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The dogs used in conditioned response experiments by a Russian scientist of the late nineteenth century, Ivan Pavlov. In these experiments, Pavlov sounded a bell while presenting food to a dog, thereby stimulating the natural flow of saliva in the dog's mouth. After the procedure was repeated several times, the dog would salivate at the sound of the bell, even when no food was presented.
Note: Someone who reacts instinctively rather than reflectively to a situation is said to be engaging in a Pavlovian reaction.