No abstract

Acquisition to a target conditioned stimulus (eS) is prevented when extra, unsignaled unconditioned stimuli (USs) are presented with sufficient frequency to remove contingency between target es and US. Acquisition occurs, however, when the extra USs are signaled by another es. According to the Rescorla-Wagner theory, signaling reduces contextual conditioning, which otherwise prevents acquisition. Results of Experiment lIed to the rejection of a rival explanation derived from scalar expectancy theory by showing that acquisition does not occur when only half ofthe extra USs are signaled. The results of Experiment 2 were, however, contrary to the RescorlaWagner theory because they showed equivalent acquisition when the stimulus used to signal the extra USs was also present concurrently with the target es. Signaling may exert its effect by converting the intertrial interval to es-.

Pigeons were autoshapcd with a keylight as the conditioned stimulus (CS) and food as the unconditioned stimulus (US). Preexposure to repeated US presentations was followed by training sessions in which a single US preceded a single CS-US pairing. Preexposure blocked conditioning to the CS only when the interval between the prior US and the US in the CS-US pairing (critical interval) was equal to the US-US interval in preexposure. Blocking was examined as a function of the length of the critical interval and the amount of preexposure. The hypothesis that blocking occurs because the prior US predicts the time of arrival of the US in the CS-US pairing was supported by a ruduclion in blocking when USs separated by intervals longer than the critical interval were added to preexposure sessions. Certain other interpretations of these results were tested and rejected. This research, undertaken in partial fulfillment of the requirements for M. J. Goddard's PhD degree (1987) at McMaster University, was supported by a Natural Sciences and Engineering Research Council of Canada grant to H. M. Jenkins. We are indebted to Professor John J. B. Ayres for critical comments on the thesis and to Maureen McCrackin for assistance in running the experiments.

No abstract

In Pavlovian conditioning, organisms can learn that a conditioned stimulus (CS) signals the delivery of an unconditioned stimulus (US). The present paper first reviews research showing that organisms can also learn that a stimulus, normally considered to be a US, signals the delivery of another US. Second, the paper shows how such signal value may contribute to three conditioning phenomena that are of interest to many psychologists: contingency, drug conditioning, and learned helplessness. In addition to showing how such signal value may contribute to these three phenomena, suggestions for further research, as well as current gaps in the knowledge base, are outlined.