Temporal relations in the concurrent schedule used in Experiment 2. Vertical lines above the solid horizontal arrow indicate reinforcers earned on the FI 60-s schedule; vertical lines below the arrow indicate reinforcers earned on the FI 20-s schedule. Blank spaces after a reinforcer indicate a 15-s intertrial interval (ITI). The reinforcer on the FI 20-s (respectively, the FI 60-s) key was scheduled 20 s (respectively, 60 s) after the beginning of a trial if the last reinforcer had been collected on the FI 60-s schedule (respectively, the FI 20-s schedule). As a consequence, the time between two successive reinforcers (the trial duration; TD) was 20 s on the FI 20-s schedule and 60 s on the FI 60-s schedule, whereas the time between two successive FI 20-s reinforcers or two successive FI 60-s reinforcers (the interreinforcement time; IT) was 95 s. FI = fixed interval.