The concept of motivation was linked inextricably with hedonic, affective, or emotional states in addiction in the context of temporal dynamics by Solomon’s opponent process theory of motivation. Solomon and Corbit (1974) postulated that hedonic, affective, or emotional states, once initiated, are automatically modulated by the central nervous system with mechanisms that reduce the intensity of hedonic feelings. The a-process includes affective or hedonic habituation (or tolerance), and the b-process includes affective or hedonic withdrawal (abstinence). The a-process in drug use consists of positive hedonic responses, occurs shortly after presentation of a stimulus, correlates closely with the intensity, quality, and duration of the reinforcer, and shows tolerance. In contrast, the b-process in drug use appears after the a-process has terminated, consists of negative hedonic responses, and is sluggish in onset, slow to build up to an asymptote, slow to decay, and gets larger with repeated exposure. The thesis here is that opponent processes begin early in drug-taking, reflect changes in the brain reward and stress systems, and later form one of the major motivations for compulsivity in drug taking.
