In psychology, the Stroop effect is a demonstration of interference in the reaction time of a task. When the name of a color (e.g., “blue”, “green”, or “red”) is printed in a color that is not denoted by the name (e.g., the word “red” printed in blue ink instead of red ink), naming the color of the word takes longer and is more prone to errors than when the color of the ink matches the name of the color. The effect is named after John Ridley Stroop, who first published the effect in English in 1935. The effect had previously been published in Germany in 1929. The original paper has been one of the most cited papers in the history of experimental psychology, leading to more than 701 replications. The effect has been used to create a psychological test (Stroop test) that is widely used in clinical practice and investigation.
Brain imaging techniques including magnetic resonance imaging (MRI), functional magnetic resonance imaging (fMRI), and positron emission tomography (PET) have shown that there are two main areas in the brain that are involved in the processing of the Stroop task. They are the anterior cingulate cortex, and the dorsolateral prefrontal cortex. More specifically, while both are activated when resolving conflicts and catching errors, the dorsolateral prefrontal cortex assists in memory and other executive functions, while the anterior cingulate cortex is used to select an appropriate response and allocate attentional resources.
The posterior dorsolateral prefrontal cortex creates the appropriate rules for the brain to accomplish the current goal. For the Stroop effect, this involves activating the areas of the brain involved in color perception, but not those involved in word encoding. It counteracts biases and irrelevant information, for instance, the fact that the semantic perception of the word is more striking than the color in which it is printed. Next, the mid-dorsolateral prefrontal cortex selects the representation that will fulfil the goal. The relevant information must be separated from irrelevant information in the task; thus, the focus is placed on the ink color and not the word. Furthermore, research has suggested that left dorsolateral prefrontal cortex activation during a Stroop task is related to an individual’s’ expectation regarding the conflicting nature of the upcoming trial, and not so much on the conflict itself. Conversely, the right dorsolateral prefrontal cortex aims to reduce the attentional conflict and is activated after the conflict is over.
Moreover, the posterior dorsal anterior cingulate cortex is responsible for what decision is made (i.e. whether you will say the incorrect answer [written word] or the correct answer [ink color]). Following the response, the anterior dorsal anterior cingulate cortex is involved in response evaluation—deciding whether the answer is correct or incorrect. Activity in this region increases when the probability of an error is higher.