Within-session response rates when reinforcement rate is changed within each session.

Three pigeons pecked keys and 5 rats pressed levers for food delivered on variable-interval schedules. During baseline conditions, subjects responded on a variable-interval 40-s schedule throughout the session. During experimental conditions, the programmed rate of reinforcement changed every 10 min in the 50-min sessions. When rats served as subjects, Herrnstein's (1970) hyperbolic equation provided a good description of the relation between rate of responding during a 10-min interval and the rate of reinforcement obtained during that interval. Responding, measured over 10-min blocks, was also approximately equally sensitive to changes in the programmed rate of reinforcement at all times in the session. Herrnstein's equation provided a poorer description of the changes in responding when pigeons served as subjects. Differences in experimental experience or differences in the absolute rates at which subjects responded may have contributed to the differences in results for these different species.