Sign Tracking, but Not Goal Tracking, is Resistant to Outcome Devaluation

Individuals make choices and prioritize goals using complex processes that assign value to rewards and associated stimuli. During Pavlovian learning, previously neutral stimuli that predict rewards can acquire motivational properties, whereby they themselves become attractive and desirable incentive stimuli. But individuals differ in whether a cue acts solely as a predictor that evokes a conditional response, or also serves as an incentive stimulus, and this determines the degree to which a cue might bias choice or even promote maladaptive behavior. Here we use rats that differ in the incentive motivational properties they attribute to food cues to probe the role of the neurotransmitter dopamine in stimulus-reward learning. We show that intact dopamine transmission is not required for all forms of learning in which reward cues become effective predictors. Rather, dopamine acts selectively in a form of reward learning in which “incentive salience” is assigned to reward cues. In individuals with a propensity for this form of learning, reward cues come to powerfully motivate and control behavior. This work provides insight into the neurobiology of a form of reward learning that confers increased susceptibility to disorders of impulse control.

Concepts of motivation are vital to progress in behavioral neuroscience. Motivational concepts help us to understand what limbic brain systems are chiefly evolved to do, i.e., to mediate psychological processes that guide real behavior. This article evaluates some major motivation concepts that have historic importance or have influenced the interpretation of behavioral neuroscience research. These concepts include homeostasis, setpoints and settling points, intervening variables, hydraulic drives, drive reduction, appetitive and consummatory behavior, opponent processes, hedonic reactions, incentive motivation, drive centers, dedicated drive neurons (and drive neuropeptides and receptors), neural hierarchies, and new concepts from affective neuroscience such as allostasis, cognitive incentives, and reward 'liking' versus 'wanting'. Copyright 2004 Elsevier Inc.

Drugs of abuse acquire different degrees of control over thoughts and actions based not only on the effects of drugs themselves, but also on predispositions of the individual. Those individuals who become addicted are unable to shift their thoughts and actions away from drugs and drug-associated stimuli. Thus in addicts, exposure to places or things (cues) that has been previously associated with drug-taking often instigates renewed drug-taking. We and others have postulated that drug-associated cues acquire the ability to maintain and instigate drug-taking behavior in part because they acquire incentive motivational properties through Pavlovian (stimulus-stimulus) learning. In the case of compulsive behavioral disorders, including addiction, such cues may be attributed with pathological incentive value ("incentive salience"). For this reason, we have recently begun to explore individual differences in the tendency to attribute incentive salience to cues that predict rewards. When discrete cues are associated with the non-contingent delivery of food or drug rewards some animals come to quickly approach and engage the cue even if it is located at a distance from where the reward will be delivered. In these animals the reward-predictive cue itself becomes attractive, eliciting approach towards it, presumably because it is attributed with incentive salience. Animals that develop this type of conditional response are called "sign-trackers". Other animals, "goal-trackers", do not approach the reward-predictive cue, but upon cue presentation they immediately go to the location where food will be delivered (the "goal"). For goal-trackers the reward-predictive cue is not attractive, presumably because it is not attributed with incentive salience. We review here preliminary data suggesting that these individual differences in the tendency to attribute incentive salience to cues predictive of reward may confer vulnerability or resistance to compulsive behavioral disorders, including addiction. It will be important, therefore, to study how environmental, neurobiological and genetic interactions determine the extent to which individuals attribute incentive value to reward-predictive stimuli.