Stimulus selection in the rat

Comparison of “blocking” and “overshadowing” effects in compound conditioning

Abstract

Three experiments examined the competing effects of CS salience and pretraining upon stimulus selection in rats (Wistar strain) by measuring conditioned suppression of water licking. Rats were initially conditioned to a faint tone (stage 1), and then were shifted to conditioning to a salient light compounded with the tone (stage 2). At the final testing stage the two stimuli were presented separately without US. In Experiment I, the tone suppressed licking only incompletely at the end of the pretraining stage. The results of the test suggest that the presence of light through stage 2 reduced the amount of conditioning to the tone. The tone, on the other hand, did not block the learning to the light. Experiment II, in which the number of pretraining trials was increased to produce more suppression to the weak tone, showed the same results as those of Experiment I. In Experiment III, in which the intensity of light was reduced to the level that it could not overshadow the tone, the pretrained tone effectively blocked the conditioning to the light during compound conditioning. Possible explanations of these findings were discussed.