Associate learning can be studied in the conditioned-response experiment originated by the Russian physiologist and Nobel Prize winner Ivan Pavlov. White studying the relatively automatic reflexes associated with digestion, Pavlov noticed that a dog salivated not just when food was placed as an unlearned response, or, as he called it, an unconditioned response.
But surely, he light, the response to the sight of food has to be learned or conditioned response. Pavlov taught to salivate to various signals, such as the onset of a light or tone, thereby proving to his satisfaction at a new stimulus-response association could be formed in the laboratory.