Individual and species survival demand that organisms find and obtain needed resources (e.g., food and shelter) and opportunities for mating despite costs and risks. Such survival-relevant natural goals act as “rewards,” i.e., they are pursued with the anticipation that their consumption (or consummation) will produce desired outcomes (i.e., will “make things better”). Behaviors with rewarding goals tend to persist strongly to a conclusion and increase over time (i.e., they are positively reinforcing) (21). Internal motivational states, such as hunger, thirst, and sexual arousal, increase the incentive value of goal-related cues and of the goal objects themselves and also increase the pleasure of consumption (e.g., food tastes better when one is hungry) (22). External cues related to rewards (incentive stimuli), such as the sight or odor of food or the odor of an estrous female, can initiate or strengthen motivational states, increasing the likelihood that complex and often difficult behavioral sequences, such as foraging or hunting for food, will be brought to a successful conclusion, even in the face of obstacles. The behavioral sequences involved in obtaining desired rewards (e.g., sequences involved in hunting or foraging) become overlearned. As a result, complex action sequences can be performed smoothly and efficiently, much as an athlete learns routines to the point that they are automatic but still flexible enough to respond to many contingencies. Such prepo tent, automatized behavioral repertoires can also be activated by cues predictive of reward (19, 23). Addictive drugs elicit patterns of behavior reminiscent of those elicited by natural rewards, although the patterns of behavior associated with drugs are distinguished by their power to supplant almost all other goals. Like natural rewards, drugs are sought in anticipation of positive outcomes (notwithstanding the harmful reality), but as individuals fall deeper into addiction, drug seeking takes on such power that it can motivate parents to neglect children, previously law-abiding individuals to commit crimes, and individuals with painful alcohol- or tobacco-related illnesses to keep drinking and smoking (24). With repetitive drug taking comes homeostatic adaptations that produce dependence, which in the case of alcohol and opioids can lead to distressing withdrawal syndromes with drug cessation. Withdrawal, especially the affective component, can be considered to constitute a motivational state (25) and can thus be analogized to hunger or thirst. Although avoidance or termination of withdrawal symptoms increases the incentive to obtain drugs (26), dependence and withdrawal do not explain addiction (7, 19). In animal models, reinstatement of drug self-administration after drug cessation is more potently motivated by reexposure to the drug than by withdrawal (27). Perhaps more significantly, dependence and withdrawal cannot explain the characteristic persistence of relapse risk long after detoxification (6, 7, 19).
References
Hyman, Steven E. 2005. “Addiction: a disease of learning and memory.” The American Journal of Psychiatry 162 (8): 1414–22. doi:10.1176/appi.ajp.162.8.1414.