In Ivan Pavlov’s classical-conditioning experiments, dogs learned to salivate when they heard a bell.
Pavlov originally studied digestion and noticed that dogs began salivating before food reached their mouths. He then repeatedly paired a previously neutral stimulus, such as a metronome or bell, with food. After enough pairings, the sound alone produced salivation.
Food naturally causes salivation, so it was called the unconditioned stimulus, and the resulting salivation was an unconditioned response. After learning, the bell became a conditioned stimulus and salivation became a conditioned response. The experiment established a major form of associative learning.
Popular accounts often say Pavlov used only a bell, but his laboratory used several signals, including metronomes. The dogs did not learn to salivate because the sound directly produced hunger; they learned an association between the sound and the expected food.