Background: Differentiating bipolar disorder (BD) from major depressive disorder (MDD) during a depressive episode is an important clinical issue. Although reward system abnormalities have been highlighted as biological bases of BD and MDD, few studies have directly compared reward system abnormalities during remission and depression periods.
 Methods: We performed a functional MRI study of 33 BD patients, 32 MDD patients, and 33 healthy controls (HCs) using a Monetary Incentive Delay Task. In 21 regions of interest (ROIs) related to reward anticipation, differences in brain activation (beta values) between highly rewarded and un-rewarded conditions were extracted, and differences by diagnosis were analyzed. We also examined differences in the relationship between the severity of depression and brain activation by diagnosis in the ROIs that showed significant differences among the disease groups and HCs.
 Results: BD patients showed significantly reduced brain activation during reward anticipation in the anterior cingulate cortex, anterior insula, and putamen compared to HCs, and MDD patients showed significantly reduced brain activation in the anterior insula and brainstem compared to HCs. A trend-level difference in the relationship between depression severity and brain activation between BD and MDD was found in the right brainstem and left anterior insula.
 Conclusions: Our findings suggest differences in the effects of depression on the reward system between BD and MDD. Further research is needed to determine whether behavioral tasks that reflect the function of the reward system and structural imaging of brain regions involved in the reward system that are implemented in clinical practice can help differentiate between BD and MDD.
 Author's abstract