The effectiveness of external and drug produced internal stimuli in the discriminative control of operant behavior

A comparison was made between the effectiveness of external cues and drug produced internal cues to exercise stimulus control over food reinforced operant behavior in the albino rat. Two external stimulus groups used either the presence or absence of a change in illumination provided by turning on two cue lights or the presence or absence of a 1000 Hz. tone as cues. Two drug groups whose cues were provided by the ip. injection of 1.0 mg/kg dl-amphetamine sulfate or saline in one group and 10.0 mg/kg sodium pentobarbital or saline in the other were used. A mixed schedule control group was also included. Two operant discrimination procedures were utilized. The first was a two lever multiple schedule with each of the two cues in each group associated with responding on one of the levers. The animals were reinforced on a differential reinforcement of low rate (DRL) 15 seconds schedule for correct lever responses. Schedule components changed randomly on a session to session basis. Following 32 days of training, tests for discriminative control over lever choice were made with each cue condition. These tests were carried out during extinction. The results of these tests indicated that the control group and the external stimulus groups did not differ, whereas both drug groups demonstrated more effective stimulus control when compared to the other groups. The Ss were then transferred to a two lever mixed schedule of reinforcement during which correct lever responses were reinforced on a variable interval (VI) 30 seconds schedule of reinforcement. Following 16 days of this interpolated training during which most Ss attained a high rate of responding, probe tests for residual stimulus control were carried out during extinction. For these tests the presence of the cue used during original training was reinstated. Stimulus control by the external stimuli was again not observed. The pentobarbital produced cues demonstrated weak but significant stimulus control over lever choice. The amphetamine produced cues demonstrated strong residual stimulus control over both lever choice and response patterning as evidenced by the paced responding on the drug-appropriate lever characteristic of DRL extinction. The results of this study were discussed in terms of the importance of reinforcement feedback cues which apparently interfered with the development of stimulus control by the external stimuli. A second study was carried out to see if these external stimuli were capable of becoming effective cues in this paradigm. In addition, it was suggested that more frequent changes in cue conditions during original training would diminish the role of reinforcement feedback and thus result in more effective stimulus control. This proved to be the case, although stimulus control still proved to be weak and no evidence was seen for residual stimulus control for these stimuli. Possible explanations for the superior effectiveness of drug cues in this experiment are discussed and the relationship of this finding to a conditioning model of drug abuse was considered.Psychology, Department o