[Executive functioning in unipolar depression: a review].

Abstract

While several neuropsychological studies have demonstrated that cognitive deficits are seen across a broad range of cognitive domains, executive deficits associated with frontal lobe dysfunction may be prominent in depression. Executive function refers to cognitive processes that control and integrate other cognitive activities such as episodic memory. These executive functions involve a set of cognitive behaviors which include: dealing with novelty, selecting strategies, inhibiting incorrect responses, monitoring performance and using feedback to adjust future responding. The measurement of executive function relies mainly on the use of neuropsychological tests known to be sensitive to frontal lobe damage such as the Wisconsin and California Card Sorting Tests, verbal fluency tests, Stroop-test, Tower of London Task and Trail Making Test. The present review focuses on studies investigating executive functions in primary unipolar depression with these neuropsychological tasks. Unipolar depressed patients mainly exhibit cognitive inhibition deficits, problem-solving impairments and planning deficits. Cognitive inhibition deficits in depressed patients have been related to a reduction of cognitive resources and psychomotor retardation. Inhibition disturbance could lead depressed patients to process irrelevant information and consequently reduce their capacity to control transient mood changes. Several studies have found evidence of problem solving impairments in depressed patients. Depressed subjects show with card sorting tests difficulties in hypothesis testing with a loss of spontaneous and reactive cognitive flexibility. The cognitive rigidity and hypothesis-testing associated with dorsolateral prefrontal dysfunction in depression may prevent patients to cope with life events and lead to a perpetuation of depressed mood by a continuation of stress exposure. Planning tasks, such as the Tower of London Test, also demonstrate that depressed patients fail to use negative feedback as a motivational boost to improve their performance. Both trait and state factors influence the executive level of depressed patients. Executive deficits have been reported in more severely depressed subjects with melancholic or psychotic features. Executive functioning also might predict a poorer outcome in depression. Thus initiation and perseveration scores - a measure of cognitive flexibility - is associated with relapse and recurrence of depression and residual depressive symptoms. Brain imaging studies show that reduced blood flow, particularly in medial prefrontal cortex and dorsal anterior cingulate cortex subserve executive impairments in depression. However neuroimaging studies underscore the importance of mood-cognitive interactions in depression. A recent working model of depression (Mayberg et al., 1999) implicates failure of the coordinated interactions of distributed cortical-limbic pathways in the neuropsychopathology of depression. According to this model, neocortical (prefrontal and parietal regions) and superior limbic elements (dorsal anterior cingulate) are postulated to mediate impaired attention and executive function, whereas ventral limbic regions (ventral anterior cingulate, subcortical structures) are postulated to mediate circadian and vegetative aspects of depression. Further studies are needed to validate this model at the neuropsychological level as well as the brain level and to elucidate the complex interactions between mood, cognitive resources and executive function in depression.