In 1935, John Ridley Stroop published a study demonstrating one of the most robust and easily replicable findings in cognitive psychology: that automatic processes interfere with controlled processes.

The Task

Try to name the color of ink in which each word is printed — not the word itself — as fast as possible:

• BLUE — you want to say "blue" but must say "red"
• RED — you want to say "red" but must say "green"

The Finding

People are significantly slower and make more errors when the word meaning conflicts with the ink color. Reading is so automatic that it cannot be suppressed — it interferes with the deliberate task of naming ink color.

What It Reveals

• Automatic processing (reading) runs faster than controlled processing (color naming)
• Cognitive interference occurs when automatic and controlled processes conflict
• Attention is limited and selective suppression is effortful

Applications

The Stroop effect is used to study attention, cognitive control, brain damage, ADHD, aging, and the effects of drugs and fatigue on executive function.

Reference:

Wikipedia: Stroop Effect

https://en.wikipedia.org/wiki/Stroop_effect