<qa>  What is the name of the two key diagnostic criteria for major depressive disorder?Anhedonia has been one of the two key diagnostic criteria for major depressive disorder MDD since the publication of The Diagnostic and Statistical Manual of Mental Disorders, Third Edition, yet little is known about its neural substrates. Neuroimaging studies have identified numerous brain regions that are thought to be involved with MDD. Most studies dealt with MDD as a syndromal entity, and not surprisingly, yielded quite variable results with respect to the areas of the brain identified, the nature of the functional changes i.e., decrease or increase in activities, lateralization, and correlation with clinical symptoms. Clinical heterogeneity and lack of symptom-specific targets are presumably among the factors contributing to the variability. The hypothesis that a functionally impaired mesolimbic dopaminergic pathway may comprise a part of neural substrate underlying core MDD symptoms of anhedonia and loss of motivation has been proposed. Nevertheless, the roles of brain reward mechanisms in mediating anhedonia in MDD remain unclear. Availability of appropriate experimental paradigms that can be used empirically to measure anhedonia is a prerequisite to test such a hypothesis. Recent studies using monetary incentive paradigms coupled with neuroimaging techniques have identified hemodynamic responses in structures that serve as part of the mesolimbic dopaminergic pathway during processing rewards in healthy humans. 