The Stroop effect is named after John Stroop, who published his study describing this effect in 1935 in the “Journal of Experimental Psychology.” In his test, the subject is presented with randomized color names such as blue, red and green that are printed in colored ink. The subject’s task is to name the color in which the words are printed while ignoring their verbal content. It has been shown that when the name of a color is printed in a color that does not match its name, it takes much longer for the subjects to name the word and subjects make more mistakes than when the color of the ink matches the name of the color. The Stroop test measures selective attention and how easily a person can suppress a habitual response, such as reading, in favor of a less familiar task, such as naming the color.
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Anterior Cingulate Cortex
Imaging methods such as MRI and PET have shown that the Stroop test causes the subject’s frontal lobe to become activated during the task. Mainly two different areas in the frontal lobe get activated. These are the anterior cingulate cortex, or ACC, and the dorsolateral prefrontal cortex, or DLPFC. Both of these areas are responsible for conflict monitoring and resolution. When there is competing information, ACC is used to select an appropriate response and allocate attentional resources. This has been demonstrated, for example, by G. Bush and colleagues, who published their study in “Human Brain Mapping” journal in 1998. The subjects of this study were performing the Stroop task while their brain activity was measured by an MRI scanner. It was discovered that there was a significant activity over the ACC area during the task. Interestingly, as the subjects learned to do the task better, the activity in this area diminished.
Left Dorsolateral Prefrontal Cortex
Another area that has been shown to be crucial for the Stroop task is the dorsolateral prefrontal cortex, or DLPFC. This area increases attention to task-relevant information so that future conflicts could be avoided. Interestingly, left and right sides of the DLPFC seem to have different functions. M.A. Vanderhasselt from Ghent University, Belgium, reviewed articles on DLPFC and the Stroop task and published the findings in the “Psychonomic Bulletin & Review” journal in 2009. The authors concluded that according to most studies the challenging tasks and complicated instructions caused left DLPFC activation in the subjects while they were performing Stroop tasks. The authors stated that the left DLPFC activation was related to participants’ expectancies regarding the conflicting nature of the upcoming trial, and not so much on the conflict itself.
Right Dorsolateral Prefrontal Cortex
The right DLPFC, on the other hand, seems to aim to reduce attentional conflict. It is typical for this area to get activated after a conflict. Right DLPFC aims to reduce the effect of all the irrelevant information on task performance, as stated by Vanderhasselt. It seems that the right DLPFC activation is more for general tasks than the left DLPFC. For example, in their blood flow study published in the “Neuroimage” journal in 1997, S.F. Taylor and colleagues from the University of Michigan stated that activation in the left DLPFC reflected processing more specific to the Stroop task, while right DLPFC activation was more nonspecific.