Stroop effect

Demonstration

Say aloud the colors of these each of these words, as fast as you can:

''' Green Red Blue Yellow Blue Yellow '''

''' Blue Yellow Red Green Yellow Green '''

If naming the first group of colors is easier and quicker than the second, then your performance exhibits the Stroop effect.

In psychology, the Stroop effect is a demonstration of interference in the reaction time of a task. When a word such as blue, green, red, etc. is printed in a color differing from the color expressed by the word's semantic meaning (e.g. the word "red" printed in blue ink), a delay occurs in the processing of the word's color, leading to slower test reaction times and an increase in mistakes. The effect is named after John Ridley Stroop who first published the effect in English in 1935. The effect had previously been published by Jaensch in 1929, but only in German. The original paper has been one of the most cited papers in the history of experimental psychology, leading to over 700 replications.

Original experiment
In his experiment, J. Ridley Stroop administered several variations of two main tests. Stroop referred to his tests as RCN, to stand for "Reading Color Names", where participants were required to repeat the written meaning of words with differing colored fonts, and NCW, to stand for "Naming Colored Words", in which participants were asked to verbally identify the color of each printed color name. Additionally Stroop tested his participants at different stages of practice with each task, to account for the effects of association. Stroop identified a large increase on the time taken by participants to complete the NCW (Naming Colored Words) tasks, an effect still pronounced despite continued practice at each task. This interference is thought to have been caused by the automation of reading, where the mind automatically determines the semantic meaning of the word, and then must override this first impression with the identification of the color of the word, a process which is not automatized.

Further development
Edith Kaplan's group (developer of the Delis-Kaplan neuropsychological test battery) developed the task further by separating the task into four stages: naming color fields, congruent color words, incongruent color words, and combined. The additional strain on the executive function of the brain allows for a more precise diagnosis.

The Stroop task (effect) is also employed to study frontal function and attention in brain imaging studies. Speaking is not possible in the scanner because it moves the head, so a number theme is often used instead. For instance, three words may be displayed that read "two" and the participant must press three on their button box. Another variant is to present emotional pictures in the scanner (again using the number paradigm) to determine their effect on frontal inhibition. The use of pictures in general and particularly in studying post-traumatic stress disorder has been controversial, especially regarding bias effects in stimulus creation.

Since its development, the Stroop task, a measure of the effect of interference on performance of a color identification task, has utilized the Stroop effect to investigate aspects of such varied psychological disorders as Attention Deficit Hyperactivity Disorder, Schizophrenia, and Anorexia. EEG and fMRI studies of the Stroop effect have revealed selective activation of the anterior cingulate cortex during a stroop task, a prefrontal structure (see frontal lobe) in the brain which is hypothesized to be responsible for conflict monitoring. J. Ridley Stroop's original word color identification test has additionally been modified to include other sensory modalities and variables.

In synesthetes
A similar effect has been observed in individuals with grapheme-color synesthesia - people who perceive colors when seeing certain numbers and letters. If a number or letter is presented to such an individual in a color other than what they would perceive, there is a delay in determining what color the character actually is. According to V.S. Ramachandran and Edward M. Hubbard:

Stroop effect in popular culture
The Brain Age: Train Your Brain in Minutes a Day! software program, produced by Ryūta Kawashima for the Nintendo DS portable video game system, contains an automated Stroop Test administrator module, translated into game form.

A Nova episode used the Stroop Effect to illustrate the subtle changes of the mental flexibility of Mt. Everest climbers in relation to altitude. 

A Perplex City card (#043) entitled "Use your Anterior Cingulate" asks who discovered this phenomenon.