Stimuli which are present when a response is reinforced become the occasion for this response on subsequent presentations of these stimuli. When a critical stimulus is altered, a decrement in the response is usually observed. A generalization gradient is produced, and the amount of decrement is a function of the degree of change in the stimulus. That internal stimuli become the occasion for responding has been demonstrafed by the classical "drive discrimination" studies (HULL, 1933; LE~a, 1935) in which animals were trained to make different responses based on "drive" stimuli, inferred from food or water deprivation. The use of chemical agents provides a direct and relatively rapid means of altering internal stimuli and makes possible a more precise evaluation of their role in the formation of stimulus-response-reinforcement relationships.
A response acquired in the presence of a particular set of internal conditions should occur most frequently when these conditions are repeated. Changing these conditions by injection of drugs should result in a response decrement. Likewise, a response acquired under a given drug condition should occur less frequently when this drug is not present. The experiment reported here was designed to investigate these relationships by training rats to respond under particular drug conditions (morphine, dl-amphetamine or placebo) and comparing the number of ex. tinction responses made when these conditions were held constant, with the number made when the drug conditions were changed.
Method
Subjects. Ninety-eight male, }Vistar strain albino rats between four and five months old were used as subjects. Eighteen were used to determine the time course of the effects of each of the drugs, while * This paper is based on a Ph.D.