Stroop Color Naming

J. Ridley Stroop, a theologist with a side interest in psychology, sought an experimental method that would enable him to measure the interference of one stimulus dimension on attempts to process another. In

1935, he published a seminal paper describing three experiments. The second experiment asked subjects to name the color of the ink of color words (e.g., the word "red" printed in green ink) or the color of colored squares. The first condition is called "incongruent" and the second "neutral." Stroop found that the words interfered with naming the color. That is, RT to the incongruent condition was slower than RT to the neutral condition. Later, researchers added a congruent condition to the task (e.g., the word "green" printed in green ink). The congruent condition enabled one to look at facilitation, which is the difference in RT between congruent and neutral trials. It is commonly found that although facilitation in the congruent condition is small and on many occasions not significant, interference in the incongruent condition is robust and significant. Since 1935, this type of conflict has been studied extensively using variations of Stroop's color-naming task and in other Stroop-like situations with a wide variety of different stimuli and task demands. All studies converge on the conclusion that people cannot suppress the irrelevant dimension if it is heavily practiced and consequently overlearned (like reading words). Hence, the Stroop effect is considered a powerful example of automatic processing.

Nevertheless, several studies have shown that readers can modulate and partially control the impact of the word. Increasing the proportion of congruent trials relative to incongruent trials produces a larger Stroop effect. Moreover, even when the numbers of congruent and incongruent trials are kept constant while the proportion of neutral trials changes, the effect can be altered. It seems that the expectation to face a relatively large proportion of conflict trials prompts the adoption of a strategy that helps reduce interference. In addition, it seems that language competence may modulate the effect. When bilinguals are tested, under certain conditions they can reduce the effect in their first language but not in their second language. Note that they experience interference in both their first and their second language, but they are better able to control reading (i.e., reduce Stroop interference) in the language in which they are more competent.

Neuropsychological studies of brain-injured individuals suggest that the left frontal lobe is crucial to successful performance of the Stroop task. In particular, it has been reported that injury to the left dorsolateral prefrontal cortex results in enlarged Stroop interference. This result suggests that the Stroop interference presented by noninjured individuals is an underestimate of the potential interference.

Stroop interference presented by the noninjured individuals is the product of automatic intrusion of the irrelevant word and their ability (admittedly not perfect) to inhibit this reading. In addition, it points to the involvement of the left dorsolateral prefrontal cortex in the control processes by which Stroop interfence is modulated. Consistent with this lesion evidence, neuroimaging studies of blood flow and changes in blood oxygenation during task performance show that the incongruent condition of the Stroop task activates left dorsolateral prefrontal cortex more than the neutral or congruent conditions. Even more noticeable in neuroimaging studies is differential activation of the anterior cingulate gyrus. Barch, Braver, Sabb, and Noll suggested that this medial-frontal structure is also active in a variety of other tasks in which selections must be made among competing stimuli, stimulus properties, and responses to them.

Of course, the crucial question in the present context is whether resolution of conflict in the Stroop task involves inhibition. Many theorists interpret the task in this way, although a well-known computational model of Stroop performance suggested by Cohen, Dunbar, and McClelland is able to account for many aspects of performance in the Stroop task by facilitation of the less automated process of color naming rather than inhibition of the more automated process of word reading. The phenomenon discussed next offers a more demanding and therefore more analytic test.

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