Blocking Effect

In Kamin's blocking effect, the conditioning of an association between two stimuli, a conditioned stimulus and an unconditioned stimulus, is impaired if, during the conditioning process, that conditioned stimulus is presented together with a second conditioned stimulus that has already been associated with the same unconditioned stimulus. In a typical demonstration, an animal is first conditioned to associate a light with food until it reliably salivates at the light alone, and only then is a tone added alongside the light in further trials; when tested afterward, the animal does not salivate to the tone by itself, because the light had already come to fully predict the food and blocked anything being learned about the added tone. The effect is best explained by the Rescorla-Wagner model, which holds that once one conditioned stimulus already fully predicts an outcome, nothing further is learned about a second stimulus introduced alongside it.