John Garcia provided one type of stimulus of sight, taste and sight to the rats. This process is called neutral stimulation. After several months he exposed the rats with different drugs and different type of radiation. He named these stimuli as unconditional stimuli. Due to this treatment rats became sick.
Through this experiment he discovered that if rates provide with new stimuli different than neutral stimuli, they may become sick but after recovery from nuisance they started avoiding that unconditional stimuli.
These results challenged the belief of strict behaviorists that unconditional stimuli play little influence in how organisms could be conditioned.