Discrimination of fixed-interval schedules by humans

A matching procedure was used to examine whether humans discriminate fixed-interval (FI) schedules. Fixed-ratio (FR), FI, or differentialreinforcement- of-low-rate (DRL) schedules were in effect during a sample schedule link. Responses meeting the schedule requirement produced a choice link in which each of three circles was presented with a unique color. The choice was reinforced depending on the schedule of the sample link. In Experiment 1, a correct response equal to or higher than 94.4% was obtained for each sample schedule in each of the four undergraduates. Post hoc analyses suggested that time spent with the sample schedule link may have functioned as a discriminative stimulus of the choices. In Experiment 2, after replicating the schedule discrimination with additional four participants, the FI value was changed to equate to the mean time spent in the FR or DRL schedule link. With this modified FI value, the accuracy was deteriorated but eventually recovered, whereas the lengths of time spent in the sample schedule links diverged across the schedules. Previous studies have shown that humans were insensitive to reinforcement schedules, especially to the FI schedule. These results demonstrate that humans discriminated the FI schedule and suggest that human schedule insensitivity is due to other than discriminative properties of the schedules.