Conditioned flavour preference negatively reinforced by caffeine in human volunteers

This study examined whether 100 mg caffeine could reinforce preference for the flavour of a novel drink in moderate caffeine users, both after overnight caffeine abstinence and 2 h after receiving 100 mg caffeine, using a two-stage between-groups procedure with 36 volunteers. In the first stage, liking for a test drink (fruit tea) was assessed at breakfast following overnight caffeine abstinence, with half the subjects receiving caffeine. Liking for the tea increased significantly over four trials for subjects receiving caffeine, and decreased significantly in those without caffeine. These effects were greatest in subjects who rated the drink as highly novel. In stage two, subjects evaluated a second drink (fruit-juice) 2 h after receiving the tea, and again half the subjects received caffeine Those subjects who received caffeine in stage two but not stage one showed a significant increase in liking for the fruit-juice over the 4 test days, whereas subjects who did not receive caffeine at either stage showed a progressive decrease in liking for this drink. In contrast, no significant change in liking for the fruit-juice was seen at stage two for subjects who had received caffeine in stage one, regardless of the presence or absence of caffeine at stage two. Caffeine at breakfast increased ratings of energetic and lively, and energetic ratings also increased following caffeine in the fruit-juice in subjects who had not had caffeine at breakfast. Overall, these data are consistent with a negative reinforcement model of caffeine reinforcement, and demonstrate further the utility of the conditioned flavour preference method for evaluating reinforcing effects of drugs in humans.