Review
Cognitive impairment in major depression

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Abstract

In the past decade, a growing bulk of evidence has accumulated to suggest that patients suffering from major depression (MD) present some cognitive disturbances, such as impairment in attention, working memory, and executive function, including cognitive inhibition, problem- and task-planning. If the results of short-term memory assessment in depressed patients are equivocal, a general consensus exists that memory problems are secondary to attentional dysfunctions, and reflect the inability to concentrate. Moreover, both unipolar and bipolar patients show evidence of impaired verbal learning that has been commonly interpreted as reflecting an inability to transfer information from short-term to long-term storage. According to some authors, there would be a gender-related as well age-related specificity of some disturbances. Depressed patients also show impairments of executive functions and their recent exploration through brain imaging techniques has recently permitted to formulate some general hypotheses on the possible involvement of different brain areas in MD.

Introduction

Major depression (MD) unquestionably may affect the ability to think, concentrate, make decision, formulate ideas, reason, and remember. Other cognitive symptoms typical of MD are represented by negative self-evaluation, worthlessness, thoughts of death, suicidal ideation, ruminative thinking over minor past failings, delusions (50% of patients tend to focus on fixed ideas of guilt and sinfulness, poverty, somatic concerns, and feelings of persecution), and hallucinations (auditory are more common than the visual one, in 20% of the patients).

Less clear are the nature and extent of such disturbances, as well as their specificity to affective illness, their etiology, or their existence before the onset of the symptoms. The degree of progression, chronicity, and reversibility of cognitive impairment in treated or untreated MD patients is, likewise, inadequately understood. Problems of particular significance arise when attempting to distinguish between general factors that might account for intellectual impairment across psychiatric, psychological or medical conditions — e.g., distractibility, attention, fatigue, or motivation — and those specific to MD, like lateralization differences.

Cognitive dysfunctions are generally assessed by a neuropsychological examination including an interview of the patient's background and present situation, a behavioral observation, and the administration of a battery of neuropsychological tests. Data obtained are useful to treatment planning, to assess the efficacy of the treatment, as well as to differentiate deficits which are state-dependent from those that may be trait-related or vulnerability markers of the disorder.

The aim of this paper is to review the main data on cognitive disturbances in MD, with a focus on findings derived from neuropsychological tests exploring general intelligence, verbal memory, non-verbal memory, psychomotor skills, attention and executive functions.

Section snippets

General intelligence

The most used test batteries to estimate general intelligence are the Intelligence Quotient (IQ), the Wechsler Adult Intelligence Scale (WAIS), the Wide Range Achievement Test (WRAT), and the National Adult Reading Test (NART). General intelligence seems to be impaired in depression, however, it is not considered a primary disturbance, but rather the result of impairment of memory, psychomotor skills or attention. Data on premorbid intellectual level in patients affected by mood disorders and

Memory

Verbal memory is usually assessed by the California Verbal Learning Test (CVLT), the Wechsler Memory Scale (WMS) and the Rey Auditory Verbal Learning Test (RAVLT). Non-verbal (visuo-spatial) memory can be evaluated by two neuropsychological tests, the Rey–Osterrieth Complex Figure Test (ROCFT) and the Benton Visual Retention Test (BRVT). These instruments assess visual perception, visual memory, and visuo-constructive abilities, as well as perception of spatial relations and memory for newly

Psychomotor skills

Psychomotor retardation is not necessarily typical of depression, however, when present, it is one of its most dramatic symptoms. It can be demonstrated in depressed patients in different ways, including reaction time measures, information processing speed, writing and drawing tasks, and other measures.

Cognitive and psychomotor speed are commonly slowed in depression and seem more typical of older than younger patients (Sabbe et al., 1999, Caligiuri and Ellwanger, 2000, Tsourtos et al., 2002,

Executive functions

The executive system is thought to be involved in handling novel situations outside the domain of some of automatic psychological processes that could be explained by reproduction of learned schemas or set behaviors. The novel situations are those that involve planning or decision making; those that involve error correction or troubleshooting; situations where responses are not well-learned or contain novel sequences of action; dangerous or technically difficult situations; and situations which

Cognitive impairment and age or gender

Several studies reported that young depressed patients show deficits in some components of executive functioning, such as attention, short-term and working memory, and in psychomotor skills, while data on verbal memory and learning functions are controversial (Savard et al., 1980, Mormont, 1984, Rubinow et al., 1984, Castaneda et al., 2008). On the contrary, in elderly patients suffering from MD, memory difficulties may be the chief complaint and may be mistaken for early signs of a dementia

Conclusions

There is now a consistent, albeit not exhaustive, literature on cognitive impairments in MD. The most consistent findings are those related to memory disturbances as a result of attention deficit, and of executive functions that highlight a certain rigidity in shifting the focus. A wide agreement exists also on the notion that there exists an age-related specificity in some cognitive symptoms, as older patients may show a marked psychomotor retardation and memory difficulties.

Interestingly, a

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