variables in stroop effect experiment

Stroop Effect Experiment in Psychology

Charlotte Ruhl

Research Assistant & Psychology Graduate

BA (Hons) Psychology, Harvard University

Charlotte Ruhl, a psychology graduate from Harvard College, boasts over six years of research experience in clinical and social psychology. During her tenure at Harvard, she contributed to the Decision Science Lab, administering numerous studies in behavioral economics and social psychology.

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Saul McLeod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul McLeod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

On This Page:

The Stroop effect is a psychological phenomenon demonstrating interference in reaction time of a task. It occurs when the name of a color is printed in a color not denoted by the name, making it difficult for participants to identify the color of the word quickly and accurately.

Take-home Messages

• In psychology, the Stroop effect is the delay in reaction time between automatic and controlled processing of information, in which the names of words interfere with the ability to name the color of ink used to print the words.
• The Stroop test requires individuals to view a list of words printed in a different color than the word’s meaning. Participants are tasked with naming the color of the word, not the word itself, as fast as they can.
• For example, when presented with the word “green” written in red ink, it is much easier to name the word that is spelled instead of the color ink in which the word is written.
• The interference, or the delay in response time, is measured by comparing results from the conflict condition (word and color mismatch) to a neutral condition (e.g., a block of color or a color word with matching ink). Subtracting the results from these two conditions helps to eliminate the influence of general motor responses.
• Reading, a more powerful automatic process, takes some precedence over color naming, which requires higher cognitive demands.
• Since psychologist John Ridley Stroop first developed this paradigm back in 1935, the Stroop task has since been modified to help understand additional brain mechanisms and expanded to aid in brain damage and psychopathology research.

What Is The Stroop Effect?

The Stroop effect refers to a delay in reaction times between congruent and incongruent stimuli (MacLeod, 1991).

Congruency, or agreement, occurs when a word’s meaning and font color are the same. For example, if the word “green” is printed in green.

Incongruent stimuli are just the opposite. That is the word’s meaning and the color in which it is written do not align. For example, the word “green” might be printed in red ink.

The Stroop task asks individuals to name the color of the word instead of reading the word itself.

The delay in reaction time reveals that it is much harder to name the color of a word when the word itself spells another color (the incongruent stimuli) than it is to name the color of the word when the word itself spells that same color (the congruent stimuli).

The First Stroop Experiment

The Stroop effect was first published in 1935 by American psychologist John Ridley Stroop, although discoveries of this phenomenon date back to the nineteenth century (Stroop, 1935).

Building off previous research, Stroop had two main aims in his groundbreaking paper:

• To examine how incongruency between the color of the word and the word’s content will impair the ability to name the color.
• To measure what effect practicing reacting to color stimuli in the presence of conflicting word stimuli would have upon the reaction times.

To empirically study these two major aims, Stroop ran three different experiments:

1) Experiment 1 :

Participants (70 college undergraduates) were tasked with reading the word aloud, irrespective of its color. In other words, participants must read aloud the word “green” even if written in a different color.

2) Experiment 2 :

The second experiment was the opposite of the first. Participants (100 college students) were first asked to name the color of individual squares (instead of the color of words) as a training mechanism for the subsequent task. Afterward, participants had to say the color of the word, regardless of its meaning – the opposite of the experiment 1 procedure.

3) Experiment 3 :

The third and final experiment integrated all of the previously mentioned tests with an undergraduate population of 32 participants.

The independent variable (IV) was the congruency of the font name and color.

• Congruent (word name and font color are the same)
• Incongruent (word name and font color are different)

The dependent variable (DV) was reaction time (ms) in reporting the letter color.

After running the three experiments, Stroop drew two main conclusions:

• The interference of conflicting word stimuli upon the time for naming colors caused an increase of 47.0 seconds or 74.3 percent of the normal time for naming colors printed in just squares.
• The interference of conflicting color stimuli upon the time for reading words caused an increase of only 2.3 seconds or 5.6 percent over the normal time for reading the same words printed in black.

These tests demonstrate a disparity in the speed of naming colors and reading the names of colors, which may be explained by a difference in training in the two activities.

The word stimulus has been associated with the specific response “to read,” while the color stimulus has been associated with various responses: “to admire,” “to name,” etc.

The observed results might reflect the fact that people have more experience consciously reading words than consciously labeling colors, illustrating a difference in the mechanisms that control these two processes.

How the Stroop Effect Works

Why does the Stroop effect occur? We can tell our brain to do lots of things – store memories, sleep, think, etc. – so why can’t we tell it to do something as easy as naming a color? Isn’t that something we learn to do at a very young age?

Researchers have analyzed this question and come up with multiple different theories that seek to explain the occurrence of the Stroop effect (Sahinoglu & Dogan, 2016).

Speed of processing theory:

The processing speed theory claims that people can read words much faster than they can name colors (i.e., word processing is much faster than color processing).

When we look at the incongruent stimuli (the word “green” printed in red, for example), our brain first reads the word, making it much more difficult to then have to name the color.

As a result, a delay occurs when trying to name the color because doing so is not our brain’s first instinct (McMahon, 2013).

Selective attention theory:

The theory of selective attention holds that recognizing colors, compared to reading words, requires more attention.

Because of this, the brain needs to use more attention when attempting to name a color, making this process take slightly longer (McMahon, 2013).

Automaticity:

A prevalent explanation for the Stroop effect is the automatic nature of reading. When we see a word, its meaning is almost instantly recognized. Thus, when presented with a conflicting color, there’s interference between the automatic reading process and the task of naming the ink color.

This theory argues that recognizing colors is not an automatic process , and thus there is a slight hesitancy when carrying out this action.

Automatic processing is processed in the mind that is relatively fast and requires few cognitive resources.

This type of information processing generally occurs outside of conscious awareness and is common when undertaking familiar and highly practiced tasks.

However, the brain is able to automatically understand the meaning of a word as a result of habitual reading (think back to Stroop’s initial study in 1935 – this theory explains why he wanted to test the effects of practice on the ability to name colors).

Word reading, being more automatic and faster than color naming, results in involuntary intrusions during the color-naming task. Conversely, reading isn’t affected by the conflicting print color.

Researchers in support of this theory posit that automatic reading does not need controlled attention but still uses enough of the brain’s attentional resources to reduce the amount left for color processing (Monahan, 2001).

In a way, this parallels the brain’s dueling modes of thinking – that of “System 1” and “System 2.” Whereas the former is more automatic and instinctive, the latter is slower and more controlled (Kahneman, 2011).

This is similar to the Stroop effect, in which we see a more automatic process trying to dominate over a more deliberative one. The interference occurs when we try to use System 2 to override System 1, thus producing that delay in reaction time.

Parallel distributed processing:

The fourth and final theory proposes that unique pathways are developed when the brain completes different tasks. Some of these pathways, such as reading words, are stronger than others, such as naming colors (Cohen et al., 1990).

Thus, interference is not an issue of processing speed, attention, or automaticity but rather a battle between the stronger and weaker neural pathways.

Additional Research

John Ridley Stroop helped lay the groundwork for future research in this field.

Numerous studies have tried to identify the specific brain regions responsible for this phenomenon, identifying two key regions: the anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLFPC).

Both MRI and fMRI scans show activity in the ACC and DLPFC while completing the Stroop test or related tasks (Milham et al., 2003).

The DLPFC assists with memory and executive functioning, and its role during the task are to activate color perception and inhibit word encoding. The ACC is responsible for selecting the appropriate response and properly allocating attentional resources (Banich et al., 2000).

Countless studies that repeatedly test the Stroop effect reveal a few key recurring findings (van Maanen et al., 2009):

• Semantic interference : Naming the ink color of neutral stimuli (where the color is only shown in blocks, not as a written word) is faster than incongruent stimuli (where the word differs from its printed color).
• Semantic facilitation : Naming the ink of congruent stimuli (where the word and its printed color are in agreement) is faster than for neutral stimuli.
• Stroop asynchrony : The previous two findings disappear when reading the word, not naming the color, is the task at hand – supporting the claim that it is much more automatic to read words than to name colors.

Other experiments have slightly modified the original Stroop test paradigm to provide additional findings.

One study found that participants were slower to name the color of emotion words as opposed to neutral words (Larsen et al., 2006).

Another experiment examined the differences between participants with panic disorder and OCD. Even with using threat words as stimuli, they found that there was no difference among panic disorder, OCD, and neutral participants’ ability to process colors (Kampman et al., 2002).

A third experiment investigated the relationship between duration and numerosity processing instead of word and color processing.

Participants were shown two series of dots in succession and asked either (1) which series contained more dots or (2) which series lasted longer from the appearance of the first to the last dots of the series.

The incongruency occurred when fewer dots were shown on the screen for longer, and a congruent series was marked by a series with more dots that lasted longer.

The researchers found that numerical cues interfered with duration processing. That is, when fewer dots were shown for longer, it was harder for participants to figure out which set of dots appeared on the screen for longer (Dormal et al., 2006).

Thus, there is a difference between the processing of numerosity and duration. Together, these experiments illustrate not only all of the doors of research that Stroop’s initial work opened but also shed light on all of the intricate processing associations that occur in our brains.

Other Uses and Versions

The purpose of the Stroop task is to measure interference that occurs in the brain. The initial paradigm has since been adopted in several different ways to measure other forms of interference (such as duration and numerosity, as mentioned earlier).

Additional variations measure interference between picture and word processing, direction and word processing, digit and numerosity processing, and central vs. peripheral letter identification (MacLeod, 2015).

The below figure provides illustrations for these four variations:

The Stroop task is also used as a mechanism for measuring selective attention, processing speed, and cognitive flexibility (Howieson et al., 2004).

The Stroop task has also been utilized to study populations with brain damage or mental disorders, such as dementia, depression, or ADHD (Lansbergen et al., 2007; Spreen & Strauss, 1998).

For individuals with depression, an emotional Stroop task (where negative words, such as “grief,” “violence,” and “pain,” are used in conjunction with more neutral words, such as “clock,” “door,” and “shoe”) has been developed.

Research reveals that individuals who struggle with depression are more likely to say the color of a negative word slower than that of a neutral word (Frings et al., 2010).

The versatility of the Stroop task paradigm lends itself to be useful in a wide variety of fields within psychology. What was once a test that only examined the relationship between word and color processing has since been expanded to investigate additional processing interferences and to contribute to the fields of psychopathology and brain damage.

The development of the Stroop task not only provides novel insights into the ways in which our brain mechanisms operate but also sheds light on the power of psychology to expand and build on past research methods as we continue to uncover more and more about ourselves.

Critical Evaluation

Dishon-Berkovits and Algom (2000) argue that the Stroop effect is not a result of automatic processes but is due to incidental correlations between the word and its color across stimuli.

They suggest that participants unconsciously recognize these correlations, using word cues to anticipate the correct color hue they should name.

When testing with word-word stimuli, Dishon-Berkovits and Algom created positive, negative, and zero correlations.

They observed that zero correlations nearly eliminated Stroop effects, implying that the effects might be more about the way stimuli are presented rather than true indicators of automaticity or attention.

However, their methodology raised concerns:

• They had difficulty creating zero correlations with color-hue situations.
• Their study didn’t include a neutral condition, which means interference and facilitation were not examined.
• There’s a general finding that facilitation effects are smaller than interference effects, which their findings don’t necessarily support

Despite these considerations, the correlational approach does not invalidate Stroop’s original paradigm or the many studies based on it.

Stroop-based findings have been instrumental in understanding various clinical conditions like anxiety, schizophrenia, ADHD, dyslexia, PTSD, racial attributions, and others.

The takeaway is that while the theory proposed by Dishon-Berkovits and Algom introduces a fresh perspective, it does not negate the established findings and implications of the Stroop effect.

Instead, it encourages a deeper examination of how automaticity and attention might be influenced by certain environmental factors and correlations.

Describe why the Stroop test is challenging for us.

The Stroop test is challenging due to the cognitive conflict it creates between two mental processes: reading and color recognition. Reading is a well-learned, automatic process, whereas color recognition requires more cognitive effort.

When the word’s color and its semantic meaning don’t match, our brain’s automatic response to reading the word interferes with naming the color, causing a delay in response time and an increase in mistakes. This is known as the Stroop effect.

Banich, M. T., Milham, M. P., Atchley, R., Cohen, N. J., Webb, A., Wszalek, T., … & Magin, R. (2000). fMRI studies of Stroop tasks reveal unique roles of anterior and posterior brain systems in attentional selection . Journal of cognitive neuroscience, 12 (6), 988-1000.

Cohen, J. D., Dunbar, K., & McClelland, J. L. (1990). On the control of automatic processes: a parallel distributed processing account of the Stroop effect . Psychological Review, 97 (3), 332.

Dishon-Berkovits, M., & Algom, D. (2000). The Stroop effect: It is not the robust phenomenon that you have thought it to be .  Memory & Cognition ,  28 , 1437-1449.

Dormal, V., Seron, X., & Pesenti, M. (2006). Numerosity-duration interference: A Stroop experiment . Acta psychologica, 121 (2), 109-124.

Frings, C., Englert, J., Wentura, D., & Bermeitinger, C. (2010). Decomposing the emotional Stroop effect . Quarterly journal of experimental psychology, 63 (1), 42-49.

Howieson, D. B., Lezak, M. D., & Loring, D. W. (2004). Orientation and attention. Neuropsychological assessment , 365-367.

Kahneman, D. (2011). Thinking, fast and slow . Macmillan.

Kampman, M., Keijsers, G. P., Verbraak, M. J., Näring, G., & Hoogduin, C. A. (2002). The emotional Stroop: a comparison of panic disorder patients, obsessive–compulsive patients, and normal controls, in two experiments. Journal of anxiety disorders, 16 (4), 425-441.

Lansbergen, M. M., Kenemans, J. L., & Van Engeland, H. (2007). Stroop interference and attention-deficit/hyperactivity disorder: a review and meta-analysis . Neuropsychology, 21 (2), 251.

Larsen, R. J., Mercer, K. A., & Balota, D. A. (2006). Lexical characteristics of words used in emotional Stroop experiments . Emotion, 6 (1), 62.

MacLeod, C. M. (1991). Half a century of research on the Stroop effect: an integrative review . Psychological bulletin, 109 (2), 163.

MacLeod, C. M. (2015). The stroop effect. Encyclopedia of Color Science and Technology.

McMahon, M. (2013). What Is the Stroop Effect. Retrieved November, 11 .

Milham, M. P., Banich, M. T., Claus, E. D., & Cohen, N. J. (2003). Practice-related effects demonstrate complementary roles of anterior cingulate and prefrontal cortices in attentional control . Neuroimage, 18 (2), 483-493.

Monahan, J. S. (2001). Coloring single Stroop elements: Reducing automaticity or slowing color processing? . The Journal of general psychology, 128 (1), 98-112.

Sahinoglu B, Dogan G. (2016). Event-Related Potentials and the Stroop Effect. Eurasian J Med , 48(1), 53‐57.

Spreen, O., & Strauss, E. (1998). A compendium of neuropsychological tests: Administration, norms, and commentary . Oxford University Press.

Stroop, J. R. (1935). Studies of interference in serial verbal reactions . Journal of experimental psychology, 18 (6), 643.

van Maanen, L., van Rijn, H., & Borst, J. P. (2009). Stroop and picture—word interference are two sides of the same coin . Psychonomic bulletin & review, 16 (6), 987-999.

Further information

• Exampe of a stroop effect lab report
• Picture-word interference is a Stroop effect: A theoretical analysis and new empirical findings

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How the Stroop Effect Works

Naming a Color but Not the Word

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Shereen Lehman, MS, is a healthcare journalist and fact checker. She has co-authored two books for the popular Dummies Series (as Shereen Jegtvig).

Performing Your Own Stroop Test

Terms and key questions, frequently asked questions.

The Stroop effect is a phenomenon that occurs when the name of a color doesn't match the color in which it's printed (e.g., the word "red" appears in blue text rather than red). In such a color test (aka a Stroop test or task), you'd likely take longer to name the color (and be more likely to get it wrong) than if the color of the ink matched the word.

Although it might sound simple, the Stroop effect refers to the delayed reaction times when the color of the word doesn't match the name of the word. It's easier to say the color of a word if it matches the semantic meaning of the word.

For example, if someone asked you to say the color of the word "black" that was also printed in black ink, it would be much easier to say the correct color than if it were printed in green ink.

The task demonstrates the effect that interference can have when it comes to reaction time. It was first described during the 1930s by American psychologist John Ridley Stroop for whom the phenomenon is named. His original paper describing the effect has become one of the most famous, as well as one of the most frequently cited, in the history of psychology. The effect has been replicated hundreds of times by other researchers.

For students of psychology looking for a relatively easy and interesting experiment to try on their own, replicating the Stroop effect can be a great option.

Theories of the Stroop Effect

Researchers don't yet know why words interfere with naming a color in this way, but researchers have proposed several theories:

• Selective attention theory : According to this theory, naming the actual color of the words requires much more attention than simply reading the text.
• Speed of processing theory : This theory states that people can read words much faster than they can name colors. The speed at which we read makes it much more difficult to name the color of the word after we've read the word.
• Automaticity :   This theory proposes that automatic reading doesn't require focused attention . Instead, the brain simply engages in it automatically. Recognizing colors, on the other hand, may be less of an automated process. While the brain registers written meaning automatically, it does require a certain amount of attentional resources to process color, making it more difficult to process color information and therefore slowing down reaction times.
• Parallel Distributed Processing : Word recognition is an unconscious process that's better described as "contextually controlled" rather than automatic.

Other Uses of the Stroop Test

Over time, researchers have altered the Stroop test to help study populations with brain damage and mental disorders such as dementia, depression, and attention-deficit/ hyperactivity disorder (ADHD).

For example, in studying people with depression, researchers present negative words such as "grief" and pain" along with neutral words such as "paper" and "window." Typically, these people speak the color of a negative word more slowly than they do a neutral word.

The original Stroop test included two parts. In the first, the written color name is printed in a different color of ink, and the participant is asked to speak the written word. In the second, the participant is asked to name the ink color.

There are a number of different approaches you could take in conducting your own Stroop effect experiment.

• Compare reaction times among different groups of participants. Have a control group say the colors of words that match their written meaning. Black would be written in black, blue written in blue, etc. Then, have another group say the colors of words that differ from their written meaning. Finally, ask a third group of participants to say the colors of random words that don't relate to colors. Then, compare your results.
• Try the experiment with a young child who has not yet learned to read. How does the child's reaction time compare to that of an older child who has learned to read?
• Try the experiment with uncommon color names, such as lavender or chartreuse. How do the results differ from those who were shown the standard color names?

Before you begin your experiment, you should understand these concepts:

• Selective attention : This is the way we focus on a particular item for a selected period of time.
• Control group : In an experiment, the control group doesn't receive the experimental treatment. This group is extremely important when comparing it to the experimental group to see how or if they differ. 
• Independent variable : This is the part of an experiment that's changed. In a Stroop effect experiment, this would be the colors of the words. 
• Dependent variable : The part of an experiment that's measured. In a Stroop effect experiment, it would be reaction times.
• Other variables :   Consider what other variables might impact reaction times and experiment with those.

The Stroop test helps researchers evaluate the level of your attention capacity and abilities, and how fast you can apply them. It's particularly helpful in assessing attention-deficit/hyperactivity disorder (ADHD) and executive functioning in people with traumatic brain injuries (TBIs).

The Stroop test helps researchers measure the part of the brain that handles planning, decision-making, and dealing with distraction.

There are many possible combinations of scores on the first and second tasks. They might indicate speech problems, reading skill deficits, brain injury. color blindness, emotional upset, or low intelligence. Likewise, they might mean that your brain is able to handle conflicting information well and has adequate cognitive adaptability and skills.

Stroop JR.  Studies of interference in serial verbal reactions . J. Exp. Psychol. Gen. 1935;18;643-662. doi:10.1037/h0054651

Sahinoglu B, Dogan G. Event-Related Potentials and the Stroop Effect .  Eurasian J Med . 2016;48(1):53‐57. doi:10.5152/eurasianjmed.2016.16012

Besner D, Stolz JA. Unconsciously controlled processing: the stroop effect reconsidered .  Psychonomic Bulletin & Review . 1999;6(3):449-455. doi:10.3758/BF03210834

Frings C, Englert J, Wentura D, Bermeitinger C. Decomposing the emotional Stroop effect .  Quarterly Journal of Experimental Psychology . 2010;63(1):42-49. doi:10.1080/17470210903156594

By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

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3 Lab 3. Stroop Interference: Pay No Attention to the Man Behind the Curtain

CogLab Exercise 13

The Stroop interference task is one of the most familiar—and most intriguing—experiments in the history of cognitive psychology. It is appealing because of its simplicity, and for its ability to consistently trip up subjects performing the test. The most standard version of the Stroop test requires subjects to say aloud the color of ink that a word is printed in. The tricky part is that the words are color words different from the ink they’re printed in. For example, if the word BLUE was written in red ink, subjects would have to say “red” instead of “blue.”

It sounds simple, but when attempting to read a list of such incongruent color words quickly, the task becomes extremely difficult. Understanding what exactly makes the task so challenging has intrigued cognitive psychologists since John Ridley Stroop first introduced the experiment in 1935(Stroop, 1935).

When Stroop designed his experiment, it was a logical yet creative synthesis of all prior research on the concept of interference. Numerous others before him had shown that subjects could read the word “blue” more quickly than they could look at a blue square and say that it was blue. James Cattell, in 1886, claimed that this is because we are trained to read words when we see them, but we are not in the practice of naming the colors of things when we first encounter them. (We’re more likely to think “apple” than “red”). So, the delay in naming the color of a square (compared to reading the word for the color) was explained by our natural tendency to identify the square as a square before we identify its color.

This notion is the essence of one of the primary theories explaining the Stroop effect: automaticity. For any literate person, reading is an automatic process. When we encounter a word, we read it. It’s hard not to read a word, even when we’re asked not to. For example, try not reading the bold words in the next sentence. I bet you weren’t able to ignore the bold words, were you? You had to look at the word to see if it was bold, and by then you had already read it. The same situation is created by the Stroop task. Subjects are asked to ignore the meaning of the word YELLOW and focus only on the color of the ink it is printed in. But, just like with the apple, before they notice the color they notice the word itself. Thus the interference.

Stroop was the first one who came up with the idea of combining the color naming task and the word reading task. Though he is most renowned for this portion of his study, it actually included other experiments. His first experiment sought to discover whether or not interference worked in the opposite direction. That is, if subjects would have trouble reading the word YELLOW when it was printed in blue ink. Both Stroop’s data and subsequent replications show that incongruent colors cause no significant interference on the reading of the words. This supports the automaticity hypothesis.

The other primary theory explaining the Stroop effect is a speed of processing hypothesis, arguing that both the word-reading and color-naming take place simultaneously, but that word naming occurs faster and thus interferes with the color naming. It is assumed that the faster process can interfere with the slower process, but not vice versa.

Both theories have enjoyed rigorous support, and both have been easily refuted. The speed of processing theory is challenged when the words are presented either backwards or upside down (Ex: WOLLEY). As would be expected, it takes considerably longer to read these distorted words. In this case, identifying the word’s color happens faster than reading the word, so the roles are reversed. Despite it taking longer to read the words, the incongruent colors still interfere significantly with reporting the color of the print. Because the slower process was then affecting the faster one, the speed of processing theory had to be reconsidered.

The automaticity theory was challenged by Kahneman and Chajczyk (1983). They determined that the automaticity effect could be “diluted” by including an additional “non-color” word in the stimulus. This had the effect of splitting the subjects’ attention and actually reduced the interference of incongruent colors. They essentially discovered that reading a word was less automatic when the subjects had to choose which of two words to read first. This introduced a sliding scale of automaticity, which is counter-intuitive and challenged the entire theory. How can something be partly automatic? However, because a lessened form of interference still existed, the theory had to be modified rather than dismissed altogether. The modification produced a “ continuum of automaticity,” claiming that actions only became automatic with continued practice, and what was automatic could be altered over time. In essence, automatic was redefined as “well-practiced.”

Interestingly, Stroop’s often-overlooked third experiment in his 1935 study addressed this very issue. Stroop was convinced that subjects could overcome the interference effects of the task if they simply practiced focusing on the color of the ink instead of the word itself. He had 32 subjects do just this for eight days, and found that the mean amount of time it took them to name the colors of 50 words dropped from 49.6 seconds to 32.8 seconds. This amount of practice also produced interference on reading the words rather than naming their colors. The mean time jumped from 19.4 seconds to 34.8 seconds after the eight days of training. This was the first report of what is now called the “reverse Stroop effect.” However, this reverse effect disappeared quickly. In a second reading immediately following the first, the time had recovered to a near baseline 22.0 seconds. Overall, Stroop accepted the explanation that our everyday lives leave us more prepared to read words than to analyze their colors.

The effect Stroop discovered and publicized is an important one. It is so replicable that it is often included on tests given to assess the extent of mental disorders and learning disabilities. One of the more intriguing frameworks in which the effect has been analyzed is that of working memory. Working memory is a form of short-term memory that allows us to keep bits of information cued up while solving a problem. (Remembering that you’re supposed to focus on the color of ink instead of what the word says is a perfect example). Long and Prat (2002)theorized that subjects with lower working memory capacities would succumb more easily to the interference of the Stroop effect than would subjects with high capacities. They found that their theory was correct, but only when the words presented were consistently incongruent. When congruent words were frequently mixed in (ex: RED printed in red ink), the high capacity subjects performed just as poorly as the low capacity subjects. This shows that the high capacity subjects were actively employing a strategy to complete the task, but that they would only rely on such a strategy when it was obvious that most or all of the words were incongruent.

Long and Prat (2002) wondered if these high capacity subjects were merely ignoring the meaning of the words, or if they were reading them and actively suppressing them. The theory of automaticity suggests they should be unable to look at the words without reading them, and this was indeed the case. Long and Prat found that if the color they were ignoring in one trial was the color they were supposed to name in the next trial, the high capacity subjects struggled. Look at the table below:

Because subjects were forced to suppress the word BLUE in trial one, they found it harder to say BLUE in the immediately following trial two. Long and Prat call this phenomenon negative priming. Interestingly, the low-capacity subjects showed considerably less interference as a result of negative priming, presumably because they had less capability to hold the circumstances of the previous trial in their working memory.

As you can imagine, there are numerous explanations of the Stroop effect, depending on which viewpoint one chooses to approach it from. This is why it is still a pertinent and interesting phenomenon to study seventy years after it was first discovered (MacLeod, 1991; MacLeod & Dunbar, 1988). Participants are asked to count how many characters are presented (either 1, 2, 3, or 4). It’s easy when the characters are x’s (i.e. XXX—correct answer: 3), but more challenging when they are digits (i.e. 444—correct answer: 3).

Our lab involves the classical version of the Stroop test with same (congruent) and different (incongruent) colors. Even though you now understand what the Stroop test is about, it will almost certainly still be a challenge.( 3)

At this time, complete the experiment stroop effect in CogLab . Instructions can be found in Lab 13 of the COGLAB Website .

Questions for Lab 3

1. What are the independent and dependent variables in this experiment?

2. Graph both your data and the class data. Did we achieve a Stroop effect? Why or why not? If your results are significantly different from the class data, how would you explain this difference?

3. Is there a speed-accuracy tradeoff in this experiment? In other words, did your accuracy increase as you took more time to make your answers? Why or why not? Is this the expected relationship between speed and accuracy?

4. What if H. M. was a subject in Stroop’s experiment 3, which was designed to see if days and days of practice eliminated the Stroop interference effect? How do you think he would perform compared to the other subjects? (Hint: look back at the information in Lab 1 regarding case studies for some information regarding H.M.’s memory. What type of memory would be involved in the Stroop task? Was that type of memory disrupted in H.M.’s case?)

5. Can you think of a real-world situation where the Stroop effect might take place? How about a reverse Stroop effect? (Just to get you thinking…researchers have documented what is known as an emotional Stroop effect. This happens, for example, when subjects who are deathly afraid of spiders are asked to name the color of ink used to print words such as “crawl,” “venom,” and “web.” It takes them longer to perform the task with these words than with words that are more generally associated with fear, such as “death.” So, can you think of some other possibilities?)

6. Given the theory of automaticity, how would you expect subjects to perform if the words were in a language they were barely familiar with? For example, I know the Spanish color words, but I certainly don’t use them everyday. What would happen if the test included items such as AZUL printed in green ink?

7. At a cross walk, there is often a signal that tells pedestrians when to walk and when not to walk. Typically these signals use an upright hand to represent do not walk and a human figure to indicate when walking is permitted. What colors are usually associated with these symbols? Do the colors and symbols seem congruent (support a correct response) or incongruent (inhibit a correct response)? Explain.

Data Sheet for Lab 3

NAME: ____________________________

Report Mean Reaction Time (ms):

Graphs for Lab 3

Individual Data

Name: _____________________________

Laboratory in Cognition Student Manual Copyright © by Charles Weaver, III. All Rights Reserved.

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A full documentation of the Stroop experiment

What the Stroop task is

See it yourself, example bitmap instruction1.png, example bitmap yellowgreen.png, code explained, feedback to participants explained..

The Stroop task is one of the best known cognitive psychological experiments. First make sure you understand what it is about by reading the lesson about the phenomenon itself .

In short, in this task you need to name the "ink" color of the words. Thus, if you see the word "red" written in green ink, you need to say "green", and not "red". It turns out this is not at all easy.

This lesson not only teaches you how to present the Stroop task itself, but also how to use feedback, which is a bit sophisticated.

First run the example to see what it does (this is the same as in the other Stroop lesson).

Click here to run a demo of the Stroop task

Download the code

Images = Bitmaps

Almost every experiment will have some images to be shown. This is a key aspect of every experiment. The computer needs a name for these. Typically, images have an "extension", such as ".jpg" for jpeg files, or ".png" for png files. Most digital photos you take are in jpeg format, but many pictures on the internet are in png format. PsyToolkit can work with any format, but png is the default format.

Below is an example of two pictures:

The image below is made with Inkscape and has a lot of the instructions. The reason why PsyToolkit relies on Inkscape made images so much is that Inkscape is just a great program to make images with. Of course, you can use other software as well, you can even make png images with PowerPoint.

The picture below is one of the import stimuli use. For each color word, such as yellow printed in green ink, there is one bitmap.

Below, you see the experiment source code with annotations. Before you start reading, note that there are a few important things to note about PsyToolkit experiment code:

1) There are different "sections". Sections such as "bitmaps", "task", "table", "block" are all separated from one another by an empty line.

2) When the computer runs the experiment, it starts really with the first "block" section there is. So in order to understand how the experiment is carried out, you should scroll down a bit and look for a line starting with "block".

3) The "task" section describes one trial. The computer executes this line for line. Some lines do stuff, such as showing an image on the screen, whereas other lines just tell the computer on what information it has to have (e.g., which keyboard keys are being used in this task).

4) The "block" section tells the computer how many trials there are. If you have instructions or feedback after the trials have been done, this all should be in the block.

The code has the following code as the last bit of the block. It is not necessary to do the experiment, but it is actually really nice to have participant feedback for participants.

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Stroop Effect experiment

Brain, behaviour and cognition (psych100), university of waikato, recommended for you.

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Laboratory Experiment 3 – The Stroop Effect

Introduction: The ‘Stroop Effect’ experiment is a very well-known study in psychology, still used today to explore a large variety of cognitive processes. Based on the previous research of John Ridley Stroop. Stroop discovered that people find it more difficult to name the object or its properties, but reading and speaking the name of a particular item came without thought. Stroop was quite interested in what he previously found and so he produced the study of the Stroop effect experiment – involving a variety of colour-word conditions. The experiment’s name originated from Stroop’s PhD thesis back in 1935. [ CITATION Str35 \l 1033 ]

The Stroop effect experiment begins with participants looking at a computer screen, and when a word appears, they are to name the colour of the word, not the word itself. Other conditions included the name of a colour being presented in a different colour, proving to be quite challenging for the participants. The hypothesis of this study, was to find out whether our independent variable - the different types of words made a difference with our dependent variable – the accuracy and speed of the colours being spoken.

As seen in the experiment, the confusion led to either prolonged answers or many errors if they answered it too quickly. Automaticity is a cognitive process, where someone can speaks or do something without realisation and minimal effort. With automaticity, comes action slips, where the person ends up automatically doing something without actually thinking about it, which can lead to making mistakes. An example of this occurring in everyday life is, you could be putting the milk back in the fridge after making yourself a cup of tea, and you end up placing the teapot back in there too. And in today’s learning, the Stroop effect is often

spoken of, as another example of this process. Researchers interpreted the Stroop Effect, as a Stage Model - models which have been created to describe numerous other information processing effects.

Method: To test the hypothesis above, the procedure began with all participants in the psychology computer room. It involved two people pairing up – one who spoke and the other who pressed the ‘space bar’ and selected the answer, after they had heard a response. The main objective was to state the colour of the printed word as soon as it appeared on the screen. Each person went through 60 different conditions, answering each one as quickly and accurately as they could, then we switched roles.

Results: Through the differences obtained by the time and accuracy measures, we can see they are statistically significant. The significance for the first graph, for time was like this; F(2,152) = 135, p <.001. And for the second graph for accuracy, the significance was (F(2,152) = 14, p <.001. With both factors having 2 degrees of freedom (2). The ‘p’ statistic representing <.001, shows there is no chance of our results being acquired by chance. The non-sense word type had the quickest response time and the highest amount of correct answers from the participant. The common word type appears to be in the middle, showing it had a faster response than the colour word type but still took longer than the non-sense word type. Lastly, the colour word type took the longest to figure out and had the lowest percentage of correct answers.

Figure 2. Graph for Accuracy

Table 2: Tests of Within-Subjects Effects for Accuracy

Discussion: From what we can see in the graphs and tables above, it definitely supported our hypothesis of whether or not the different types of words made a difference on the speed and accuracy of the colours being spoken. This experiment has shown us that regardless of the different types of words (at different difficulty levels), there is a significant effect on the speed at which a

participant will respond and the accuracy of that particular answer. These results relate back to what John Ridley found in his Stroop dissertation back in 1935. A limitation I personally think that could impose and influence the results of this experiment is distractions in the lab room. This type of experiment requires a person’s full attention and focus and the speed and accuracy of the answers could be inexact, if the participant is surrounded by loud talking/discussion.

References: Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18.

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Course : Brain, Behaviour and Cognition (PSYCH100)

University : university of waikato.

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In a Stroop task, participants are presented with a list of words, with each word displayed in a color of ink. The participant’s task is to say out loud the color of the ink in which the word is printed. The task has two conditions: a congruent words condition, and an incongruent words condition. In the congruent words condition, the words being displayed are color words whose names match the colors in which they are printed: for example RED, BLUE. In the incongruent words condition, the words displayed are color words whose names do not match the colors in which they are printed: for example PURPLE, ORANGE. In each case, we measure the time it takes to name the ink colors in equally-sized lists. Each participant will go through and record a time from each condition.

My test results:

Congruent words - 10.905

Incongruent words - 34.41

1. What is our independent variable? What is our dependent variable?

Independent variable is: The word/colour congruency is the varable being manipulated in the experiment

Dependent variable is: The time it takes to recognize/name the ink colors of the mismatch word/colour congruency

2. What is an appropriate set of hypotheses for this task? What kind of

statistical test do you expect to perform? Justify your choices. Null Hypothsis, H0 - The mismatch of color to word will have no effect or decrease time to recognize and say the color

Alternate Hypothesis, H1 - The mismatch of color to word will increase time to recognize and say the color

H0: μi ≤ μc (μi - population mean of incongruent values, μc - population mean of congruent values)

H1: μi > μc (μi - population mean of incongruent values, μc - population mean of congruent values)

statistical test choices and assumptions

95% confidence interval

Paired one tail t-test -> with two tests per participant this test show if the mean of incongruent words is statistically significantly different from the congruent words at an alpha of 0.05.

assumptions/why : I will be using a t-test instead of a z-test because 1) the population standard deviation is unknown and 2) the sample set is less than 30. The t-test will be a one tailed t-test i.e. my directional alternative hypothesis is that participant’s incongruent sample mean will be larger than the participant’s congruent sample mean

A paired t-test (or dependent sample test), will be used because the data set is of one group of participants tested twice under different conditions (word/colour congruency). This will also facilitate either rejecting or accepting the null hypothesis.

3. Report some descriptive statistics regarding this dataset. Include at

least one measure of central tendency and at least one measure of variability.

sample size = 24

mean: $xbar = \Sigma{x}/n$ (where xbar is the sample mean, x is the value and n is the number of samples)

Congruent: 14.05, Incogruent: 22.02

median: as the data seems slightly positively skewed, median is a better representation of central tendency

Congruent: 14.3565, Incongruent: 21.0175

sample std. deviation: $\sigma = \Sigma{(x - xbar)^2}/n$

Congruent: 3.56, Incongruent: 4.80

4. Provide one or two visualizations that show the distribution of the

sample data. Write one or two sentences noting what you observe about the plot or plots.

Please see below a boxplot and histogram which show the distribution of data from both congruent and incongruent conditions.

Observations

From the boxplot, there are two somewhat obvious outliers or extraneous data which would possible skew the true mean of incongruent values. And from the histogram plots, although both graphs visually appear somewhat positively skewed, the mean is pretty close to the peak in both graphs which would indicate a normal distribution. Provided these are samples from the population, the sampling mean would be similar to the population mean.

5. Now, perform the statistical test and report your results. What is your

confidence level and your critical statistic value? Do you reject the null hypothesis or fail to reject it? Come to a conclusion in terms of the experiment task. Did the results match up with your expectations?

mean difference , dbar = $\Sigma (yi − xc)/n$ = 7.964

yi is incongruent, xc is the congruent values, n is the sample set

standard deviation , sd = 4.86

standard error of the difference , SE(dbar) = sd/√n = 4.86/√24 = 0.99

t-statistic , T = dbar/SE(dbar) = 7.964/0.99 = 8.04 on 23df

t-distribution with n-1 degrees of freedom (df = 23). Using the t-distribution table to find p-value…

The value of p is < 0.0001 . The result is significant at p < 0.05%

I reject the null hypothesis, the word/colour incongruent does cause a greater time response

The results match my expectations.

6. Optional: What do you think is responsible for the effects observed? Can

you think of an alternative or similar task that would result in a similar effect? Some research about the problem will be helpful for thinking about these two questions!

I believe it’s a subconscious or habitual behaviour where commonly used words are glanced over and easily recognized in your mind and therefore, because the color and words match, it takes little effort to say the word/colour. However, when the word and colour are mismatched, we first think of the word and then need to correct ourselves to say the colour causing either errors or more time to provide the correct response.

I imagine replacing letters with numbers in the experiment would yeild the same results.

Another similar task: being shown a list of words and asking the user to type (on a qwerty keyboard) commonly used words. First, use a list words with no spelling mistakes and then the same list of words with obviously misspelling or a number in the middle of the word. The reflects of knowing where the keys are positioned and the knoweledge of the correctly spelt word would allow cause the users to produce a fast time for correctly spelt words and a longer time to recognize that the word is spelt incorrectly and having to change their habitual behaviour, causing a lag and longer time to complet the word.

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

http://www.statisticshowto.com/when-to-use-a-t-score-vs-z-score/

http://www.biostathandbook.com/testchoice.html

http://www.ats.ucla.edu/stat/stata/whatstat/whatstat.htm

http://www.statstutor.ac.uk/resources/uploaded/paired-t-test.pdf

https://www.stat.tamu.edu/~lzhou/stat302/T-Table.pdf