The Stroop Effect: Not as automatic as was once thought

A continuation of our "greatest hits" from past Cognitive Daily postings:
[originally posted on May 9, 2006]

The Stroop Effect is one of the most-studied phenomena in psychology. The test is easy to administer, and works in a variety of contexts. The simplest way to see how it works is just to look the following two lists. Don't read them, instead say the color each word is displayed in, as quickly as you can:


If English is your native language, you should be much quicker at naming the colors of the first list than the second list. Why? Even though the task is to identify the colors, proficient readers can't stop themselves from reading the words, which slows color identification in cases where the color is different from the word.

But recently, Amir Raz and colleagues noticed that they could reduce and even eliminate the Stroop Effect by hypnotizing participants and suggesting to them that the words were in a foreign language, so they could focus solely on color. In a new experiment, Raz and three other researchers attempted to see if the hypnosis itself was necessary.

Research has shown that hypnosis is only effective when the person being hypnotized is already susceptible to hypnosis. In order for hypnosis to work, the person being hypnotized must be highly suggestible. But what if highly suggestible individuals are offered suggestions without first being hypnotized? Raz's team reasoned that perhaps the same effect would result.

The team identified 25 highly suggestible individuals, then divided them into two groups. Each group practiced the Stroop task for a few minutes: a word was flashed on the screen, and participants had to press a key on the computer corresponding to the color it was displayed in. Then one group was hypnotized, but the other was not. After hypnosis (or a break lasting the same amount of time), each group received the same suggestion:

Very soon you will be playing a computer game. When I clap my hands, meaningless symbols will appear in the middle of the screen. They will feel like characters in a foreign language that you do not know, and you will not attempt to attribute any meaning to them. This gibberish will be printed in one of four ink colors: red, blue, green, or yellow. Although you will only be able to attend to the symbols' ink color, you will look straight at the scrambled signs and crisply see all of them. Your job is to quickly and accurately depress the key that corresponds to the color shown. You will find that you can play this game easily and effortlessly. When I clap my hands twice, you will regain your normal reading abilities.

For half the task, they operated under the influence of the suggestion, and for the remainder of the time, they did the standard Stroop task, with no suggestion (half the participants were given the suggestion for the first half of the task, and for the other participants, the order was reversed). This chart shows the level of Stroop Effect for each condition:


Whether or not participants were hypnotized, all showed a diminished Stroop Effect when it was suggested that the words were gibberish. There was no significant difference in the results between hypnotized and non-hypnotized participants.

While the Stroop effect was not completely eliminated in this task, Raz's team argues that this experiment demonstrates that reading is not entirely involuntary. The experiment is an example of a simple way that individuals who have not been hypnotized can voluntarily reduce the tendency to automatically read the word they are looking at.

Raz, A., Kirsch, I., Pollard, J., & Nitkin-Kaner, Y. (2006). Suggestion reduces the Stroop Effect. Psychological Science, 17(2), 91-95

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Was there any probing to reveal the strategies that participants used in the suggestion/hypnosis conditions?

Could a person defocus their eyes to reduce the ease of reading the words, but still be able to perceive the colors?

Or could people respond to the suggestion by narrowing their focus to a small portion of one letter, and thus reduce their chances of attending to the entire word?

It is not unusual for participants to adopt strategies that will allow them to behave in the way they think the experimenter wants, and we have to careful when we produce findings like this to make sure we understand what the participants are doing.

I totally agree with Andrew. There are many possible cognitive strategies to dealing with the Stroop (personally, I cross my eyes a little). The administration of hypnosis was not blinded (perhaps can not be blinded), and there's no way of knowing how extensively participant biases may have played into the results. Not only is there often an inclination to please the experimenters, but many suggestible individuals will report being "proud" of their susceptibility to hypnosis.

A partial control for that was that half of them received the suggestion first, and half them afterwards.

I agree though that people will adopt strategies that help them to identify the colours. This may be unconciously, or conciously, perhaps to please the testers, perhaps just to try and beat your own best score (cf video games).

to test for it - see if the subject's performance improves over time
improve ov

Re: Andrew's commment on strategies.

I almost automatically, reading the instructions at the beginning of this post, transferred my attention from the whole word to the last letter, thereby reducing the tendency to read the word.

Result: I hesitated only once, "reading" the second list.

When I did it the second time, reading normally, I did stumble several times.

Very soon you will be playing a computer game. When I clap my hands, meaningless symbols will appear in the middle of the screen.

Hopefully they did not actually use the video game Brain Age to administer this test, as doing so would have impacted the results.


Aww.. That was fun! :) Though I must agree that I was slower when reading the second list compared to the first, but I think I did great for somebody whose native language isn't English ;)

Equally interesting, the study reveals that hypnotism is unnecessary and has no effect greater than ordinary suggestion. I'd love to know if this may be generally true in other areas in which hypnotism is touted.

If they had told the control group that they, too, would ony be able to attend to the word's color, even though they could read it, that might have made a difference. The ONLY difference should have been whether the subjects could read the words or not. The hypnosis suggested to the experimental subjects that they shift their attention completely, but only one group recieved that suggestion. Of course they got different results. If they had hypnotized both groups and given them the same exact instructions, with the *only* difference being whether they could read the words or not, the results would be more believable to me.



By MELONIE MENDOZA (not verified) on 26 Oct 2008 #permalink

this is dum

By john jaock (not verified) on 25 Feb 2009 #permalink