In the course of normal behavior, we balance the desirability of reaching a goal or obtaining a reward against the burden or aversiveness of having to work for the reward. This is closely related to the level of motivation. Motivation, the desire to act, is thought to arise within the limbic system. We are studying the biological basis of this balance in monkeys using a task that manipulates motivation. In this task, monkeys must complete a work schedule wherein the workload varies. The work is quantified by the number of times that the monkey must detect when a red spot changes to green. The monkeys work 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. Previously we learned that single neuronal responses in the ventral striatum and perirhinal cortex are directly related to the associatively learned meaning of the cue. 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. In contrast, we found that neurons in the anterior cingulate cortex that respond more strongly as more trials have elapsed. This is a provocative finding because the cingulate cortex shows abnormal activation in imaging with humans having disorders of reward expectancy such as obsessive-compulsive disorder and drug abuse. In the baso-lateral complex of the amygdala, two types of neuronal responses were observed; precue activity and responses starting at cue appearance in the rewarded trial. In the 1 trial schedule, these responses close enough in time so that they appear to be one response, i.e., the responses in the one trial schedule look like the two different types of activity seen in the longer schedules are abutted with 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. We have now been examining the responses of orbitofrontal neurons because orbitofrontal cortex is implicated in memory and evaluation of reward value. In these neuronal recordings a large proportion of the neurons respond differentially according to whether the monkey knows when the reward will be given or not. This difference, which persists across many trials, is present whether or not the visual cue instructing the monkeys about the remaining work load remains present throughout a trial or not. In studying the anterior insula, a region that is activated during drug craving, we find that the neuronal signals are strongly related to whether or not the current trial will be rewarded. These signals are generally strong around the time of the behavioral response. Thus, it appears that the anterior cingulate is involved in measuring time from the beginning of a trial schedule, and the insula seems to be signaling an immediately expected reward, two important components of reward expectancy. 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 strriatum are strongly interconnected, with both amygdala and ventral striatum receiiving 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 werre 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 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 mimicing 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.
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