinson and Coursey, 2002 and Iverson et al., 2006), and has the strongest correlation with Full Scale IQ (TPC, 1997), so the Verbal Comprehension Index (VCI) was used as a marker of general cognitive ability. We hypothesized that VCI scores would be higher than each of the other five cognitive abilities for a significantly larger proportion of people with schizophrenia relative to the proportion among NC subjects. Given the effect sizes for episodic memory in means comparisons (Heinrichs and Zakzanis, 1998), and long-held suspicion of the frontal and temporal regions in neurogenesis of schizophrenia (Palmer et al., 2009), as well as cogent models of working memory as a core deficit underlying many facets of schizophrenia (Goldman-Rakic, 1994), we hypothesized that discrepancies with VCI would be particularly common among schizophrenia patients when evaluated in reference to episodic memory and working memory.
