Limbic over-activity in depression during preserved performance on the n-back task

The profile of cognitive dysfunction observed in patients with major depressive disorder (MDD) may be partially attributed to a deficit in the central executive component of working memory (WM). This could be the consequence of a functional deficit in regions of cortex that are associated with WM function in healthy adults. In order to investigate this assertion, ten patients with a diagnosis of MDD and ten matched healthy controls undertook a parametric WM task (i.e. the n-back task) during the acquisition of blood oxygen level dependent echo planar magnetic resonance images (BOLD EPI fMRI). There was no significant difference in the behavioral performance of depressed patients and controls. This was true for both accuracy and reaction time on the n-back task. Random effects analysis of the functional imaging data (using SPM99) revealed a significant difference in load-dependent activation in the medial orbitofrontal cortex/rostral anterior cingulate between patients and controls (cluster size (K(E))/volume = 128/1024 mm3, P(corrected) = 0.025). While both participant groups exhibited a significant decrease in activation in this region with increased task difficulty, the magnitude of this decrease was smaller in patients with MDD than in controls. Therefore, this study implies that the performance of WM tasks is associated with a dysfunctional activation of the medial orbitofrontal and rostral anterior cingulate cortex in MDD. The study thus offers a rationale for explaining depressive cognitive impairment by the abnormal fronto-limbic activation found in clinical depression.