Deficits in sustaining reward responses in subsyndromal and syndromal major depression.

Preliminary findings suggest a reduction in capacity to sustain reward responses in major depression. However, relatively little is known about the stability of reward learning over time and the effect of stress on reward responses in depressed individuals. This study aimed to evaluate sustained behaviour to maximize reward in the context of known reinforcement contingencies and to evaluate the extent to which stress influences such behaviour in clinically depressed patients (n=43), subsyndromally depressed individuals (n=43), and healthy controls (n=44). A probabilistic reward learning task with contingencies known to participants was used to evaluate the change of reward response over time in both 'stress' and 'non-stress' conditions. Stress was induced by salient negative feedback during the task performance. Questionnaires capturing subjective affect were also administered to all participants after completion of the task. Response bias to the stimulus signaling greater reward decreased significantly over time in both subsyndromally and clinically depressed participants, but not in healthy controls. Healthy controls demonstrated a trend of dysfunctional reward processing under the stress condition. Moreover, in the stress condition, the deficit in sustaining behaviour to maximize reward was associated with subjective rating of pleasure in participants with either subsyndromal depression or major depression. These findings suggest that individuals with depression have difficulty sustaining behaviour during a known reinforcement schedule. Participants with anhedonic symptoms are even less likely to sustain behaviour to maximize reward under stress.