A major issue facing the developers of the DSM-V and a topic of considerable debate in the literature is whether individuals with schizophrenia (SCZ) or psychotic mood disorders (PMD) are separate and distinct disorders, or whether they occur on a “psychosis” continuum (e.g., Allardyce et al., 2007). Some experts define this continuum as a spectrum of psychotic disorders with mood disorders, such as bipolar disorder with psychotic features (BDP), on one pole and schizophrenia (SCZ) on the other pole with schizoaffective disorder (SA) in the middle (e.g., Lake and Hurwitz, 2007 and Kempf et al., 2005). Support for this view is based on reports of increased genetic risk for the continuum of disorders among first-degree relatives of SCZ, SA, and BDP. For instance, when compared to controls, the risk for bipolar disorder is higher in the relatives of SCZ and SA, the risk of schizophrenia is higher in the relatives of BDP and SA, and the risk of schizoaffective disorder is higher in the relatives of BDP and SCZ (Valles et al., 2000, Kendler et al., 1998, Rice et al., 1987 and Tsuang et al., 1980). Linkage studies also suggest that the genes related to the risk of developing SCZ, SA, and BDP may be similar (e.g., Potash, 2006 and Hamshere et al., 2005).
The question of whether PMD, such as SA or BDP, are on the same continuum as schizophrenia has also been addressed by examining the severity of neurocognitive deficits and clinical symptoms across the disorders. Prior research has found several similarities between SCZ and PMD. For instance SA and SCZ had similar deficits in neurocognitive performance on individual tasks measuring working memory, executive functioning, and intelligence (e.g., Gooding and Tallent, 2002 and Reichenberg et al., 2002). Similarly, SCZ and BDP had similar performance deficits on working memory tasks (Glahn et al., 2006). Also, although Heinrichs and colleagues (2008) found differences between SCZ and SA on specific neurocognitive tests, they concluded that these differences were of insufficient magnitudes to validate two distinguishable disorders. However, few studies compared neurocognitive domain scores between SCZ and PMD (Evans et al., 1999).Additional research is needed to compare the performance of PMD to SCZ with respect to neurocognitive domains, which may be more reliable and robust than scores from individual tests.
Studies comparing the psychopathology of SCZ to PMD indicated that the clinical boundaries between these disorders remain unclear. For instance, some studies found that SCZ, SA, and BDP had similar levels of negative symptoms (Evans et al., 1999 and Cuesta and Peralta, 1995), while others did not share this finding (Peralta and Cuesta, 2008 and Kendler et al., 1995). The extent of positive and disorganized symptoms among SCZ, SA, and BDP also remains unclear. For instance, Peralta and Cuesta (2008) found that measures of psychosis and disorganization in SA were intermediate between SCZ and BDP, while Evans and colleagues (1999) reported that SCZ scored higher than PMD on measures of positive symptoms. Others found them to have similar ratings on positive (Cuesta and Peralta, 1995 and Kendler et al., 1995) and disorganized symptoms (Cuesta & Peralta, 1995). It is possible that these findings were mixed given that few groups used the same methods to assess psychopathology. Thus, our study will use standardized measures to assess psychopathology in SCZ and PMD.
Although existing research has examined the relationships between neurocognitive deficits and psychopathology within SCZ, research has yet to examine how these relationships might vary across different psychotic disorders. Prior research suggests that the domains of psychopathology in SCZ show various relationships to neurocognitive function (e.g., Basso et al., 1998). For example, research on SCZ suggests that positive symptoms show little relationship to neurocognitive function (e.g., Nieuwenstein et al., 2001), while disorganized symptoms have been consistently related to impairments in working memory, executive function and episodic memory (e.g., Bozikas et al., 2004 and Cameron et al., 2002). Findings for negative symptoms are mixed, but a number of studies suggest a relationship between negative symptoms and deficits in working memory, executive function and episodic memory (e.g., Nieuwenstein et al., 2001 and Palmer and Heaton, 2000). Understanding whether psychopathology and neurocognitive deficits are similarly related among SCZ and PMD will inform theories of diagnosis, psychopharmacologic treatment, and illness pathophysiology. Thus, it is necessary to examine whether the pattern of relationships between neurocognitive deficits and psychopathology is similar between SCZ and PMD.
In the present study, we examined whether individuals with schizophrenia differ from individuals with psychotic mood disorders with respect to [1] the type and severity of neurocognitive deficits, [2] level of psychopathology, and [3] the relationship between neurocognitive function and psychopathology.
