International Journal of Comparative Psychology

The experiment examined interval timing in rats during a momentary, unsignaled, increase in the time to reinforcement. A session began with intervals programmed according to a fixed interval (FI) 60 s reinforcement schedule, changed to either an FI 90 s, FI 120 s, or FI 180 s schedule at an unpredictable point, and then returned to an FI 60 s schedule after 1, 8, or 24 successive long intervals had elapsed. Overall, postreinforcement wait time duration increased with increases in the scheduled time to reinforcement. The amount by which wait time increased did not depend on the duration of longer intervals, but did depend on the number of longer intervals intercalated into a session. The results indicate that rats are sensitive to moment-by-moment changes in the time to reinforcement and support other studies showing an asymmetry between timing upward and downward shifts in the criterion for reinforcement of an FI schedule.