Interaction between discrimination of drug states and external stimuli.

Rats learned a two-choice operant response by discriminating differences between external stimuli, internal (drug-produced) stimuli, or a combination of these two types of stimuli. Separate groups of rats were used for each stimulus condition. A tactile and visual external cue was superior to the ethanol-saline cue in producing stimulus control, but the group receiving both drug and external stimulus cues performed in a manner very similar to the external cue-only group. The two stimulus sources thus did not "add" to promote more rapid or complete discrimination. After acquisition of discrimination, previously coincident drug and external stimulus states were reversed to determine which stimulus source had more behavioral control. This test for stimulus selectivity indicated that the external stimulus had essentially complete control of response choice.