Rats perceived pairings of a light + tone compound conditioned stimulus (CS) with either a high- or low-value food unconditioned stimulus (US). Conditioning to the tone was blocked if the light had previously been paired with the same-value US. Considerable conditioning occurred to the tone if the light had been paired with the other-value US before compound conditioning. However, if after conditioning of the light, the compound was first paired with the same-value US, then subsequent pairings of the compound with the other-value US generated little conditioning to the tone. These results supported theories of blocking and unblocking that emphasize changes in the processing of the CS.
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