Enhancing caffeine reinforcement by behavioral requirements following drug ingestion

Each morning eight adults with caffeine versus placebo discrimination histories ingested letter-coded capsules containing 100 mg caffeine or placebo and then engaged in a relaxation or vigilance activity. Subjects were first exposed to caffeine and placebo once each with each activity. Then each day for 10 days subjects made two choices; they chose which compound they would prefer if vigilance were scheduled and which they would prefer if relaxation were scheduled, with the restriction that they could not choose the same compound with both activities; only one choice (randomly selected) was reinforced. Eight of eight subjects always chose caffeine with vigilance. The next choice condition was identical, except that subjects were free to take either compound with both activities. Six of six subjects reliably chose caffeine with vigilance. Four reliably chose placebo with relaxation. In the final condition, each day for 10 days subjects chose between each drug and each of 52 monetary values; those choices were made separately for vigilance and relaxation; only one choice (randomly selected) was reinforced. For six of seven subjects, the maximum dollar value at which subjects chose drug over money was higher for caffeine in vigilance than for placebo in either activity. For five subjects, the maximum value at which subjects chose caffeine over money was higher in vigilance than in relaxation. Overall, this study demonstrates enhanced caffeine reinforcement when a vigilance activity followed drug ingestion.