A PROCEDURE FOR STUDYING THE WITHIN‐SESSION ONSET OF HUMAN DRUG DISCRIMINATION

The purpose of the present study was to develop a procedure for measuring the within‐session onset of human drug discrimination. During daily sessions, under double‐blind conditions, caffeine‐abstinent adults ingested a letter‐coded capsule containing 178 mg caffeine or placebo. Trials were presented at 30‐s intervals, beginning immediately after drug ingestion and continuing for 60 min. On each trial, subjects could guess which of their two letter‐coded drugs they had received by pressing a left button (for one drug) or right button (for the other drug); subjects could also press a center “no guess” button instead of guessing. Each trial ended after one button press. After each session, subjects were told which drug they had received. Subjects always earned one point (worth $0.10 per point) for each correct guess. Subjects lost either 0, 1, or 10 points for each incorrect guess; the point‐loss contingencies were varied in random order across sessions. Discrimination earnings accumulated across all sessions. The point‐loss contingencies decreased random responding and delayed the discrimination time course. Overall, this procedure provided an orderly and relatively continuous measure of the within‐session onset of drug discrimination and should have a range of applications in understanding the human behavioral pharmacology of drugs. 1994 Society for the Experimental Analysis of Behavior