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In fixed-interval (FI) and response-initiated fixed-interval (RIFI) schedules of reinforcement, a response is required after an interval has elapsed for delivery of reinforcement. In RIFI schedules, a response is required to initiate each interval as well. The objective of this experiment was a systematic comparison of performance in the two schedule types over a range of interval durations. Four pigeons were exposed to FI and RIFI schedules of 15, 30, 60, 120 and 240 s. Interfood intervals were longer and more variable in RIFI than corresponding FI schedules. In addition, response rates early in the RIFI schedules were higher than in corresponding FI schedules. However, the distribution of first-response latencies, mean breakpoints, and normalized response gradients suggest that temporal discrimination was equivalent in the two schedules. [ABSTRACT FROM AUTHOR]