An analysis of overshadowing and blocking

In classical aversive conditioning experiments, rats do not always learn about all aspects of a compound stimulus predicting shock. A strong stimulus may overshadow a weaker one; and pretraining on one component may block learning about a second component. These results have been explained either by appealing to a notion of selective attention, or by assuming that learning about one component is a function of prior response strength to the entire compound of which it forms a part. In Experiment I, overshadowing was demonstrated on the first trial of conditioning, i.e. before either component had acquired any response strength. In Experiment II, pretraining on one component resulted in complete failure to learn about a second component during compound training, but did not prevent additional learning about the first component. Both results were interpreted as supporting an attentional analysis of blocking and overshadowing.