The process of increasing an animal’s tolerance to a particular stimulusStimulus A change in the environment. If the stimulus has no effect on the animal, it is a neutral stimulus. A stimulus that stands out in the environment, that the animal notices more than other environmental stimuli, is a salient stimulus. A stimulus that causes a change of state in the animal, that causes him to perform a specific behavior, for example, is a discriminative stimulus. by gradually increasing the presence of the stimulus.