Caffeine and stroop interference.

In two experiments the hypothesis that caffeine reduces Stroop interference was tested. In the first experiment interference was measured as the reduction in performance when subjects had to indicate the numerosity of strings of incongruent digits, relative to neutral-symbol strings. In the second experiment the incongruent condition consisted of naming the color of words referring to incongruent colors, and was compared to color naming of neutral strings. In the number-digit task 250 mg caffeine reduced interference at the level of error rates, relative to placebo. In the color-word task interference was reduced at the level of reaction times. These results were obtained with blocked presentations of incongruent and neutral conditions. The color-word experiment also contained a condition in which neutral and incongruent trials were mixed within one sequence. In this mixed condition caffeine still reduced overall reaction times, but no longer specifically interference. It is argued that this dissociation reflects a caffeine-induced increase in flexibility. The results are discussed in relation to failed previous attempts to demonstrate increased selectivity under caffeine using non-Stroop tasks, the importance of including pretreatment sessions to detect artificial effects, and the possible contribution of withdrawal effects.