Neural Mechanisms Of Motivation And Reward

National Institute Of Mental Health

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Abstract

Motivation, the desire to act, is thought to arise within the limbic system. We are studying the biological basis of motivated behavior and reward expectancy in monkeys using a task that manipulates their motivation. The monkeys work faster and with fewer errors when a cue indicates that a juice reward will be delivered immediately after the next correct response than when the cue indicates that additional trials will be needed. Previously we learned that single neuronal responses in the ventral striatum and perirhinal are directly related to the associatively learned meaning of the cue in this complex behavioral task. More recently we have also examined the activity of neurons in the amygdala and anterior cingulate cortex. Neurons in ventral striatum keep track of whether the animal is at the beginning of or somewhere in the course of a behavioral sequence that ultimately leads to reward. Some neurons signal that a new series of trials is starting, others signal that a series of trials is in progress, and still others signal the rewarded trial is starting when it follows one or more unrewarded trials. Thus populations in both brain regions provide neural signals that could reinforce complex reward-seeking behavior. However, neither brain region showed a signal related to directly to reward expectancy. We have shown that neurons of the anterior cingulate cortex respond more strongly as more trials have elapsed, thus showing a signal related to expectancy. This is a provocative finding because the cingulate cortex shows abnormal activation in imaging with humans having disorders of reward expectancy such as obssessive-compulsived disorder and drug abuse. In the lateral basal nucleus of the amygdala, which receives input from the perirhinal cortex, the 2 and 3 trial schedules, two types of responses occurred, precue activity and responses starting at cue appearance in the rewarded trial. In the 1 trial schedule, these responses abutted each other.When the cue was shuffled to be independent of the schedule, no precue activity occurred and only a weak response was observed after cue onset. The results indicate that two different signals were coded sequentially in time. The first signal might be related to immediate expectation of the beginning of the next schedule and the second signal might be related to immediate expectation of the reward. 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 schdules was unexpected. To investigate whether perirhinal cortex plays a critical role in this associately learned reward schedule related behavior, 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 developing 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 and NMDA receptor proteins 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 mimicing the results after a bilateral ablation. Animals with control injections of nonspecific DNA are unimpaired. 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, and that either D2 and/or NMDA receptors are must be intact for this function. It appears that all of these brain regions carry signals that might be important for the coding of reward or goal expectation and incentive.