Drug effects on the repeated generalization of a visual discrimination acquired under one trial per day conditions

Six pigeons were trained to respond under concurrent fixed-ratio extinction schedules of food reinforcement (FR 50) on the side keys of a three key chamber. Presence or absence of continuously-flashing, colored lights on the center key signalled which key pecking response (left or right) would be reinforced over the course of an entire experimental session; thus, the lights were analogous to a drug cue in drug-discrimination situations. In test (generalization) sessions of one FR in duration, the percentage of light on the center key was varied from 0 to 100%. During these sessions, saline, lysergic acid diethylamide (LSD; 0.04-0.20 mg/kg) and morphine (3.75 mg/kg) produced orderly, log-linear light generalization gradients, but only LSD shifted the gradient to the right (by approximately 50%). These results demonstrate the potential value of the method for assessing the effects of drugs on both exteroceptive (light) and interoceptive (drug) stimulus control. The method has the additional advantage that the effects of drugs on such control (i.e., on stimulus generalization) are relatively unaffected by corresponding effects on response control (e.g., rate of responding).