How cognitive is psychomotor slowing in depression? Evidence from a meta-analysis

The current meta-analysis explores the hypothesis that many of the apparently specific deficits that depressed subjects show on speeded cognitive tasks are actually the result of general (i.e., task-independent) slowing. Results of three analyses provided strong support for this hypothesis. The reaction times (RTs) of the depressed groups were consistently slower than those of the control groups. Moreover, the degree of cognitive slowing was approximately the same regardless of the task or condition. Regression analyses revealed a proportional relation between the RTs of the two depressed and control groups, suggesting that depression slows sensory/motor and cognitive processes to approximately the same degree. The regression-based approach used in the present analyses has implications for the study of other neurological disorders and also may have important applications in the assessment of both general and specific cognitive deficits in individual patients.