Differential effects of methylphenidate on signalled and non-signalled reinforcement.

Rats were trained to press a bar with water reinforcement under a variable interval 15-sec (VI 15), variable interval 15-sec signalled-reinforcement (VI SD), or a differential-reinforcement-of-low-rate 10-sec (DRL 10) schedule. Methylphenidate (MP) had little effect on response rates or reinforcements-earned in the VI 15 group. However, MP increased response rates in a dose-related manner in the VI SD and the DRL 10 groups. These findings and additional results reported in the literature suggest that signalled reinforcement does not necessarily antagonize the rate-increasing effects of the amphetamine-type drugs.