We manipulated the discriminability of the stimuli signaling which response was more likely to produce a reinforcer in two ways: By changing the reversal time; and by changing the nature of the cue that accompanied the reversal. In some conditions, the reversal occurred 10 s after the most recent food delivery, whereas in others it occurred 30 s after the most recent food delivery. Because error in discriminating elapsed time increases in proportion to the duration to be estimated