Background: Several lines of research suggest a disturbance of reversal learning (reward and punishment processing, and affective switching) in patients with major depressive disorder (MDD). Obsessive-compulsive disorder (OCD) is also characterized by abnormal reversal learning, and is often co-morbid with MDD. However, neurobiological distinctions between the disorders are unclear. Functional neuroimaging (activation) studies comparing MDD and OCD directly are lacking.
Method: Twenty non-medicated OCD-free patients with MDD, 20 non-medicated MDD-free patients with OCD, and 27 healthy controls performed a self-paced reversal learning task in an event-related design during functional magnetic resonance imaging (fMRI).
Results: Compared with healthy controls, both MDD and OCD patients displayed prolonged mean reaction times (RTs) but normal accuracy. In MDD subjects, mean RTs were correlated with disease severity. Imaging results showed MDD-specific hyperactivity in the anterior insula during punishment processing and in the putamen during reward processing. Moreover, blood oxygen level-dependent (BOLD) responses in the dorsolateral prefrontal cortex (DLPFC) and the anterior PFC during affective switching showed a linear decrease across controls, MDD and OCD. Finally, the OCD group showed blunted responsiveness of the orbitofrontal (OFC)-striatal loop during reward, and in the OFC and anterior insula during affective switching.
Conclusions: This study shows frontal-striatal and (para)limbic functional abnormalities during reversal learning in MDD, in the context of generic psychomotor slowing. These data converge with currently influential models on the neuropathophysiology of MDD. Moreover, this study reports differential neural patterns in frontal-striatal and paralimbic structures on this task between MDD and OCD, confirming previous findings regarding the neural correlates of deficient reversal learning in OCD.