Motivation, the desire to act, thought to arise within the limbic system. So that we can study the biological basis of motivated behavior, we have developed a task to manipulate the motivation of monkeys. We have found that 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. Single neuronal responses in both the ventral striatum and perirhinal are directly related to the associatively learned meaning of the cue in this complex behavioral task. Specifically, neurons in both regions 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 , other neurons 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. Because of its close connections with the visual system, the rhinal cortex has been thought to be and has been shown to be important for normal pattern recognition behavior. Rhinal cortex and ventral striatum are strongly connected, and both show highly structured organization of dopaminergic input leading us to speculate that perirhinal cortex might show reward-related activity similar to the activity we had seen in the ventral striatum. 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, we tested both normal monkeys and monkeys with bilateral rhinal cortex lesions in the reward schedule task described above. We found that normal monkeys associate new visual cues with the schedule starting within a single training session. However, animals with bilateral rhinal cortex ablations are severely impaired in making this association, be ingunable 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 seriesof tasks which culminated in reward. One strong hypothesis about reward-related behavior is that dopamine plays an important role. To test this, we have injected DNA that is antisense to the both the D2 and NMDA receptors 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.
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