The emotional Stroop effect (ESE) is the result of longer naming latencies to ink colors of emotion words than those of neutral words. This report refers to potential sources of confounding and includes a modal experiment that provides the means to control for them.
The emotional Stroop effect (ESE) is the result of longer naming latencies to ink colors of emotion words than to ink colors of neutral words. The difference shows that people are affected by the emotional content conveyed by the carrier words even though they are irrelevant to the color-naming task at hand. The ESE has been widely deployed with patient populations, as well as with non-selected populations, because the emotion words can be selected to match the tested pathology. The ESE is a powerful tool, yet it is vulnerable to various threats to its validity. This report refers to potential sources of confounding and includes a modal experiment that provides the means to control for them. The most prevalent threat to the validity of existing ESE studies is sustained effects and habituation wrought about by repeated exposure to emotion stimuli. Consequently, the order of exposure to emotion and neutral stimuli is of utmost importance. We show that in the standard design, only one specific order produces the ESE.
Modern life is replete with emotion and stress. Who has avoided the emergency room or (witnessing) a traffic accident? In order to perform efficiently under such stressful situations, it is important to preserve one's composure by focusing on the relevant stimuli. However, research has shown that the emotional valence of the stimulus can affect attention, in particular modulate the speed of processing. In the laboratory, one of the most popular paradigms to study the effect of negative stimuli on performance is the emotional Stroop task. The typical finding is that it takes people longer to name the ink color of emotion words than that of neutral words, the Emotional Stroop Effect (ESE). There are several accounts that attempt to explain the observed slowdown attributing attention3, freezing2, or mood3, however it is still a matter of current debate.
The experimental setup of the emotional Stroop task is well known. Words in color are presented singly for view and the participant's task is to name the ink color of each word as quickly and accurately as possible. The words come from two categories of different valence. The first category includes negative words (e.g., DEATH) or words related to a specific psychopathology (e.g., GERMS with obsessive-compulsive patients or BATTLE with post-traumatic stress disorder patients). The second category includes neutral words (e.g., CHAIR). The ESE is the difference in color-naming latency between the emotional and the neutral words. The stimuli can be presented in a single block with emotion and neutral words intermixed in a random fashion or in two separate blocks defined by word category. The slowdown with emotion words is usually more pronounced when the ESE is derived in the blocked design4, 5. Therefore, the block design has become the method of choice for researches of the ESE and it is the method applied in this protocol too (see Figure 1 for an illustration of the emotional Stroop experimental setup).
Figure 1: The Experimental Array in the Emotional Stroop Task: The participant's task is to name color in which the word appears. Please click here to view a larger version of this figure.
It is important to distinguish the ESE from its older namesake, the classic Stroop effect (SE)6. In the classic Stroop task, color words such as RED or GREEN are presented in various ink colors and the participant's task is to name the ink color of the words. Despite the shared task — to identify the ink color of words — the SE and the ESE differ. Because the words are color words, the stimuli in the classic Stroop task divide into congruent (the word naming its ink color) and incongruent (word and color conflict) categories. The SE is defined as the difference in color naming performance between congruent and incongruent stimuli. Because the quality of congruity does not apply to stimuli in the ESE — the word CANCER in blue is neither more nor less congruent than the word LECTURE in brown — the SE is not defined in the environment of the ESE. The ESE documents the effect of the emotional valence of the stimuli on performance.
The ESE, just like the SE, has generated voluminous research. In fact, the ESE rivals its namesake in sheer output of experimental studies with both patient and non-patient populations (for a review, see1, 7). The task has been employed with a gamut of pathologies from generalized anxiety (e.g., 8, 9) to trait anxiety (e.g., 10, 11) to obsessive-compulsive disorders (e.g., 12, 13) to depression9, 14 to social phobia15, 16 to post-traumatic stress disorders (e.g., 17, 18). The ESE has also been studied with unselected populations (e.g., 2, 3, 19, 20), although the effects in heathy participants are not always observed and are often less pronounced. At least a portion of ESE's popularity is attributable to its objective nature as it is not based on self-report and is not intrusive. Furthermore, the emotion words can be selected to tap the specific pathology or current concern of the patient.
Below, we portray the steps required to design and perform an emotional Stroop experiment. Our purpose in this report is to describe in detail an ESE experiment with needed controls. The most important feature of this design is the control it provides against various threats to validity. The main threat treated in this design is that of habituation. Adopting these procedures renders the ESE a valid and reliable means of assessing attention under emotion.
The protocol follows the guidelines of Tel-Aviv University Helsinki human research ethics committee.
1. Word Selection and Matching
Emotional | Neutral | ||||
Word | Length | Frequency | Word | Length | Frequency |
hate | 4 | 10.7 | gate | 4 | 9.7 |
dead | 4 | 11.2 | dear | 4 | 10.29 |
poor | 4 | 10.9 | pool | 4 | 9.7 |
snake | 5 | 8.6 | shake | 5 | 8.6 |
gloom | 5 | 8.1 | bloom | 5 | 8.2 |
bomb | 4 | 9.64 | comb | 4 | 7.39 |
pox | 3 | 7.1 | box | 3 | 12.1 |
Average | 4.1 | 9.4 | Average | 4.1 | 9.4 |
2. Preparation of Experimental Design
3. Experimental Programing and Randomization
4. Subject Selection and Preparation
5. Data and Statistical Analysis
When blocks follow the neutral-emotion-neutral sequence (e.g., 3), a large ESE of 34 msec is observed via slower responses in the emotion block (mean of 791 msec) than in the first neutral block (mean of 757 msec; see Figure 2). The same group of participants was also fairly sluggish to name the ink color in the second set of (other) neutral words (mean of 778 msec). The 21 msec difference in performance between the two blocks with neutral items documents the presence of the sustained effect of exposure to negative emotional stimuli.
Figure 2: Mean RTs to Name the Ink-color of Singly Presented Words in Three Blocks of Trials with Neutral, Emotion, and More Neutral Items. The blocks with neutral items entail different matched words. Vocal responses were used in this experiment. The error bars depict one standard error around the mean. Please click here to view a larger version of this figure.
In order to verify that there is no flagging of attention or fatigue (especially with a fixed order of blocks), an auxiliary experiment entailing solely blocks of neutral items can be performed. If there is no difference in performance across successive blocks of trials, one can assume that such effects are minimal (see Figure 3).
Figure 3: Mean RTs to Name the Ink-color of Singly Presented Words in Three Blocks of Trials with Neutral Items. Each block entails a different set of matched words. Vocal responses were used in this experiment. The error bars depict one standard error around the mean. Please click here to view a larger version of this figure.
In the typical ESE study in the literature with only two blocks of trials (emotion, neutral), an ESE is only expected to emerge in the group of participants performing first in the neutral block. There is not an ESE in the reversed order of blocks. This should result in an interaction of Block Valence and Block Order in the pertinent ANOVA. Of course, this standard design is not suited to test sustained effects or fatigue and habituation. Therefore, we suggest using designs with a minimum of three blocks, neutral-emotion-neutral (Figure 4).
Figure 4: Mean RTs to Name the Ink Color of Emotion and Neutral Words Presented in Blocks of Different Order. In the Neutral-then-Emotional (NE) group, the block of neutral words preceded the block of emotion words. In the Emotional-then-Neutral (EN) group, the block of emotion words preceded that of neutral words. Vocal responses were used in this experiment. The error bars depict one standard error around the mean. This figure has been modified from3. Please click here to view a larger version of this figure.
Supplemental Code File. Please click here to download this file.
The ESE comprises a very simple task: The participant names the ink color of singly presented words. This simple task yields results of both pragmatic and theoretical consequence. The ESE documents the fact that people are sensitive to the emotional valence entailed in stimuli although this feature is completely irrelevant to the task at hand.
The ESE has evolved into an immensely popular method for probing emotion and anxiety with both patient and non-patient populations1, 74. Its appeal can be attributed to its potential as an objective (computer-based)diagnostic tool, free of potential patient-therapist interaction bias. The emotion words can be selected to match the specific pathology or current concern of the patient. Furthermore, the tool is not intrusive nor self-report based. The efficiency of the ESE is firmly established at the group level, but it has yet to be demonstrated at the individual level29, 30. Further studies are needed in order to assess the reliability of the individual patient's ESE and its relation with other known measures of anxiety and other computer based paradigms such as the dot probe31. Also, despite the prevalence of the ESE, the precise magnitude of the effect is moot, with reported effect sizes ranging from -1 to 400 msec1. This is partly due to the use of different settings (e.g., computer/ cards/ oral) or to the specific pathology group tested. However, substantial variability is still evident in studies testing well-defined population groups in similar test settings. One purpose of the current protocol is to present a clear and standardized procedure byway of removing confounding and unwanted variability. This can be achieved by employing critical steps such as avoiding word repetitions, bypassing habituation, and allowing for proper lexical control. Following these guidelines should help researchers collect valid data, draw unbiased conclusions, improve reliability, and aid with comparisons across various emotional Stroop studies.
The importance of the critical steps granted, there are many possibilities for variation. Given the possibility of different research questions, the current protocol may not be optimal for some and departures from the current protocol are possible. For example, if a researcher wishes to examine the effect of vocal emotional interference, modifications of the protocol may be necessary Researchers should decide their preferred method of administration to fit their experimental needs. Variations also apply to the number of blocks and word categories to individual word selection (e.g., controlling for additional lexical variables such as number of syllables) to determining the number of trials (words) within blocks to employing a mixed or blocked design to introducing fixed/randomized/counterbalanced order of blocks to choosing a vocal or keypress responding to choosing the colors or specifying the inter-trial and block intervals. The advantages and disadvantages of each of these considerations are addressed in the relevant protocol steps. Most can be fitted to one's needs and individual preference.
The authors have nothing to disclose.
The authors have no acknowledgements.
DirectRT software | Empirisoft | http://www.empirisoft.com/directrt.aspx | Software allows easy programing of a custom experiment in a spreadsheet application (e.g., Excel) |
Personal computer | Any | https://www.office.com/ | At least 256mb of memory, a graphics card with at least 16mb of video memory and a processor speed of at least 500mhz. |
A spreadsheet application (e.g., Excel) | Microsoft | Any version | A spreadsheet application capable of editing CSV files |
SPSS/STATISTICA software | IBM/Statsoft | http://www-01.ibm.com/software/analytics/spss/ http://www.statsoft.com/ | Statistical software |