Refer to Figure 1 below. Starting at the top and moving from left to right, say the ink color of each word. You’ll find that the task becomes much slower and takes more effort after the first two lines. The later lines are more challenging because irrelevant information at a semantic level—the meaning of the word—interferes with naming the color of the ink. Such interference in cognitive processing is known as the Stroop effect (after John Ridley Stroop, the discoverer of the phenomenon). We apparently still process the word that we’re trying so hard to ignore, implying that accurate attention to the target and exclusion of the interfering information involves late attentional selection (MacLeod, 1991). Critics argue that in this procedure it is impossible to process the relevant cue (color) without simultaneously processing the irrelevant cue (the word) because they make up the same single stimulus. However, a Stroop-like effect is observed even when the relevant and irrelevant cues are physically separated—that is, when blocks of color to be named are simply placed near the words, all of which are written in black.
MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative review. Psychological Bulletin 109: 163–203.