A partial reinforcement extinction effect despite equal rates of reinforcement during Pavlovian conditioning

In 4 experiments rats received appetitive Pavlovian conditioning followed by extinction. Food accom-panied every trial with the conditioned stimulus (CS) for the continuously reinforced groups and only half of the trials for the partially reinforced groups. In contrast to previous experiments that have compared the effects of partial and continuous reinforcement, the rate at which food was delivered during the CS was the same for both groups. The strength of the conditioned response during extinction weakened more rapidly in the continuously than in the partially reinforced groups. The results demonstrate that the partial reinforcement extinction effect is a consequence of the nonreinforced trials with the CS, rather than the rate at which the unconditioned stimulus is delivered during the CS. For many years the partial reinforcement extinction effect (PREE) was a mainstay of research in the field of animal learning and behavior. Within the context of instrumental conditioning, the PREE can be defined as a resistance to the detrimental effects of extinction following trials in which responding is intermittently rather than consistently paired with reinforcement (for a review see Mackintosh, 1974). So widespread was the PREE in studies of instrumental conditioning—and such was the paucity of evidence for the effect within Pavlovian conditioning—that for some time the PREE was thought to differentiate these two forms of condi-tioning (Kimble, 1961). However, the possibility that the PREE contradicted the single-process rule of learning (Guthrie, 1935; Hull, 1943) has not stood the test of time. Many studies of Pavlovian conditioning have now demonstrated that a conditioned stimulus (CS) that is intermittently paired with an unconditioned stimulus (US) will show a resistance to extinction relative to a CS that is consistently paired with a US (e.g., Fitzgerald, 1963