Effects of reinforcer magnitude on responding under differential-reinforcement-of-low-rate schedules of rats and pigeons

Experiment 1 investigated the effects of reinforcer magnitude on differential-reinforcement-of-low-rate (DRL) schedule performance in three phases. In Phase 1, two groups of rats (n = 6 and 5) responded under a DRL 72-s schedule with reinforcer magnitudes of either 30 or 300 RI of water. After acquisition, the water amounts were reversed for each rat. In Phase 2, the effects of the same reinforcer magnitudes on DRL 18-s schedule performance were examined across conditions. In Phase 3, each rat responded under a DRL 18-s schedule in which the water amounts alternated between 30 and 300 I,1 daily. Throughout each phase of Experiment 1, the larger reinforcer magnitude resulted in higher response rates and lower reinforcement rates. The peak of the interresponse-time distributions was at a lower value under the larger reinforcer magnitude. In Experiment 2, 3 pigeons responded under a DRL 20-s schedule in which reinforcer magnitude (1-s or 6-s access to grain) varied from session to session. Higher response rates and lower reinforcement rates occurred under the longer hopper duration. These results demonstrate that larger reinforcer magnitudes engender less efficient DRL schedule performance in both rats and pigeons, and when reinforcer magnitude was held constant between sessions or was varied daily. The present results are consistent with previous research demonstrating a decrease in efficiency as a function of increased reinforcer magnitude under procedures that require a period of time without a specified response. These findings also support the claim that DRL schedule performance is not governed solely by a timing process.