Effects of methylphenidate on responding under extinction in the presence and absence of conditioned reinforcement.

Pigeons pecked a key during sessions that began with a variable number of reinforcers under a second-order schedule of food presentation. Every 30sec, on the average, a key peck was followed immediately by one of two consequences: (a) food presentation, accompanied by a stimulus complex that consisted of houselight off, key color change, tone presentation, and hopper-light illumination, or (b) the stimulus complex alone. Following the last food presentation, 20min of one of two types of extinction began. The two types of extinction were: (a) standard extinction (key pecks had no consequence) and (b) key pecks produced, on a variable-interval schedule, the stimulus complex previously paired with food. Consequently, it was possible to study performance under extinction during which responses either did or did not result in occasional presentation of a food-paired stimulus complex. Methylphenidate (5, 10 and 20mg/kg) occasionally was administered before sessions containing each type of extinction. At moderate doses methylphenidate produced higher response rates during extinction when the stimulus complex was presented than when it was not. These results support previous findings with rats that stimulant drugs can enhance responding during extinction when responding produces conditioned reinforcers, and illustrate this effect in a novel, within-subject design.