This term refers to a clinical condition in which a human or an experimental animal cannot experience positive emotional states derived from obtaining a desired or biologically significant stimulus. Generally, certain stimuli serve as positive reinforcers in normal individuals (e.g., food, water, the company of friends). "Positive reinforcement" is a descriptive term used by behavioral scientists to denote an increase in the probability of a behavior that is contingent on the presentation of biologically significant stimuli, such as food or water.

Anhedonia may be idiopathic (of unknown cause), may occur as a side effect of certain drugs (for example, the NEUROLEPTICS), which act as dopamine-receptor antagonists, or may be an aspect of certain psychiatric disorders, such as depression. It is conjectured that a state of anhedonia may occur during the "crash" that follows a prolonged bout of drug self-administration, particularly...