Intermezzo: The Stroop Task


Before going on to the next chapters in this tutorial, it will be useful to review the type of experiment we will be creating: a Stroop Task. This type of experiment taps into a mental state known as cognitive control; below we discuss both cognitive control and what the Stroop Task is.

Cognitive Control

Cognitive Control refers to the ability to focus on one thing - an idea, an object, a sound - and to filter out or suppress irrelevant thoughts and stimuli. This often requires suppressing a habitual action as well. For example, let’s say that you are an American, and that you are writing an article for a British audience. As it so happens your article deals with the concept of “color” quite a bit, but each time you write the word, you need to consciously write it as “colour”. This is a case in which your habitual actions are in conflict with a new context, and you need to be on guard as to when your habitual actions will lead you to make a mistake.

Or, to take another example: Let’s say that you are writing a check shortly after the beginning of the new year in 2019. When writing in the year of the date, you find yourself writing 2018 - an error that comes about from the habit of having seen and written it so many times in the past fifty-two weeks. The next time you find yourself needing to write the year of the date, you take more time to consciously override your habit of writing in the wrong year. In both cases, this effort to do a task correctly by suppressing your habits is an act of cognitive control, sometimes referred to in everyday speech as willpower.

The Stroop Task

One of the most popular tasks used to elicit cognitive control is known as the Stroop Task. Developed by John Ridley Stroop and published in 1935, the Stroop task uses incongruency between two aspects of a stimulus in order to make a task more difficult. The subject is instructed to focus on one aspect of a stimulus that is incongruent with another aspect of the stimulus - usually some aspect that will evoke a predominant response.

The classic Stroop task combines words and colors to generate incongruent and congruent stimuli. An example of a congruent stimulus would be the word “blue” written in blue ink: If the subject is instructed to name the color of the ink, there is no interference from the word itself. If there is a difference between the word and the color of the ink, on the other hand, this is an incongruent condition - for example, the word “blue” written in red ink. In this case, the predominant response is to say the name of the word itself, since reading the word is the habitual response. Extra effort is required to override that impulse and instead say the name of the color, and this additional effort is measured in the extra reaction time as compared to responding to a congruent stimulus.


Example of the two conditions of the Stroop task. On the left, the word “blue” is written in blue ink; there is no incongruency between the color of the word and the word itself, making it relatively easy to say that the color of the ink is “blue”. On the right, conversely, the color of the word and the word itself are different. The reader has to override his habit of saying the name of the word, and say the name of the color instead.

Because the Stroop experiment is easy to create, simple to do, and consistently leads to higher reaction times in the Incongruent condition, it has become one of the most widely used psychological tasks. The basic principle of the Stroop effect has generated several variations, such as incongruency between the direction of an arrow and the word superimposed upon it (e.g., an arrow pointing to the right which contains the word “left”).

Next Steps

In the following tutorials, you will create your own Stroop experiment with the Congruent and Incongruent conditions outlined above. After running the experiment on yourself or your friends, you will then analyze the behavioral data to find the Stroop effect. Lastly, we will adapt the experiment to be run in an fMRI scanner, in order to detect where in the brain where there is activity that correlates with the Stroop effect.