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a–c Diphenhydramine-maintained (DPH; 0.3–3.0 mg/kg per injection) operant responding under a second-order schedule of reinforcement in rhesus monkeys (n=3). Data are represented as mean of the last three behavioral sessions during which the variance was less than 20%. Abscissa represents the component of the behavioral session and ordinate represents the rate of operant responding (responses per second)
