Reinforcement contexts effects on fixed-interval responding

Three rats responding on fixed-interval schedules received either 1 or 4 pellets at the end of 2-min intervals. Five experimental conditions manipulated the relative probabilities of these two reinforcers. Response rates following the 1-pellet reinforcer were always higher than the rates following the 4-pellet reinforcer. The rates after the 1-pellet reinforcer were also highest in those experimental conditions where it was delivered with low probability. Contrast effects were observed when two sequential fixed intervals differed in reinforcer magnitudes. It was concluded that the context of reinforcement as well as the specific reinforcer magnitude affects responding under fixed-interval schedules.