Events in a trial (feedback)

In last week’s task, the trial structure was relatively simple: Participants were shown a text stimulus and were asked to choose between two gambles. We later added a text to remind participants of which keys they could press to express their choice, as well as a “Get ready” text at the beginning of each trial.

In particular, we will focus on the feedback today and on conditional branching next week.

Feedback

Feedback is quite common both in perceptual and in value-based decision tasks. It can have a specific function (e.g., to drive learning) or is used simply to notify that a response was recorded and the trial is over (people like to receive feedback, and can be thus used as motivator).

Today, we are going to use the stimuli for a categorization task, taken from Spiering & Ashby (2008). You can download them using this Dropbox link.

In this task, participants learn to categorize these 30 patches (visual stimuli that vary across trials in bar width and bar orientation) into 2 categories, A and B (the boundaries of which are denoted by the thick line in the figure), by trial and error. The feedback of “correct” and “incorrect” is provided after each choice and guides learning in the subsequent trials.

A new script

Create a new Inquisit script file and name it wpa_4_FirstLast.iqx (with your First and Last name). This is the file you will email me at the end of the class.

Now that you have a script file, define a defaults element at the beginning of the file (as you did in WPA3).

1. Define the choice stimuli

Just like we used the item element to define our text strings last week, we can use the item element this week to load our image files. The way we do this is by specifying the filepath for each image, where we specified the text string last week. This filepath is from the location your Inquisit script is saved in. Therefore, if your image files are saved in the same location as your .iqx file, you just have to give the file name. For example:

<item example>  
/1="patch1.png" 
/2="patch2.png" 
</item>

Otherwise, if they are in a sub-folder named for example “stimuli_wpa4”, you will write:

<item example>  
/1="stimuli_wpa4/patch1.png" 
/2="stimuli_wpa4/patch2.png" 
</item>

An alternative to this method is to load stimuli lists from a text file. I prepared a stimulus list text file in the folder, in case you’d like to try that out.

Last week we had a single item element. Now we have two categories of stimuli, category A and category B. In our design, we need to be able to provide feedback to participants on whether they were correct or not during the training phase of the Experiment. This means that we need to be able to tell Inquisit which category each item is in, so that it can provide the appropriate feedback based on the participants response. This requires two separate stimuli lists, for A and B items.

Exercise 1a: Create your stimuli lists. Give each list an informative name (e.g., stimlistA and stimlistB).

Just like with the text elements, now that we have defined our stimuli, we need to specify how they should look when displayed on the screen. Because our stimuli are pictures, not text, we can do this by defining a picture element. This element has an items attribute and a select attribute, just like the text element, as well as attributes for defining location, size etc.

Exercise 1b: Based on your text code from last week, create the appropriate picture elements, so that the stimuli are displayed in the center of the screen (use the help function if you want a reminder on what the position, items, and select attributes do). You can also change other attributes of the image being displayed. I recommend changing the size of the image as well.

NOTE: We need as many picture elements as we have item elements, even though all pictures will be displayed in the same way.

2. Define the Trials

Exercise 2a: Create a trial element for each of our stimuli types, similar to the ones from last week. The valid responses for all elements should be a and b.

In the paper, there is a break of 1500ms between each trial. Build this into your trial elements. Last week we controlled how much time there was between each trial by using the stimulustimes attribute of the trial element. This is useful if we want to display something during that time (for example: a warning to get ready, or a fixation cross). However, we can also explicitly tell Inquisit to pause between trials, using either the pretrialpause attribute or the posttrialpause attribute. The first of these prepares the trial, but then waits the designated time before beginning it. This can be advantageous if it takes Inquisit some time to prepare the next trial (e.g., it has to load large stimuli, or do calculations), because it ensures that their is a consistent time gap between trials.

Exercise 2b: Try setting the pause between trials of 1500 ms using pretrialpause.

Now we have to tell Inquisit what counts as the correct response on each trial.

Exercise 2c: Look at the help file for the trial element, and see if you can work out how to set the correct response for each trial type.

Inquisit can display information that is dependent on participants’ choices in several ways. The simplest is to display a message based on the response they gave. There are 3 main message attributes, correctmessage, errormessage and responsemessage. We will use the first 2. Each of these, takes two pieces of information: the name of the stimulus that is displayed as the message, and the length of time for which it should be displayed.

This means that we need to create some stimuli for Inquisit to display. We’ll just create text stimuli for both messages. When they get a category right, I want to display the word Correct! in large font colored blue, when they get the category wrong, it should display Incorrect! in red instead.

Exercise 2d: Create the text elements to do this. Then, use the correctmessage and errormessage attributes to tell each trial how to use these two text stimuli. You’ll need to do this for each trial type.

3. Define the Block and Experiment

For simplicity, we will just have 1 block of 30 trials, where we randomize the category A and B stimuli.

Exercise 3a: Create 1 block and 1 experiment elements. To find out how to randomly select from 2 trial types, check the examples in the Inquisit help page for the block element.

Testing the code

Congratulations, you should now have a working Experiment. Try it out before you attempt the rest of the Exercises.

4. Final message

Now that you have a working experiment and you have gone through it once, you can check the data output. You should see a correct column, which is 0 when the response was incorrect and 1 when the response was correct.

We now want to summarize participants’ performance and show it at the end of the experiment in a text message.

For this purpose, we need to introduce the values element, which contains custom variables that can be retrieved and updated throughout the course of a script.

Exercise 4a: Create a variable inside the values element, call it accuracy and set it at an initial value of 50.

Now we need to update such variable after each trial, to reflect the performance up to that point. First, the trial element has 2 attributes called ontrialend and ontrialbegin, inside which we can specify operations that we want to be performed before or after each trial. In this case, we will use the ontrialend attribute alone. Second, each trial object already has a few values attached to it that can be “called” for various computations, as well as directly added as a column in the output data.

Exercise 4b: Within the squared brackets of the ontrialend attribute of trial A, type the label of that trial, followed by a dot, and press the tab key. You should be able to see now all the values that are attached to that object. Look for one that represents the percentage of correct responses and press enter. Now, within the squared brackets, redefine the accuracy variable to this value (to assign a value to a variable, use the notation: variable = value). NOTE: if you only use the percentage of correct recorded by the same trial, the final percentage correct which will be displayed will be the performance in the final trial category that the participant saw. Therefore, we need to calculate the average between the average performance in trial A and B to define the values.accuracy variable. Be sure that you do the same in both trial types.

Finally, we need to create a page, using the page element, that shows the final value of the accuracy variable:

<page final_message>
Congratulations!

^^ You finished the experiment.

^^ You maintaned a performance of <%values.accuracy%> percent.
</page>

The page element is used to define simple pages of text to be displayed as instructions using the preinstructions or postinstructions attribute (you can use these both at the block and experiment level). Tab and space characters occuring within the page definition will also appear in the instruction page as displayed. Line breaks, however, are stripped out prior to display. To force a line break, use the special character “^”.

The current value of a given measure for a given trial or block (as well as custom values, as in our case) can be inserted anywhere within the text of an instruction page by specifying the type of element, element’s name, and the name of the measure (<% type.name.property %>) or in case of a custom variable by specifying values and the name of the variable (<%values.accuracy%>).

Exercise 4c: Add this page at the end of the block, using the postinstructions attribute.

Inquisit provides built-in support for reporting accuracy, latency and response window measures at the end of a block of trials using the blockfeedback attribute (find it in the Inquisit Help).

Exercise 4d: Use the blockfeedback attribute instead of our custom page to show the percentage of correct responses at the end of the block.