The role of rewards in the survival and well-being of biological agents ranges from the control of vegetative functions to the organization of voluntary, goal-directed behavior. The brain extracts the reward information from a large variety of stimuli and events for appropriate use in the control of behavior. Recent studies revealed that neurons in a limited number of brain structures carry specific signals about past and future rewards. The neurophysiological study of reward processing within the framework of goal-directed behavior may contribute to a basic understanding of mechanisms of drug abuse and could thus have a strong medical and social.....