Monkeys were trained in a Skinner box to press a key for standard food pellets on a fixed-ratio 10 schedule for 30 min. When stable responding had been achieved, two novel foods (almond nuts and marshmallows) were introduced. In the conditioning sessions on the odd-number days, pressing a key delivered one of the two novel foods as the to-be-conditioned target food, and each monkey was injected cyclophosphamide (4 mg/kg) intravenously in the home cage 10 min after the end of the session. In the control sessions on the even-number days, the monkeys earned the other food, and were never injected. As the conditioning was repeated, the monkeys eventually stopped key-pressing for the target food in the conditioning sessions, but continued to work for and eat the other food in the control sessions.
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