Drugs of abuse have a profound effect on the response to previously neutral stimuli to which the drugs become paired. This phenomenon, called conditioned reinforcement, can be defined as when a previously neutral stimulus reinforces or strengthens behaviours through its association with a primary reinforcer and becomes a reinforcer in its own right. Such cues can be contextual and predictive, and the process of conditioned reinforcement entails not only approach to salient cues but also instrumental responding to turn on the cues, in view of their own rewarding (conditioned reinforcing) properties. As such, conditioned reinforcement as a construct preceded and laid the foundation for incentive salience.55 Incentive salience can be defined as motivation for rewards derived from both one’s physiological state and previously learned associations about a reward cue that is mediated by the mesocorticolimbic dopamine system.56 Both conditioned reinforcement and incentive salience provide constructs for what underlies cue-induced drug seeking, self-administration behaviours, and, conceivably, the transition to habit-like compulsive drug seeking.
