In each session, a series of trials was arranged, during which two keys were lit and available. An exponential (constant probability) VI schedule arranged food deliveries at the rate of 1.33 foods/min. Each trial ended in a food delivery, and the subsequent trial started immediately after the food delivery ended. Thus, the end of food marked the start of a new trial, but food deliveries did not themselves signal additional information about the current contingencies. Because food deliveries were arranged on a VI schedule, trials were not fixed in duration. The VI schedule was held constant across conditions, and hence the average trial duration also remained constant across conditions.