Fig. 1. Shown are session-by-session operative-lever (filled circles) and inoperative-lever (open circles) response rates, and reinforcement rates (triangles) for the monkeys initially exposed to the tandem FR 1 DRO 15-s schedule.

For Monkey 3511, the DRO value was reduced to 7.5 s. For this monkey, the point at which the cancellation contingency was implemented is indicated. All subsequent sessions employed the cancellation contingency. For all monkeys, the point at which the contingencies were reversed across levers also is indicated.