Neural Mechanisms Of Motivation And Reward

National Institute Of Mental Health

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Abstract

The intensity or vigor of goal-directed behavior is a correlate of the motivation underlying it. Motivation is related to the subjective value of rewards. Motivation is moderated, or even completely dissipated, if the perceived effort or discomfort seems too great. Under what circumstances do we seek a goal or a reward?[unreadable] [unreadable] To study motivated behavior in monkeys, we use several variants of a task in which monkeys must perform some work, in this case detecting when a target spot turns from red-to-green, to obtain a drop of juice. We use another visual stimulus, a cue, to indicate how much discomfort must be endured, e.g., the number of trials to be worked, to obtain the reward. The monkeys learn about the cues quickly, often after just a few trials. The number of errors becomes proportional to amount of work remaining before reward, with the monkeys working faster and with fewer errors when a visual cue indicates that the reward will be delivered immediately after the next correct response than when the cue indicates that additional red-to-green detections will be needed. This is a behavior in which the monkeys decrease their performance in response to an increased predicted workload. This task achieves our goal of manipulating motivation. [unreadable] [unreadable] In a reward postponement version of this task, the monkeys only work for one trial, but the reward is postponed by different amounts of time after the trial is completed correctly. In this task, both the reward size and the postponement time affect the error rate. The error rates are directly related to the reward size, R, and hyperbolically the delay D, (errors proportional to R/(1+D) ) These results are consistent with temporal discounting of reward with time.[unreadable] [unreadable] Because the anterior cingulate cortex is widely accepted as important role in organizing motivated behavior, we recorded from single cingulate neurons in this area from a monkey performing this task. We found strong effects of predicted delay duration on the cue response and only a small effect of predicted reward size effect on the activity. These results suggest that the anterior cingulate cortex plays a role in anticipating and monitoring the cost expended to obtain a reward, such as the burden of waiting.[unreadable] [unreadable] Because of its close connections with the visual system, the rhinal cortex has been shown to be important for normal pattern recognition behavior. Rhinal cortex, amgdala, and ventral striatum are strongly interconnected, with both amygdala and ventral striatum receiving visual information from the perirhinal cortex. Given the emphasis on the relation between perirhinal cortex and pattern-recognition behavior in the past, the prominence of the perirhinal activity related to the reward schedules was unexpected. To investigate whether perirhinal cortex plays a critical role in learning to associate viusal cues with the reward schedules, normal monkeys and monkeys with bilateral rhinal cortex lesions were studied using the reward schedule task described above. Normal monkeys associate new visual cues with the schedule starting within a single training session. In contrast, animals with bilateral rhinal cortex ablations are severely impaired in making this association, being unable to do so even after six weeks of daily training. Thus, perirhinal cortex is a critical structure for learning the associative relation between a visual cue and its meaning for reward schedules. We hypothesize that dopaminergic input provides signals sensitive to long-term progress through a planned or expected series of tasks which culminated in reward. To test this, we injected material that temporarily should block production of D2 receptor protein into the remaining rhinal cortex of monkeys with a unilateral rhinal cortex lesion. For a period of about 10 weeks these animals are unable to associate new visual cues with reward schedules, thus mimicking the results after a bilateral ablation. Animals with control injections of DNA blocking production of the NR2B subunit of the N-methyl-D-aspartate (NMDA) glutamate receptor perform like normal monkeys, showing that the D2 receptor is critical for this associative learning of cues predicting workload. These data suggest that rhinal cortex is critical for forming the associations between stimuli and their motivational/emotional meaning for predicting reward in reward schedules. It appears that all of these brain regions carry signals that might be important for the coding of reward or goal expectation and incentive.[unreadable] [unreadable] Orbitofrontal cortex is implicated in memory and evaluation of reward value. Neuronal recordings show a large proportion of the neurons respond differentially according to whether the previous trial was rewarded and whether the current trial will be rewarded. To explore OFC neurons further we recorded while carrying out classical conditioning, where one stimulus predicted a delivery of juice and the other predicted that no juice would be delivered. Neurons showed increased firing rates after CS presentation, during the CS-outcome period, and/or after the trial outcome. All of the recorded (31) neurons differentiated between trials with positive and neutral outcomes: 25 had larger responses during the rewarded trials, and 6 had larger responses during unrewarded trials. [unreadable] [unreadable] Recently we fortuitously found that the zona incerta (ZI), a nucleus located between the dorsal thalamus and the subthalamic nucleus and connected with many cortical areas and subcortical structures including cingulate cortex and substantia nigra pars compacta, two structures known to be involved in reward processes, is responsive to many events in our task. The activity observed in the zona incerta might be related to the motivational state of the animal. Because of the diversity of its connectivity, this structure could be an important relay structure in the reward circuitry.[unreadable] [unreadable] Category learning in monkeys is often studied with behavioral tasks in which one observes monkeys? category judgment through their choice of action and reinforces the correct judgments and choices. We tested monkeys using the reward postponement task (see above) with categorical cue sets (20 stimuli for each of the 3 categories, dogs, cats, and rats; or trucks, cars, and motorcycles) as predictors of postponement duration. All monkeys show significantly different error rates (p<0.01, c2-test) among categories whether natural or not. These results indicate that monkeys can quickly learn to categorize visual stimuli and generalize to novel members of a set without instruction.[unreadable] [unreadable] We have also studied dynamic re-grouping of categories without a requirement for differential motor responses. A stimulus set created from linear morphs between two given shapes (along the feature dimension ?shape?) and two given colors (?color? dimension) was separated either along the feature dimension shape or color, with un-announced, random switches during each session. The meaning of the stimulus in terms of reward contingency (long or short postponement) is determined by its current class-assignment. To contrast un-instructed with instructed re-grouping, we trained monkeys on a Go-NoGo version of the categorization task, where monkeys had to act differentially in response to the two categories for successful completion of trials. In both tasks, the monkeys could re-group the stimuli repeatedly within each session. These results show that monkeys can dynamically apply experimenter-defined category assignments that indicate reward-contingency, even when no category-motor association is required.