Drug addiction and memory systems: how neutral stimuli can gain control of behaviour.

Central to the problem of addiction is the fact that, once addicted, subjects can show intense drug craving and a high risk of relapse, even following long-term abstinence. This implies that addiction involves a number of long lasting changes in neural systems that need to be identified. Indeed, because of the robust emotional value of drug addiction, initial research into the neural bases of this phenomenon has focused mainly on the similarity existing between the circuits activated by natural (food intake, sexual or social interaction ...) vs artificial (drugs) rewards. On the basis of this initial research, several points can be made. First, a fundamental property of a natural or an artificial reward is its ability to elicit goal-directed behaviours; this ability makes it an important component of associative learning. Second, addiction is not synonymous only with pleasure. Indeed, withdrawal, drug seeking and drug craving carry overall aversive reactions that can also be associated with neutral stimuli that, in turn, can gain control of behaviour. In view of this, a considerable amount of research has been conducted to date aimed at defining the extent to