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Green Red Blue
Purple Blue Purple

Blue Purple Red
Green Purple Green

The Stroop effect refers to the fact that naming the color of the first group of words is easier and quicker than the second.

In psychology, the Stroop effect is a demonstration of interferance 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), naming the color of the word takes longer and is more prone to errors than when the meaning of the word is congruent with its ink color. The effect is named after John Ridley Stroop who first published the effect in English in 1935.[1] The effect had previously been published in 1929, but only in German. [2][3][4] The original paper has been one of the most cited papers in the history of experimental psychology, leading to over 700 replications.[4]

The effect has been used to create a psychological test which is widely used in clinical practice and investigation. The test has also been further modified to investigate very different phenomena.[4]

Original experiment

Stimulus 1: Purple Brown Red Blue Green

Stimulus 2: Purple Brown Red Blue Green

Stimulus 3: ♦♦♦♦♦ ♦♦♦♦♦ ♦♦♦♦♦ ♦♦♦♦♦ ♦♦♦♦♦

Examples of the 3 stimulus and colors used for each of the activities of the original Stroop article.[1]

Figure 1 from experiment 2 of the original description of the Stroop effect(1935). 1 is the time that takes to name the color of the dots while 2 is the time that takes to say the color when there is a conflict with the written word; which takes longer.[1]

The effect was first published in 1929, but only in German[2][3][4] However it is named after John Ridley Stroop who published the effect in English in 1935 in an article titled Studies of interference in serial verbal reactions which includes three different experiments.[1]

In the experiments he administered several variations of the same test for which three different kind of stimulus were created. In the first one names of colors appeared in black ink. In the second names of colors appeared in a different ink than the color named. Finally in the third one there were squares of a given color.[1]

In the first experiment the participants had to read the written color names of the words in color and black ink. In the second experiment participants were required to say the color of the letters independently of the written word with the second kind of stimulus and also name the color of the dot squares. Additionally Stroop in the third experiment tested his participants at different stages of practice with each task, to account for the effects of association.[1]

Stroop identified a large increase on the time taken by participants to complete the color reading in the second task compared to the naming of the color of the squares in experiment 2 while this delay did not appear in the first experiment. Such interference was explained 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.[1]

As opposed to the Stroop test most commonly used in psychological evaluation,[5] J.R Stroop never compared the time used for reading black words and the time needed for naming colors that conflicted with the written word.

Research use

The original paper of the Stroop effect has been one of the most cited papers in the history of experimental psychology, leading to over 710 replications.[4] The test has been further modified to investigate very different phenomena.

In the study of interference the most commonly used procedure has been similar to Stroop's second experiment, in which subjects are tested on naming colors of incompatible words and of control patches; however the first experiment (reading words in black versus incongruent colors) has received much less interest. In both cases the interference score is expressed as the difference between the times needed to read each of the two types of cards.[4] Usually lists of stimulus are used, but time measures for individual words permit more control on research variables.[4] Rather than naming or reading stimuli aloud, subjects have also been asked to sort stimuli into categories.[4] Different characteristics of the stimulus such as ink colors or direction of words have also been sistematically varied.[4] None of all these modifications eliminates the effect of interference.[4] Good for science projects.

The Stroop task has been employed to study frontal function and attention in brain imaging studies.[6] 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.[7]

The test has additionally been modified to include other sensory modalities and variables,[8] to study the effect of bilingualism,[9] or to investigate the effect of emotions on interference.[10] A similar effect has also 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.[11]

Clinical use

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.

Edith Kaplan's group (developer of the Delis-Kaplan neuropsychological test battery) developed the task further by separating the task into four different 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.[How to reference and link to summary or text]

Versions of the test

There are a number of versions of the test. They include:

See also

References & Bibliography

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Stroop, John Ridley (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology 18: 643-662.
  2. 2.0 2.1 Jaensch, E.R (1929). ''Grundformen menschlichen Seins, Berlin: Otto Elsner.
  3. 3.0 3.1 Jensen AR, Rohwer WD (1966). The Stroop color-word test: a review. Acta psychologica 25 (1): 36–93.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 MacLeod CM (March 1991). Half a century of research on the Stroop effect: an integrative review. Psychological bulletin 109 (2): 163–203.
  5. Golden, CJ (1978). Stroop Color and Word Test: A Manual for Clinical and Experimental Uses, 1–32, Chicago, Illinois: Skoelting.
  6. Pujol J, Vendrell P, Deus J, et al (January 2001). The effect of medial frontal and posterior parietal demyelinating lesions on stroop interference. NeuroImage 13 (1): 68–75.
  7. Kaufmann L, Ischebeck A, Weiss E, et al (October 2008). An fMRI study of the numerical Stroop task in individuals with and without minimal cognitive impairment. Cortex 44 (9): 1248–55.
  8. Roberts KL, Hall DA (June 2008). Examining a supramodal network for conflict processing: a systematic review and novel functional magnetic resonance imaging data for related visual and auditory stroop tasks. Journal of cognitive neuroscience 20 (6): 1063–78.
  9. Rosselli M, Ardila A, Santisi MN, et al (September 2002). Stroop effect in Spanish-English bilinguals. Journal of the International Neuropsychological Society : JINS 8 (6): 819–27.
  10. Kampman M, Keijsers GP, Verbraak MJ, Näring G, Hoogduin CA (2002). The emotional Stroop: a comparison of panic disorder patients, obsessive-compulsive patients, and normal controls, in two experiments. Journal of anxiety disorders 16 (4): 425–41.
  11. Ramachandran, V.S. and Edward M. Hubbard. "More Common Questions about Synesthesia. Scientific American online. April 14, 2003. URL accessed 2007-03-12.

Key texts



  • Dyer, F.N. (1973) The Stroop phenomenon and its use in the study of perceptual, cognitive and response processes, Memory and Cognition 1: 106-20.
  • Preston, M.S. and Lambert, W.E. (1969) Interlingual interference in a bilingual version of the Stroop color-word task, Journal of Verbal Learning and Verbal Behaviour 8: 295-301.
  • Pritchatt, D. (1968) An investigation into some underlying associative verbal processes of the Stroop colour effect, Quarterly Journal of Experimental Psychology 20: 351--59.
  • Scheibe, K.E., Shaver, P.R. and Carrier, S.C. (1967) Color association values and response interference on variants of the Stroop test, Acta Psychologica 26: 286-95.

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