Surprise and the attenuation of blocking.

Three experiments, employing conditioned suppression in rats, examined the extent to which pretraining on one element of a compound stimulus blocked conditioning to the other element when some feature of the reinforcer was changed on compound trials. In Experiment 1, both the addition of an unexpected second shock 8 sec after the end of each trial and the post-ponement of an expected second shock from 4 to 8 sec attenuated blocking. Experiment 2 established that this latter unblocking effect was due to the surprising postponement of the second shock on trials when the new ele-ment was added rather than to any generalization decrement from pre-training to compound training. Experiment 3 showed that blocking was attenuated not only by the addition of an unexpected second shock but also by the omission of an expected shock. In a series of experiments on conditioned suppression in rats, Kamin (1968, 1969) showed that the amount of conditioning ac-cruing to one element of a compound condi-tioned stimulus (CS) over a series of rein-forced compound trials was affected by the subject's prior experience with the other ele-ment. If one element had previously been paired with shock, it might severely attenuate or, in Kamin's words, block conditioning to the other element. Kamin interpreted his results to mean that conditioning was dependent on the predict-ability of reinforcement: No conditioning accrued to the added element on compound trials because reinforcement was already fully predicted by the presence of the pretrained element. Only surprising reinforcers are re-inforcing. Rescorla and Wagner (1972) have proposed a model of Pavlovian conditioning which is intended to capture at least part of this idea. They assume that a given reinforcer can support only a certain amount of conditioning (represented by a parameter, X) ; and increments to the associa-