In some conditions, we also enhanced the discriminability of the stimuli by using keylight color to signal which response was more likely to produce food. Conditions in which the occurrence of the reversal was signaled by elapsed time alone are termed Continuous conditions, because elapsed time is a continuously changing stimulus. In Continuous conditions, the decision to respond on either the L →H or H→ L key would be made on the basis of time elapsed since the last reinforcer. In some conditions, the reversal was additionally signaled by a discrete keylight color change. In these Discrete conditions, both keys were lit red before the reversal, but changed to yellow at the reversal; here, the decision to respond could be made on the basis of time elapsed since the most recent food delivery, and/or the color of the keys. In Early Discrete conditions, the same keylight color change occurred 20 s before the reversal (Early Discrete conditions were arranged only with a 30-s reversal time). Here, the decision to respond on either the L →H or H→ L key would be made on the basis of time elapsed since the last stimulus change (assuming that more recent time markers exert greater control; see Cowie et al., 2014). Table 1 outlines the different reversal times and stimulus types used in each condition.