Subjects with bipolar disorder showed different reward system activation than subjects with major depressive disorder in the monetary incentive delay task.

Background: Differentiating between bipolar disorder (BD) and major depressive disorder (MDD) during the depressive episode is an important clinical challenge. Reward system abnormalities have received much attention as one of the biological underpinnings of BD and MDD, but few studies have directly compared these abnormalities in remitted and depressed states.

Methods: This was a functional MRI study using the Monetary Incentive Delay task in 65 patients (BD [n = 33], MDD [n = 32]) and 33 healthy controls (HC). Regions of interest (ROI) analysis with 21 ROIs related to reward anticipation and 17 ROIs related to gain outcome were implemented, as well as whole-brain analysis. The difference in the dimensional effect of depression on brain activation was also examined.

Results: Relative to the HC group, BD patients showed significantly decreased activation during reward anticipation in the anterior cingulate cortex, anterior insula (AI), and putamen, and MDD patients showed significantly decreased activation in the AI and brainstem. The dimensional effect of depression severity showed a trend-level difference between BD and MDD in the right brainstem and left AI.

Conclusions: The current study showed a possible differential effect of depression on the reward system between MDD and BD. Further studies on reward systems might offer reliable markers to distinguish between MDD and BD patients in the depressive phase.