Anhedonia in melancholic and non-melancholic depressive disorders

Anhedonia represents a core symptom of major depression and may be a potential marker for melancholia. However, current understanding of this construct in depressive sub-types is limited.

Participants were recruited from the Black Dog Institute (Sydney) and Massachusetts General Hospital (Boston). Diagnostic groups were derived on the basis of agreement between clinician and DSM-IV diagnosis from structured interviews. Currently depressed unipolar melancholic, non-melancholic and healthy control participants were administered a probabilistic reward task (PRT) to assess a behavioural correlate of anhedonia-blunted reward-based learning. Self-reported measures of anhedonia, approach and avoidance motivation were completed by the Sydney sample.

Relative to healthy controls and non-melancholic participants, melancholic depressed participants had reduced response bias, highlighting blunted reward learning. Moreover, although non-melancholic participants were characterized by a delayed response bias, melancholic depressed participants failed to develop a bias throughout blocks. Response bias showed no associations with self-report measures of hedonic tone in depressed participants. Positive associations were observed between response bias, approach and avoidance motivation in non-melancholic participants only.

Possible medication, fatigue and anxiety effects were not controlled; small sample sizes; inclusion criteria may have excluded those with severe melancholia and led to underestimation of group differences.

Melancholia is characterised by a reduced ability to modulate behaviour as a function of reward, and the motivational salience of rewarding stimuli may differ across depressive sub-types. Results support the view that melancholia is a distinct sub-type. Further exploration of reward system functioning in depressive sub-types is warranted.