The Stroop effect

This tutorial will walk you through creation of a full experiment to test the Stroop effect (Stroop, 1935) [1]. This example will be based on Experiment 2, where stimulus conflict-words and color patches are presented in random order. The task requires participants to name the color of each stimulus (i.e. the color of the word's "ink" or of the color patch), ignoring the color names spelled out by the words themselves.

Tip: Throughout the tutorial there will be links to other documentation to explain concepts in detail. If you follow a link you can return to this tutorial by hitting the back button in your browser. Alternatively, you can right-click a link and open it in a new tab.

Objectives

At the end of this tutorial you will have covered the following concepts:

Design

One-factor, three-level experiment using randomly blended stimuli (color-word and color-patch). When presented, each color-word will have a random font color applied.

Method

  • Create a new study

    1. Create a new study, call it Stroop and choose New as the creation option.
    2. Add a new factor to the study and call it Congruence.
    3. Add three new levels to the study:
      • Incongruent with a description such as ink color is different from the color referred to by the word
      • Congruent with the description such as ink color is the same as the color referred to by the word
      • Neutral with the description stimuli in which only a color-patch is displayed

  • Create stimuli for the study

    1. Click on the Stimuli tab in the study tab bar to go to the stimulus manager and then create a new stimulus set and call it Stimuli.
    2. Click Components in the stimulus set tree and add a new group called color with the component group type Blended.
    3. Navigate to the color group by clicking on its name in the table or in the stimulus set tree.
    4. Add two new groups to the color group:
      • a discrete group called word with the component type Text.
      • a discrete group called patch with the component type Image.

      At this point, you have created a blended component group (color) containing two groups to be blended (word and patch). By default, blending strategy is set to random and that's how we'll leave it for this exercise. Your stimulus set tree should now look like this (Figure 1).

      Figure 1. Example stimulus set tree (Stroop)

      Figure 1. Example stimulus set tree (Stroop)
    5. Navigate to the word group by clicking on its name in the table or in the stimulus set tree.
    6. Create a CSV file containing your color words making sure to wrap each item in a variable called ink-color, then import the CSV file. For convenience, you can download one I prepared earlier. The CSV file should look like this (Figure 2).

      Figure 2. CSV file of color words each wrapped in a variable (Stroop)

      Figure 2. CSV file of color words each wrapped in a variable (Stroop)
    7. Once imported, the word group should look like this (Figure 3).

      Figure 3. List of color words each wrapped in a variable (Stroop)

      Figure 3. List of color words each wrapped in a variable (Stroop)

      Note the items are highlighted red as something isn't right. Hovering the mouse over the list will show a detailed explanation of how to fix things. In this case, we are yet to create our ink-color variable, so let's go ahead and do that now.

    8. Create a CSV file containing your ink-color variable then import the CSV file by clicking Import variables. For convenience, you can download one I prepared earlier. The CSV file should look like this (Figure 4).

      Figure 4. CSV file of font color variables (Stroop)

      Figure 4. CSV file of font color variables (Stroop)

      Once your variables have been imported you should end up with something like this (Figure 5).

      Figure 5. Color-word items (Stroop)

      Figure 5. Color-word items (Stroop)

      We have now completed building the color-word items of our stimuli. Now it's time to build our color-patches.

    9. Navigate to the patch group.
    10. Import your patch images and set their levels to Neutral. For convenience, you can download some color-patches I prepared earlier (you'll need to unzip them after download).

    11. We have now finished building our stimuli resulting in a randomly blended group of color-words and color-patches. A random font color will be applied to each color-word via the ink-color variable whose strategy is set to Exclusive to ensure each variable value is used only once per stimulus cycle.

  • Create the trials timeline

    1. Click on Study in the study tab bar to go to the study screen and then navigate to the Study sequence by clicking on it in the study tree.
    2. Create a new trial block, name it Trial block, choose New as the creation option and then navigate to it by clicking on its name in the table or in the study tree.
    3. Create a new trials construct, name it Trials, choose New as the creation option and then navigate to it. This is the trials timeline.
    4. Set the stimulus set for the trials to operate on by clicking Stimulus set: nil and choose Stimuli from Existing sets (this is the stimulus set we created earlier).
    5. It's time to build our trials timeline. First we'll add a stimulus component by clicking New item and selecting Stimulus component and choosing color (this is the stimulus component we created earlier).
    6. Click New item and add a keyboard response by selecting Response - keyboard, choosing New and naming it keys.
    7. Click on the keys item in the trials timeline to add keys and values to the response. You are now in edit mode.
    8. Add the following using the Add key button:
      • Key: r, Value: red
      • Key: b, Value: blue
      • Key: p, Value: purple
      • Key: w, Value: brown
      • Key: g, Value: green
      Hit Save in the bottom right corner when you've finished.
    9. Add a fixation at the start of the trial by clicking New item, selecting Fixation, choosing New and naming it fixation.
    10. Click on the fixation item in the trials timeline to enter edit mode, set the Timeout (ms) to 100 and set the fixation Type to Cross (vertical).
    11. Now add a New Interstimulus interval called ISI, setting the Start time (ms) to 100 and setting the Timeout (ms) to 100.
    12. Now adjust color and keys to start after ISI by setting their Start time (ms) to 200.

      The top-to-bottom order of items on the timeline doesn't reflect their timing order, so if you'd prefer, you can reorder the items by dragging and dropping them until you have something like this (Figure 6).

      Figure 6. Trials timeline (Stroop)

      Figure 6. Trials timeline (Stroop)

      We have now finished building our trials timeline.

  • Create the trials screen

    1. Navigate to the trials screen by clicking Screen in the trials tab bar.
    2. Add fixation to the screen by hovering the mouse over the top of the grid and selecting the cell and then selecting fixation from the Insert item list.
    3. Select the 200ms interval on the screen timeline and add color to the screen using the same method you used to add fixation in the previous step.

      You can preview your work so far by clicking Preview study at the far right-hand end of the study tab bar. The stimulus text items (color-words) are a bit too small for my liking, so...

    4. In the trials timeline click on color and change the font size to 64px using the rich text editor provided.

      We have now finished building our trials screen.

  • Create the trials logic

    1. Navigate to the trials logic section by clicking Logic in the trials tab bar.
    2. Add the rules shown in (Figure 7).

      Figure 7. Stroop rules

      Figure 7. Stroop rules

      As shown above in (Figure 7), we have made the following assertions:

      • Rule 1: Sets the default value of correct to false. This means that if no other rule for correctness matches for this trial, the response (including no response) is deemed to be incorrect. This type of rule is referred to as an assignment.
      • Rule 2: Sets the default level of the Congruence factor to Incongruent if its level is not Neutral. This means that if no other rule for level matches for this trial, the level is deemed to be Incongruent (where it was not already Neutral). This type of rule is referred to as a conditional.
      • Rules 3, 4, 5, 6 and 7: Set the level of the Congruence factor to Congruent if both the ink-color and word are the same.
      • Rules 8, 9, 10, 11 and 12: Set correct to true if the key pressed (response) is the key that corresponds to either the ink-color or the color patch.

      We have now finished building our trials logic, and with that, our experiment design is complete.

  • Finishing touches

    Although our experiment design is complete, it's not very friendly, so let's fix that.

    1. Navigate to Trial block using the study tree and add new instructions, name it Instructions and choose New as the creation option.
    2. Drag Instructions to the top position in the trial block table to make it appear before Trials.
    3. Add some introductory text (Figure 8).

      Figure 8. Trial instructions (Stroop)

      Figure 8. Trial instructions (Stroop)

      Note: we could have created Instructions in the study sequence above Trial block but bundling Instructions within a trial block provides the following advantages:

    4. Add a keyboard response to your instructions by clicking New response, selecting Response - keyboard, New as the creation option and name it key.
    5. Click Edit on key and add a new key. Press the Space bar to add the space key, set the value to anything you like (e.g. next), then hit Add to add the key and then hit Save to exit edit mode.
    6. Navigate to Study sequence and add new instructions, name it Thankyou message and choose New as the creation option. Navigate to it and write a lovely thankyou message, including some instructions about the keyboard, such as Hit the space bar to end the study
    7. Add a keyboard response to your thankyou message. We can save ourselves some work by copying a keyboard we created earlier. Click New response, select Response - keyboard, select Copy as the creation option, select key (the keyboard we created in earlier instructions) and name it key. Note: The last screen in your study must include a buttons response or a keyboard response to end the study. Set the response Event to Go to next study item or trial.

Congratulations

You now have a fully functional experiment with advanced stimulus generation. Hit Preview study and try it out! To activate the study and allow participants to take part, hit the Distribution tab in the study tab bar. To download results, hit the Results tab in the study tab bar.

References

  1. Stroop, John Ridley (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology. 18(6): 643–662. doi:10.1037/h0054651. Retrieved 2018-12-04.


keywords: experiment test task example logic