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

The purpose of the present study was to develop a procedure for measur ing the within-session onset of human drug discrimination. During dail y sessions, under double-blind conditions, caffeine-abstinent adults i ngested a letter-coded capsule containing 178 mg caffeine or placebo. Trials were presented at 30-s intervals, beginning immediately after d rug ingestion and continuing for 60 min. On each trial, subjects could guess which of their two letter-coded drugs they had received by pres sing a left button (for one drug) or right burton (for the other drug) ; subjects could also press a center ''no guess'' button instead of gu essing. Each trial ended after one burton press. After each session, s ubjects were told which drug they had received. Subjects always earned one point (worth $0.10 per point) for each correct guess. Subjects lo st either 0, 1, or 10 points for each incorrect guess; the point-loss contingencies were varied in random order across sessions. Discriminat ion earnings accumulated across all sessions. The point-loss contingen cies decreased random responding and delayed the discrimination time c ourse. Overall, this procedure provided an orderly and relatively cont inuous measure of the within-session onset of drug discrimination and should have a range of applications in understanding the human behavio ral pharmacology of drugs.