Background: Cognitive deficits are common in major depressive disorder, but their nature is unclear. The effort hypothesis states that performance on effortful tasks is disproportionately impaired compared with the performance on automatic tasks. The cognitive speed hypothesis states that depression is characterized by cognitive slowness, which is a source of cognitive dysfunctioning. The present study investigated both theories in unmedicated adult depressive patients. It was also investigated whether the cognitive deficits can be attributed to more general physical illness-related factors or specifically to depressive disorder.
Method: Thirty non-psychotic depressive out-patients were compared with 38 healthy control subjects and 25 patients with severe allergic rhinitis. The effects of group on more automatic and more effortful aspects of cognitive tasks measuring cognitive speed (Concept Shifting Task, Stroop Colour Word Test, Memory Scanning Test) and memory retrieval (Visual Verbal Learning Task, Verbal Fluency Test) were evaluated by MANCOVA. Age, sex, education and pre-morbid intelligence were treated as covariates.
Results: The depressive group had cognitive deficits in the automatic processing subtask of the Stroop, memory scanning and memory span. Performance on more effortful tasks was not impaired.
Conclusions: Our results are more consistent with the cognitive speed hypothesis. Cognitive functioning in depressive disorder seems to be characterized by a reduced speed of information processing in automatic subtasks.