Effect of coffee on the speed of subject-paced information processing.

A continuously subject-paced and monetarily reinforced information-processing task was used to test whether caffeine might stimulate performance beyond the ceiling imposed by fatigue. A 3 X 2 design was adopted with the three doses 0, 150, and 450 mg caffeine and with decaffeinated coffee and fruit juice as vehicles. Two groups of 10 subjects selected to represent extremes on the extraversion-introversion scale participated in the experiment. The results revealed no differences between the two groups nor between the two drug vehicles, but caffeine at both dose levels improved performance significantly. Fatigue-induced performance decrements across the sessions reached significance, but no interaction was obtained between fatigue and drug action, supporting the view that caffeine can improve performance beyond a mere restoration of fatigue.