The overshadowing of instrumental conditioning by a stimulus that predicts reinforcement better than the response

In an experiment designed to demonstrate the overshadowing of appetitive instrumental conditioning, three groups of rats were given 10 sessions of RI (random interval) training in which reinforcement was delivered 450 msec following the response. The correlated group experienced a stimulus during the response-reinforcer delay interval, while an uncorrelated control group experienced a similar brief stimulus occasionally following responses, but these were not responses that typically produced reinforcement. A no-stimulus control group did not experience the brief response-produced stimulus. The experiment was run in two replications. The first employed a light as the critical stimulus, the second a tone. Over the 10 RI sessions, subjects in the two control conditions increased their rate of responding significantly faster than subjects in the correlated condition in both replications. This finding was interpreted as an instance of the overshadowing of the acquisition of signal value by a response because of the presence of the stimulus, which, in the correlated condition, was a more reliable predictor of reinforcement than the response. A subsequent conditioned reinforcement test confirmed that, in the correlated condition, the stimulus had, indeed, become a signal for reinforcement as a function of RI training.