Figure 2
FIG. 2. Oral self-administration of caffeine capsules in each of three male volunteers with histories of heavy coffee drinking who resided in a residential laboratory. Subjects had 13 opportunities each day to self-administer either a caffeine (100 mg) or a placebo capsule. Vertical lines indicate a change in condition: Either novel double-blind color codes associated with caffeine and placebo capsules were introduced or a period of capsule unavailability was initiated. (Redrawn from ref. 50.)

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published 2000