This tutorial will walk you through creation of a full experiment to test the Stroop effect (Stroop, 1935). 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.
At the end of this tutorial you will have covered the following concepts:
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.
Stroop
and choose New
as the creation option.Congruence
.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
Stimuli
tab in the study tab bar to go to the stimulus manager and then create a new stimulus set and call it Stimuli
.Components
in the stimulus set tree and add a new group called color
with the component group type Blended
.color
group by clicking on its name in the table or in the stimulus set tree.color
group:
word
with the component type Text
.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).
word
group by clicking on its name in the table or in the stimulus set tree.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).
Once imported, the word
group should look like this (Figure 3).
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.
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).
Once your variables have been imported you should end up with something like this (Figure 5).
We have now completed building the color-word items of our stimuli. Now it's time to build our color-patches.
patch
group.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).
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.
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.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.Trials
, choose New
as the creation option and then navigate to it. This is the trials timeline.Stimuli
from Existing sets
(this is the stimulus set we created earlier).Stimulus component
and choosing color
(this is the stimulus component we created earlier).Response - keyboard
, choosing New
and naming it keys
.
keys
item in the trials timeline to add keys and values to the response. You are now in edit mode.Key
: r, Value
: redKey
: b, Value
: blueKey
: p, Value
: purpleKey
: w, Value
: brownKey
: g, Value
: greenFixation
, choosing New
and naming it fixation
.fixation
item in the trials timeline to enter edit mode, set the Timeout (ms)
to 100 and set the fixation Type
to Cross (vertical)
.New
Interstimulus interval
called ISI
, setting the Start time (ms)
to 100 and setting the Timeout (ms)
to 100.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).
We have now finished building our trials timeline.
Screen
in the trials tab bar.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.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...
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.
Logic
in the trials tab bar.Add the rules shown in (Figure 7).
As shown above in (Figure 7), we have made the following assertions:
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.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.Congruence
factor to Congruent
if both the ink-color
and word
are the same.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.
Although our experiment design is complete, it's not very friendly, so let's fix that.
Trial block
using the study tree and add new instructions, name it Instructions
and choose New
as the creation option.Instructions
to the top position in the trial block table to make it appear before Trials
.Add some introductory text (Figure 8).
Note: we could have created Instructions
in the study sequence above Trial block
but bundling Instructions
within a trial block provides the following advantages:
Response - keyboard
, New
as the creation option and name it key
.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.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
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
.
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.