Between conditions, we changed either the left or right location of the L →H key (Fig. 2), or the reversal time (Fig. 3). Tran-sitions involving a change in the location of the L →H key (Fig. 2) were termed Same-Discriminability transitions, because the stimulus conditions remained the same across consecutive conditions. The reversal time remained unchanged fol-lowing a Same-Discriminability transition. Transitions involving a change in the reversal time, and hence a change in the discriminability of the stimulus conditions (Fig. 3), were termed Different-Discriminability transitions. For these tran-sitions, the physical location of the H→ L key remained unchanged between conditions, but the reversal time was either increased from 10 s to 30 s, or decreased from 30 s to 10 s. In Fig. 3, an increase in the reversal time would be a change from the contingencies in Condition N to those in Condition N + 1, whereas a decrease in the reversal time would be a change from the contingencies in Condition N + 1 to those in Condition N. Thus, the stimulus conditions became either less (10-to–30 s Different-Discriminability transition) or more (30-to-10-s Different-Discriminability transition) discriminable. Table 1 outlines the nature of the transition between each of the conditions.