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Classic experiments explained

Stroop Task

In short

The Stroop task is one of the most well-known experiments in Psychology, and it was first carried out by J. R. Stroop (1935), who also gave it its name. It measures the average reaction time between incongruent and congruent trials and is used to study populations with neurological and/or mental health problems.

The Stroop task

In the original Stroop task, words designating colour like ‘blue’ or ‘red’ are displayed either in the colour they indicate (congruent/compatible trials, e.g. BLUE in blue ink) or in a different colour ink (incongruent/incompatible trials, e.g. BLUE in red ink). In the original Stroop Colour Word Test participants were given card sheets containing lists of congruent or incongruent words and asked to verbally indicate the colour of the font and to ignore the word itself. Their accuracy and reaction times were recorded.

Typical Stroop task with a block of congruent and incongruent word-colour stimuli

While the original Stroop task was carried out this way, it is now more common to display words individually on a computer screen and, instead of participants saying colour out loud, record keyboard responses.

There are a lot of variations of the Stroop task, for example, the Emotional Stroop task or the Spatial Stroop task. All of them are based on interference effects between a target and a distracter element of the stimulus and measure time differences and/or differences in accuracy.

What is the Stroop Effect?

The Stroop Effect is the calculated difference between measured average reaction times to incongruent and congruent trials. It shows that the automated reading process of the word interferes with the correct and goal-directed reporting of the colour, as response times to congruent stimuli are generally a lot faster than for incongruent stimuli. The Stroop Effect is sometimes referred to as a compatibility, interference, or congruency/incongruency effect, and similar effects are seen in other tasks which contain congruent and incongruent stimuli and induce stimulus-response conflict, such as in the Eriksen Flanker task.

What is the theory behind the Stroop Effect?

There are several theories behind the Stroop Effect, mainly regarding executive function, such as attentional processes, speed of processing, and inhibition. Indeed, participants have to inhibit their first and automated reaction (responding to the written word) and instead respond to the colour. Our brain has to deal with the interference from contradicting stimuli and respond with the less obvious answer.

Why is the Stroop Effect/stimulus-compatibility effect important?

The Stroop task can be used as a behavioural test to study populations with neurological and mental health problems. For example, while healthy older adults usually just respond more slowly to congruent and incongruent stimuli than younger adults, older adults with dementia additionally make more mistakes, as their executive function is impaired. The Stroop task has also been used with other patient populations, such as people with depression, schizophrenia, and ADHD.

Can I use the Stroop test in online research?

Yes, absolutely! In fact, other researchers have already used the Stroop experiment online and found similar effects compared to lab-based research (Linnman et al., 2006).

How does it work in Gorilla?

You can try out and clone our sample of the Stroop task. We have a desktop version, which uses keyboard input, and a mobile version, which uses touch buttons. Of course, you can also tweak these samples, use your own stimuli, change the mode of responding and feedback, for example.

If you want to have a look: Try the Stroop Task in Gorilla

Are there any papers Gorilla users have written using the Stroop task?

Yes, there are! Have a look at the following article:

The role of valence in word processing: Evidence from lexical decision and emotional Stroop tasks


Linnman, C., Carlbring, P., Åhman, Å., Andersson, H., & Andersson, G. (2006). The Stroop effect on the internet. Computers in Human Behavior, 22(3), 448-445.

Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18(6), 643-662.