 We hypothesize that anhedonia in MDD is associated with impairment of brain reward mechanisms such that dysfunction of the orbital and ventromedial frontal cortices involved in the impaired hedonic attribution capacity, while dysfunction of the ventral striatum area that contains the nucleus accumbens is involved with the reduced or lack of reactivity to rewarding environmental stimuli in patients with MDD. Our hypothesis is to link specific neural substrates to the two psychiatric components of anhedonia, i.e., loss of interest and lack of reactivity, as defined in the diagnostic criteria for MDD. We plan to operationally test our hypothesis by using empirical measurement of reward responses in MDD patients with and without significant anhedonia using a conditioning task assessing appetitive and aversive learning without any performance aspects and spatial delayed response task assessing the relationship between reward, performance and mood with the event-related functional magnetic resonance imaging method. We expect to find reduced activation of the ventral striatum, orbital and ventromedial frontal cortices in response to monetary incentive stimuli in MDD patients with significant anhedonia relative to MDD patients without anhedonia and healthy control. 