Selective attention is a fundamental cognitive ability. Focusing on one stimulus while ignoring another is an ability studied by many. J. Ridley Stroop (1935) found subjects to exhibit great interference and errors when asked to name the ink color of a color-word; however, they showed almost no interference when asked to read the color-word printed in a different ink color. This phenomenon has become known as the stroop effect, and has been widely studied since.
The results of the stroop effect have been replicated many times in many variations. A study by Liotti et al. (2000) found significant effects between congruent (same color-word and ink color) and incongruent (different ink color than color-word) trials using a mixed-trial stroop task. Similar results were found in an auditory stroop effect task (Barber and Green, 1981). With stimuli presented on the basis of the speed of speaker gender judgments, the study found significant results supporting the reliability and robustness of the stroop effect.
While many studies have replicated the findings of the stroop effect, the cause of the phenomenon is highly debated. Several theories have been proposed, but there are two that seem plausible. The first is the response-competition hypothesis (Klein, 1964). In order for one to give the correct response on an incongruent stimulus, one would have to distinguish between two possible responses, the ink color and the word. This determination requires extra time resulting in the delays found in incongruent trials. Morten and Chambers (1973) expanded on such a late-selection theory claiming that the processing of the color and word occurs in parallel, but at different speeds. Since the word is processed faster it reaches the lone response channel first and is thus accepted. This theory accounts for the mistakes found in incongruent trials. Cohen et al. (1990) also proposed a parallel processing model in which the word travels faster due to the greater practice of word reading.