The extent of the damage in each case was plotted on drawings of a standard rhesus monkey brain at ~1-mm intervals, and the lesions were reconstructed (Fig. in the Rh-OFC disconnection is that the monkeys are unable to retrieve information about the relative values of the rewards. To test for a failure in retrieval requires that the testing be instituted in a manner that allows repeated periods of disconnection after the initial acquisition. Here, a disconnection was accomplished by implementing the DREADD chemogenetic technique in monkeys. We performed the study as follows: two nave monkeys (1) received unilateral removal of Rh cortex followed by a recovery period, (2) were trained to perform a stimulus-reward association task, (3) received unilateral viral vector injections into OFC contralateral to the Rh removal, and (4) were tested on the stimulus-reward association task with or without CNO (Fig. 1). Monkeys were trained on the association task as described infigure 1A. For the viral vector injections, a lentiviral vector expressing an hM4Di-CFP (DREADD) fusion protein under a neuron-specific promoter was generated (method insupplemental text DNA constructs). When the lenitviral vector was used to express hM4Di in murine cellsin vitro, binding of the systemic inducer, CNO, induced silencing of neuronal activity (Fig. S1AD). Injections of the lentiviral vector (46 for monkey D; 71 Haloperidol D4′ for monkey S) were placed in OFC in an open surgery, in the hemisphere contralateral to the Rh ablation. Animals recovered without incident. After the injections there was a 6-week minimum waiting period for vector expression to become fully established before testing resumed on the task. Both monkeys began discriminating between the cues within the first testing session. The error rates of the monkeys decreased with increasing drop size (LME; reward size, z= 12. 74, p < Haloperidol D4' 1015), with performance indistinguishable from unoperated controls2. Because sensory and motor demands were trial invariant, we interpret the differences in performance across reward size as reflecting the subjective valuation of the expected reward by the monkey. Monkeys were tested on 5-day cycles. On days 1, 2, 4 and 5 there was no drug treatment. On day 3 the monkeys were given an intramuscular CNO injection (10 mg/kg). The experimental cycle was repeated 4 times for monkey D and 5 times for monkey S. CNO treatment induced a reduction in overall error rate (main Haloperidol D4' effect of treatment) and the pattern of errors no longer reflected the reward size (treatment x reward size interaction); there was a reduction or loss in discrimination among expected reward sizes, for both monkeys (Fig. 2)(linear mixed effects model with binomial link function (LME) - CNO vs . Haloperidol D4' baseline: Monkey D; treatment, z= 4. 62, p < 104, treatment x reward size, z= 3. 87, p < 103; Monkey S; treatment, z= 7. 12, p < 1011, treatment x reward size, z= 2 . 93, p < 0. 005). Performance returned to baseline level the day after CNO induction of neuronal silencing (LME - baseline vs . treatment day +1: Monkey D; main effect, z= 0. 5, p> 0. 05, interaction with reward size, z= 0. 002, p> 0. 05; Monkey S; main effect, z= 1 . 29, p> 0. 05, interaction with reward size, z= 0. 17, p> 0. 05). CNO injections did not alter reaction times, total trials completed or total reward earned (Fig. S2). Vehicle injections in monkey D, and CNO administered prior to virus injection in monkey S were without effect (Fig. ID1 S3). All behavioral testing was conducted during a period in which CNO concentration in the cerebrospinal fluid (CSF) was well above baseline level (Fig. S1) == Fig. 1 . == Task and experimental design. (A) A trial in the reward size task – at the beginning of each trial, Haloperidol D4′ a visual cue signaled the amount of reward (1, 2,.