Conditioning of a discriminative drug stimulus: Overshadowing and blocking like procedures

Interactions between a drug discriminative stimulus ( D , 17.5 mg/kg of pentobarbital vs. N , saline) and exteroceptive stimulus conditions (light vs. dark) were examined in a T-maze, shock-escape task using conditioning procedures pertaining to the phenomena of "overshadowing" and "blocking". From an operational point of view, in the "overshadowing" procedure the stimulus compound is introduced from the outset of the training, whereas in the "blocking" procedure the stimuli components are introduced sequentially. When the compound discriminations were established, dose-generalization tests with various doses of pentobarbital (range 5.6–17.5 mg/kg) as well as saline (1 ml/kg) were carried out under both elements (light and dark) of the exteroceptive stimulus dimension. Prior training with drug ( D vs. N ) neutralized the potential influence of the exteroceptive dimension (light vs. dark); conversely training with the exteroceptive stimuli prior to the drug training accentuated the control over behavior by the visual stimuli. Dose-generalization results intermediate to those described above were observed in the group trained to discriminate the stimulus compound ( D plus light vs. N plus dark) from the outset of training. It may therefore be concluded that exteroceptive stimuli can compete with interoceptive drug stimuli for associative strength in the procedure used. Thus, the formal similarities between drug discriminative stimuli and more conventionally studied exteroceptive, sensory signals are furthered by the data.