Blocking effect

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In Kamin's blocking effect^[1] the conditioning of an association between two stimuli, a conditioned stimulus (CS) and an unconditioned stimulus (US) is impaired if, during the conditioning process, the CS is presented together with a second CS that has already been associated with the unconditioned stimulus.

For example, an agent (such as a mouse in the figure) is exposed to a light (the first conditioned stimulus, CS1), together with food (the unconditioned stimulus, US). After repeated pairings of CS1 and US, the agent salivates when the light comes on (conditioned response, CR). Then, there are more conditioning trials, this time with the light (CS1) and a tone (CS2) together with the US. Now, when tested, the agent does not salivate to the tone (CS2). In other words, an association between the tone CS2 and the US has been "blocked" because the CS1–US association already exists.

This effect was most famously explained by the Rescorla–Wagner model. The model says, essentially, that if one CS (here the light) already fully predicts that the US will come, nothing will be learned about a second CS (here the tone) that accompanies the first CS. Blocking is an outcome of other models that also base learning on the difference between what is predicted and what actually happens.^[2]