A pigeon is presented with a blue key and a red key alternatively. In the presence of the blue key, it earns food on a VI 30-sec schedule. In the presence of the red key, it earns food on a VI 15-sec schedule. The VI 30-sec schedule is then changed to a VI 60-sec schedule. As a result, the rate of response on the VI 15-sec schedule , illustrating a contrast effect.