The answer is "John Garcia".
During the examinations rats were given one taste, view, sound as a neutral stimuli. Later the rats would be presented to radiation or medications (the unconditioned stimulus), which would influence the rats to get sick. Through these examinations, Garcia found that if a rat ended up noticeably disgusted after given another taste, regardless of the possibility that the sickness happened few hours after the fact, the rat would maintain a strategic distance from that taste.