Pecking was maintained on each of two keys by a different, food-associated, conditioned reinforcer, orange illumination of the chamber for the left key and blue for the right key. Then, with pecking prevented, the pigeons were introduced directly into alternating orange and blue illumination and were shocked electrically only during orange. After this procedure, with pecking allowed but without further presentations of either conditioned reinforcer, the rate of pecking on the left of the two keys was selectively decreased.
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