Table 1. Neuropsychological performance of patients with major depression and bipolar disorder (manic phase) on memory tests taken from the Cambridge

Neuropsychological performance of patients with major depression and bipolar disorder (manic phase) on memory tests taken from the Cambridge Neuropsychological Test Automated Battery (CANTAB)

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Fig. 1

Performance of patients with mania (triangles), depression (circles) and control subjects (squares) as a function of difficulty level on the one-touch Tower of London task. The dependent measures shown are (a) mean percentage of problems solved correctly by first response and (b) mean latency to first response. Data for patients with mania and depression are taken from Murphy et al (1999) and Elliott et al (1996), respectively.

The cognitive impairments observed in both groups of patients in these studies were interpreted as evidence for relatively global neuropsychological dysfunction (Elliott et al, 1996; Murphy et al, 1999). The deficits observed in patients with mania and depression when tested on object recognition memory were comparable to those previously reported in patients with posterior dysfunction, such as temporal lobe lesions (Owen et al, 1995a) or mild Alzheimer's dementia (Sahakian et al, 1988). The deficits seen on tests of spatial recognition memory and planning ability, however, were similar to those in patients with frontal dysfunction (Owen et al, 1995b) or basal ganglia disorders such as Parkinson's disease (Owen et al, 1995b), in which there is disrupted functioning of frontostriatal ‘loops’ (Alexander et al, 1986). At first glance, these findings suggest that patients with mania and depression are similarly impaired on a range of cognitive tasks subserved by different neural regions, and that a single common underlying mechanism may account for the noted deficits in both groups. Investigators of depression have suggested that the pervasive deficits observed could be due to reduced motivation (Miller, 1975; Seligman, 1975; Richards & Ruff, 1989), a conservative response style (Johnson & Magaro, 1987; Williams et al, 1997), diminished cognitive capacity and processing resources (Hasher & Zacks, 1979), or a narrowing of attentional focus to depression-relevant or task-irrelevant thoughts (Ellis & Ashbrook, 1988). To date, few investigators have considered mania-related deficits within these or similar frameworks.

The bulk of research suggests that in both mania and depression, patients are impaired on a range of cognitive tasks subserved by different neural regions. In addition, although the few studies that actually compare mania and depression employ a limited range of tasks, it appears that conventional neuropsychological tests of attention, memory and executive function are unable to discriminate between patients with mania and depression. Together, these findings suggest that global pathological change, rather than factors unique to either disorder, may account for the observed deficits, and that similar processes may be involved despite markedly different clinical presentations.