A shock that's already explained teaches a rat nothing new

Leon Kamin trained rats until a tone reliably predicted a shock, then added a light alongside that same tone-shock pairing. Tested alone, the light produced almost no fear response — the tone had already explained the danger, so there was nothing left to learn. Kamin's 1968 blocking effect showed conditioning runs on surprise and new information, not just repeated pairing.

— Leon J. Kamin, "Attention-like" Processes in Classical Conditioning, in Miami Symposium on the Prediction of Behavior: Aversive Stimulation — University of Miami Press, 1968