While numerous studies have objective quantitative and qualitative deficits of memory in depressed patients, the mechanisms of these impairments are not well established. The study reported here was designed to assess encoding and retrieval processes in depression and to define the specific nature of memory failure associated with this disorder.
Methods: Ten inpatients with major depression responding to DSM III-R criteria and ten normal controls were included in this study. All subjects were assessed with a neuropsychological battery including: a) subtests of Weschsler memory scale (digit span, logical memory); b) verbal fluency (letter); c) two tasks assessing executive functions (cognitive estimate, Nelson's test); d) two explicit tasks of verbal learning (California Verbal Learning Test and Grober & Buschke's procedure) which measure memory performance in various conditions of encoding (incidental vs controlled) and of recollection (free recall, cued recall and recognition). Severity of depression was assessed with the MADRS and the Retardation Rating Scale for Depression.
Results: Although there was no difference between patients and controls on digit span and logical memory tasks, depressed patients exhibited a deficit in verbal learning with CVLT and Grober & Buschke's procedure. On California Verbal Learning Test, depressive subjects performed poorly in free recall and demonstrated poor consistency. Patients show free recall improvement across trials 1 or 5 and this learning effect didn't differ from controls. Score of patients and controls on cued recall and recognition were at the same level. Grober & Buschke's procedure confirmed these results. Despite a control of encoding processes during the initial presentation of the words, free recall measure revealed significant difference between groups. Like controls, patients recalled almost all items when semantic cues was provided and their recognition results showed a ceiling effect. Consistency indexes of free recall and cued recall differed significantly between groups. Verbal fluency and frontal tasks didn't allow to distinguish the depressive patients from controls.
Discussion: Depressive subjects exhibited a deficit in free recall and poor consistency while cued recall and recognition were normal. Patient's results in free recall are characterized by difficulties in planning and in maintaining retrieval strategies. These findings suggest that memory failure in depression could reflect an impairment in retrieval processes depending on executive functions controlled by the subcortical structures.