Stroop Test : how this test measures concentration and executive functions

Try this: read aloud the color of the ink of each of these words — not the word itself. BLUE   RED   YELLOW. If you hesitated, slowed down, or pronounced the word instead of the color — you just experienced the Stroop effect.

This seemingly simple phenomenon — this interference between the meaning of the word and the color of the ink — is one of the most solid and widely used discoveries in all of cognitive psychology. The Stroop test, published in 1935 by American psychologist John Ridley Stroop, is today one of the most administered neuropsychological assessment tools in the world. It is used in ADHD assessments, dementia evaluations, studies on depression, schizophrenia, frontal lesions — and increasingly as a cognitive training tool.

But why does this simple test reveal so much? What does it actually measure? And what can be done with this information — whether you are a parent of a child struggling in school, a healthcare professional, or simply curious about your own cognitive functioning?

1. The history of the Stroop test

John Ridley Stroop did not intend to change the history of neuroscience. In 1935, he wrote his doctoral thesis at George Peabody College in Nashville, on what he calls "studies of interference in serial verbal reactions." In his experiment, participants had to name the color of the ink of incongruently printed color words — the word "RED" written in blue ink, for example. The results are striking: this type of task takes significantly longer and produces more errors than simply reading the words or naming colors without text.

The thesis went unnoticed for several years. Then, in the post-war decades, with the rise of cognitive psychology and the early theories of information processing, the Stroop effect became a central paradigm. It gradually became clear that it reveals something fundamental about how the brain processes competing information — and more specifically about the ability to inhibit an automatic response in favor of a more deliberate one.

2. The principle of the test: the three conditions

In its classic form, the Stroop test consists of three conditions presented successively, each measuring something slightly different.

Condition 1 — Color: Name the color of colored dots or sequences of X (XXXX) printed in different colors. This condition establishes the baseline speed of color naming without any lexical interference.

Condition 2 — Reading: Read aloud color words (RED, BLUE, GREEN…) printed in black ink. Reading words is a highly automated skill in literate adults — this condition measures the baseline reading speed.

Condition 3 — Interference: Name the color of the ink of incongruent color words (the word RED printed in blue). This is the critical condition that generates the Stroop effect. The participant must inhibit the automatic response (reading the word) to produce the requested response (name the color).

The key score of the test is the interference effect, calculated as the difference in reaction time (and errors) between condition 3 and conditions 1 or 2. The larger this gap, the stronger the interference — and the harder cognitive inhibition had to work (or failed).

3. The Stroop effect: why the brain gets stuck

Automatic reading as a source of conflict

The Stroop effect reveals a fundamental property of the literate human brain: reading is automatic. For an adult who can read, seeing a word automatically and involuntarily triggers its semantic processing — the meaning of the word is activated even when one is not trying to read. One cannot "not read" a word that enters our visual field, just as one cannot "not hear" a loud enough sound.

Color naming, on the other hand, is not as automatic — it requires more controlled, deliberate processing. When the two processes conflict (the word says "red" but the ink is blue), the brain must resolve this competition. It must inhibit the dominant response (the word) to produce the correct response (the color). It is this inhibition effort that slows down reaction time and generates errors.

Theoretical models of the Stroop effect

Several theories have been proposed to precisely explain why reading interferes with color naming. The relative speed of processing model proposes that reading is simply faster than color naming — lexical processing "arrives" first and therefore must be inhibited. The pathway strength model suggests that the connections between words and their pronunciation are stronger than the connections between colors and their names, because we have practiced reading much more than color naming.

The most influential model today is the activation competition model: both processes (reading and naming) activate simultaneously and in parallel, and their relative activation determines which of the two responses "wins." Cognitive inhibition is the mechanism that allows modulating this competition in favor of the correct response.

4. Cognitive inhibition: the measured skill

What the Stroop test fundamentally measures is cognitive inhibition — one of the three central components of executive functions, along with mental flexibility and working memory. Understanding what cognitive inhibition is helps explain why this test has such clinical relevance.

What is cognitive inhibition?

Cognitive inhibition is the ability to suppress or restrain thoughts, responses, or information that are automatic, dominant, or irrelevant, in order to maintain goal-oriented processing. In other words, it is the ability to not do what the brain would spontaneously want to do — and instead do what the situation requires.

This skill is necessary in a number of everyday situations that we are generally unaware of. Not looking at your phone during an important meeting (inhibiting the appeal of the notification), not interrupting someone who is speaking even when you have an urgent idea to share (inhibiting the verbal impulse), not getting angry in response to provocation (inhibiting the automatic emotional reaction) — all these situations require cognitive inhibition.

The role of the prefrontal cortex

Cognitive inhibition is a function primarily underpinned by the prefrontal cortex — and more specifically by its dorsolateral and ventrolateral regions. The prefrontal cortex is the region of the brain that matures the latest (up to around 25 years), which explains why inhibition capacities are limited in young children and gradually improve with age. Lesions of the prefrontal cortex — whether due to traumatic brain injury, a Stroke, dementia, or another pathology — produce measurable inhibition deficits on the Stroop test.

5. Variants of the Stroop test

Since the original publication in 1935, dozens of variants of the Stroop test have been developed to target specific populations or measure particular aspects of cognitive control.