Abstract
Eight rats were trained on a variable-interval schedule in the presence of a light (Phase I). Responding was then extinguished in the presence of darkness that alternated with the light (Phase II). Reinforcement was then introduced in the presence of darkness (Phase III). Several rats were then returned to the condition of Phase II (Phase IV) and then to that of Phase III (Phase V). The responding of most rats showed clear behavioral contrast in Phase II-i.e., an increase in responding in the presence of the light. When, for three rats, Phase III was introduced early after the occurrence of positive contrast, either positive induction occurred, i.e., an increase in responding in the presence of the light, or there was little change. Negative contrast did not occur. It was further shown that positive contrast dissipates over time, thus replicating a result previously obtained with pigeons and that the positive induction effect seems also to dissipate over time. The introduction of reinforcement in the presence of darkness (Phases III and V) after the dissipation of positive contrast seemed to have little consistent effect.