The Neuropsychology of Mood Disorders:
Affect, Cognition and Neural Circuitry
Aprajita Mohanty and Wendy Heller
Research in the past decade has greatly enhanced our understanding of the neural processes that implement cognitive, emotional and physiological functions in mood disorders. Abnormalities or disturbances in these functions have been shown to be associated with corresponding abnormalities in regional brain functions (Heller and Nitschke, 1998). This has led to increased research efforts focused on linking theories of cognitive neuropsychology to the anatomy and physiology of related brain function in mood disorders.
Despite extensive evidence indicating impairment of cognitive functioning in depression, this area is largely ignored, especially in diagnostic evaluation (Austin et al., 1992). The current Diagnostic and Statistical Manual of Mental Disorders (DSMIV; American Psychiatric Association, 1994) identifies cognitive factors (e.g. indecisiveness, difficulties in thought and concentration) as fundamental components of depressive episodes and dysthymia. Neuropsychological studies have identified a variety of other cognitive characteristics associated with depressed moods. For example, depression is associated with deficits in executive functioning (Channon and Green, 1999; Freidman, 1964; Goodwin, 1997; Raskin et al., 1982; Silberman et al., 1983), memory (Burt et al., 1995), attention (Mialet et al., 1996), and visuospatial processing (Asthana et al., 1998). Depression-related cognitive deficits range in severity from mild subclinical impairments to pervasive global deficits, often referred to as pseudodementia (Abrams and Taylor, 1987; Golinkoff and Sweeny, 1989; Watts et al., 1990). The term pseudodementia was coined to describe patients with affective disorders who display an unusually large number of cognitive deficits that are typically associated with organic brain disease (Kiloh, 1961). The conjunction of depression and cognitive impairments of various types poses a clinical diagnostic problem because it increases the likelihood of a variety of misdiagnoses, including degenerative dementia, stroke or learning disability (Marsden and Harrison, 1972).
Abnormal patterns of brain activity and function associated with depression have been demonstrated in research using a variety of techniques, such as neuropsychological testing (Miller et al., 1995; Rubinow and Post, 1992; Silberman and Weingartner, 1986), lesion techniques (Lipsey et al., 1983; Robinson and Price, 1982; Robinson et al., 1984), electrophysiological techniques (Deldin et al., 2000; Heller et al., 1995; Henriques and Davidson, 1990), and haemodynamic techniques (Baxter et al., 1989; Bench et al., 1992, 1993; George et al., 1994a). Parallel findings to those obtained in depressed populations have been described in normal individuals following induction of depressed mood (Davidson et al., 1979, 1985) as well as individuals scoring high on measures of depressed affect (Tomarken et al., 1992). Despite substantial evidence indicating that the same brain regions implicated as abnormal in depression are also fundamental for various aspects of cognitive processing, the implications of these neuropsychological findings for cognitive processing in depression have rarely been studied (Rubinow and Post, 1992).
In this chapter, we review the evidence and describe the nature of the cognitive characteristics that have been identified in depression. In addition, we provide a brief discussion of the neural mechanisms likely to be associated with these cognitive characteristics. Before discussing the nature of cognition in depression we review a number of important factors that are likely to impact cognitive function in depression. These factors can introduce important methodological confounds in studies examining the relationship between depression and cognitive impairment, thus weakening the conclusions drawn from the results of clinical neuropsychological studies (Murphy and Sahakian, 2001).
AND CHARACTERISTICS OF DEPRESSION
Cognitive deficits in depression are influenced by the clinical characteristics of the disorder, such as the presence of specific symptoms, depression subtype and symptom severity. For example, cognitive deficits in depression have been found to vary with the presence of psychotic symptoms.
Studies show that depressed patients with psychotic features such as delusions and hallucinations have more structural brain abnormalities than normal controls (Lesser et al., 1991), show neuropsychological performance comparable to that of patients with schizophrenia (Jesteet al., 1996; Nelson et al., 1998) and are more impaired than patients with non-psychotic depression and normal controls (Basso and Bornstein, 1999; Jeste et al., 1996; Lesser et al., 1991; Nelson et al., 1998) on a broad range of neuropsychological measures such as attention, response inhibition, verbal declarative memory and visuospatial abilities. It is important to keep in mind that drug effects can be a possible confounding factor in these studies as the selected clinical groups differed in the types and dosages of psychotropic medications with which they were being treated. Studies have shown that some psychotropic medications can impair or, conversely, improve neuropsychological test performance (Spohn and Strauss, 1989). Furthermore, individuals with psychotic depression may have more severe depressive symptoms, more frequent recurrence of depressive episodes and longer episodes, poorer response to pharmacotherapy, more hyperactive hypothalamus-pituitary-adrenal (HPA) activity (Coryell et al.,