Pecking of pigeons was reinforced under a modified interval-percentile procedure that allowed independent manipulation of overall reinforcement rate and the degree to which reinforcement depended on interresponse-time duration. Increasing the contingency, as measured by the phi coefficient, between reinforcement and long interresponse times while controlling the overall rate of reinforcement systematically increased the frequency of those interresponse times and decreased response rate under both of the reinforcement rates studied. Increasing reinforcement rate also generally increased response rate, particularly under weaker interresponse-time contingencies. Random-interval schedules with comparable reinforcement rates generated response rates and interresponse-time distributions similar to those obtained with moderate-to-high interresponse-time reinforcement contingencies. These results suggest that interresponse-time reinforcement contingencies inherent in random-interval and constant-probability variable-interval schedules exercise substantial control over responding independent of overall reinforcement rate effects. The interresponse-time reinforcement contingencies inherent in these schedules may actually mask the effects of overall reinforcement rate; thus differences in response rate as a function of reinforcement rate when interresponse-time reinforcement is eliminated may be underestimated.
