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Cognitive impairment in depression




Until fairly recently it was thought that even severe forms of depression were associated with only minor impairments in cognitive function. An important and comprehensive review by Miller (1975) challenged this belief by suggesting that both mild and severe forms of depression are associated with pronounced deficits on cognitive, motor, perceptual and communication tasks. Since then, many studies have demonstrated the presence of wide-ranging neuropsychological deficits in patients with depression (Weingartner et al, 1981; Brown et al, 1994; Beats et al, 1996; Elliott et al, 1996), with current investigation focusing on the relationship of these now established deficits to clinical and neurobiological dimensions of the disorder.

Although patients with depression have been studied using a wide range of neuropsychological tests, researchers have focused on memory and executive function, as the neuroanatomical regions thought to subserve these cognitive domains are fairly well specified (see Elliott, 1998). Given that patients with depression frequently complain of memory difficulties, it is perhaps not surprising that these subjects demonstrate impairments on a range of memory tasks (see Blaney, 1986; Johnson & Magaro, 1987; Burt et al, 1995, for reviews). Deficits have been reported on tests of short-term memory, verbal and visual recognition memory, spatial working memory and immediate or delayed recall (Austin et al, 1992; Brown et al, 1994; Ilsley et al, 1995; Beats et al, 1996; Elliott et al, 1996). As such a broad spectrum of findings may suggest, there has been much debate over the precise nature of memory impairment, and a number of distinct formulations have been offered to explain the observed deficits (see Robbins et al, 1992, for discussion).

Executive abilities are also compromised in these patients, and it has been argued that of the neuropsychological tasks showing impairment, tests of executive function may be the most sensitive. These high-level tasks, of which the Wisconsin Card Sorting Test (WCST) (Grant & Berg, 1948) and the Tower of London test of planning ability (Shallice, 1982) are classic examples, require the coordination of cognitive processes for their successful completion, and are thought to depend on intact functioning of the prefrontal cortex. Indeed, patients with major depressive disorder have been shown to be impaired on both of these tests (Martin et al, 1991; Franke et al, 1993; Elliott et al, 1996), leading some researchers to postulate the importance of prefrontal dysfunction in the pathogenesis of clinical depression (e.g. Elliott, 1998).


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