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 reduc tion in performance when subjects had to indicate the numerosity of strings of incongruent digits, relative to neutral-symbol strings. In the second e xperiment the incongruent condition consisted of naming the color of words referring to incongruent colors, and was compared to color naming of neutra l 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 inte rference was reduced at the level of reaction times. These results were obt ained with blocked presentations of incongruent and neutral conditions. The color word experiment also contained a condition in which neutral and inco ngruent trials were mixed within one sequence. In this mixed condition caff eine still reduced overall reaction times, but no longer specifically inter ference. It is argued that this dissociation reflects a caffeine-induced in crease in flexibility. The results are discussed in relation to failed prev ious 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. (C ) 1999 Elsevier Science Inc.