Association of symptoms and executive function in schizophrenia and bipolar disorder

Abstract

The extent to which cognitive impairment in psychosis is related to the particular disorder or the pattern of symptoms is unclear. We examined executive function in two groups of schizophrenia patients with predominant symptoms of disorganisation (n=15) and psychomotor poverty (n=15), respectively, two groups of bipolar I disorder patients with predominant symptoms of mania (n=15) and depression (n=15), respectively, and 30 healthy controls. We predicted that the pattern of symptoms (‘excess’ [disorganisation/mania] or ‘deficiency’ [negative symptoms/depression]) would be more related to executive ability than the underlying disorder. The patient groups showed partially overlapping executive dysfunctions relative to the control group. There were no significant differences between groups with ‘excess’ symptoms (schizophrenia patients with thought disorder and bipolar patients with mania), or between groups with ‘deficiency’ symptoms (schizophrenia patients with negative symptoms and bipolar patients with depression). In contrast, differences were noted between groups with the same diagnosis: Schizophrenia patients with disorganisation were less accurate in semantic verbal fluency than those with negative symptoms; and bipolar patients with mania tended to be faster, but less accurate, in sentence completion than those with depression. A statistical comparison of the associations of ‘diagnosis’ and the ‘excess-deficiency’ dimension with executive function revealed a trend for a greater association of the latter with two measures of performance accuracy. Executive dysfunction in patients with psychotic disorders may be more related to their symptom profile than their diagnosis.

Introduction

Schizophrenia and bipolar disorder show overlapping cognitive deficits (Albus et al., 1996, Zihl et al., 1998, Martínez-Arán et al., 2002, Seidman et al., 2002), but these may be more severe in schizophrenia (Mojtabai et al., 2000, Verdoux and Liraud, 2000, Liu et al., 2002). As the two disorders also overlap in clinical phenomenology (Van Os et al., 1999), it is unclear whether the respective deficits are related to the particular disorder or the type of symptoms.

Factor analytic studies have identified three symptom clusters in schizophrenia: Disorganisation (thought disorder), which involves illogical thinking, incoherent speech and bizarre behaviour; reality distortion, characterised by delusions and hallucinations; and psychomotor poverty (negative symptoms), which involves motor retardation, emotional and volitional blunting, and reduced speech output (Andreasen and Olsen, 1982, Liddle, 1987, Liddle et al., 1989, Liddle and Morris, 1991, Eaton et al., 1995). These factors extend the concept of positive and negative schizophrenia (equivalent to disorganisation/reality distortion and psychomotor poverty, respectively) (Crow, 1980, Andreasen and Olsen, 1982), which originated from writings in the 19th century. For example, Hughling Jackson referred to the dual nature of ‘insanity’, distinguishing a positive element of ‘irritable excesses of action’, and a negative element of ‘a deficiency or paralysis of mental phenomena’ (Taylor, 1932).

Disorganisation, negative symptoms and reality distortion have been associated with altered perfusion in different loci of the prefrontal and medial temporal lobes, and different cognitive profiles (Liddle and Morris, 1991, Liddle et al., 1992, Allen et al., 1993, Johnstone and Frith, 1996, Berman et al., 1997, O'Leary et al., 2000). Interestingly, an association of psychomotor slowing with decreased activity in the left dorsolateral prefrontal cortex has been demonstrated in both schizophrenia and depression (Dolan et al., 1993). These findings suggest that some functional disturbances in schizophrenia and bipolar disorder may be more related to the pattern of symptoms than to diagnosis.

Despite a replication of the positive–negative dichotomy in bipolar disorder (Toomey et al., 1998), no study has applied the distinction between behavioural signs that are abnormal by their presence and those that represent a loss of normal function to a comparison of cognitive ability in schizophrenia and bipolar illness. Such comparison would be interesting: Disordered thought and negative symptoms have not only shown dissociable correlations with cognitive deficits in schizophrenia, but are also common in bipolar patients with mania and depression, respectively (Andreasen, 1979, Martínez-Arán et al., 2000).

The present study compared the associations of diagnosis and symptom pattern with executive function in schizophrenia patients with disorganisation or negative symptoms, and bipolar disorder patients with either of their putative symptom analogues: Mania and depression. Executive function, primarily sub-served by frontal lobe regions (Andreasen et al., 1996), refers to the selection, integration and monitoring of cognitive and behavioural processes (Royall et al., 1993), and formed our focus for several reasons: Firstly, hypofrontality has been consistently reported in both schizophrenia and bipolar disorder (Bearden et al., 2001). Secondly, executive dysfunctions may be common trait vulnerability markers in the two disorders (Zalla et al., 2004). Thirdly, frontal lobe lesions have been associated both with a lack of responses and a failure to suppress inappropriate ones (Shallice, 1988), deficits which were central to our hypotheses: We predicted that schizophrenia patients with disorganisation and bipolar patients with mania would show increased error intrusion compared to healthy controls and patients with negative symptoms or depression. We further predicted that schizophrenia patients with negative symptoms and bipolar patients with depression would show reduced verbal output and prolonged response times. In the remainder of this paper, for simplicity, we will adopt Jackson's descriptive terms of ‘excess’ symptoms to refer to disorganisation and mania, and ‘deficiency’ symptoms to refer to negative symptoms and depression.