Chapter 8 LIVECODE experiments
This chapter gives a brief tutorial on using the free and open-source programming language LIVECODE for programming behavioral experiments for cognitive research. The tutorial will cover some basic LIVECODE operations, programming a Stroop experiment, and scripts for preliminary data-analysis.
8.1 Download LIVECODE
The community edition of LIVECODE can be downloaded for free from the LIVECODE website:
Beginners to LIVECODE should peruse the website for helpful tutorials, and should read through the manual.
8.2 What is LIVECODE
LIVECODE is a powerful and easy to learn language for creating applications. The language is cross-platform and can be used on macs, pcs, linux machines, and even mobile devices (ios and android). LIVECODE makes it easy to create the visual layout for your program, and to add elements (text fields, buttons, etc.) by dragging and dropping. All of the elements in a LIVECODE program can be scripted.
8.3 Basic LIVECODE concepts
8.3.1 Stacks, Cards and Elements
The starting point for creating a program is the stack. When you create a new stack, you will see a new window appear. The window can be resized to any size that you wish. When you a user loads your program, they will see the stack load up as the this window.
Stacks are composed of cards. Each stack can have many cards. Cards are like individual slides in a powerpoint or keynote presentation. New cards can be added by selecting New Card from the Object menu.
Many different kinds of elements can be added to cards by dragging and dropping. These elements are accessed through the tools pallette (Tools>Tools Pallette). Typical elements are text fields (for displaying and entering text), buttons (for receiving user input), media (for showing images, playing movies and sound clips), scrollbars, graphic objects, and so on. The size and position of these objects on each card can be changed using the mouse, much like elements inside a powerpoint.
The stack, cards, and elements can all be given unique names using the property inspector. The property inspector is accessed by double-clicking on the element of choice (but make sure you are using the pointer tool, Tools>Pointer Tool).
Scripts are used to implemement algorithms and to control how part of your stack work. Almost every thing in LIVECODE is scriptable. The stack itself is an entity that can be scripted. Each card is its own entity that be scripted. And all of the elements on each card are their own entities that can be scripted. It is important to realize that LIVECODE scripts work in a hierarchical fasion. The stack script is at the top of the hierarchy, the card scripts are next, and the element scripts are at the bottom. Generally, the scripts that are written at each level are specific to that level. However, writing a function at a higher level (in the stack script), allows that function to be available for use at the lower levels. The reverse is not true. Writing a function inside a button script that is on a card does not mean that function will be available to other elements on the same card or other cards. Similarly, variables that are created inside scripts for the stack, cards, or elements, are local to those entities, but they can be made global (accesible to all) if this is specified in the script.
You can access the scripting editor for the stack by Object>Stack Script. Similarly, you can access the scripting editor for the card by Object>Card Script. If you drag a text field and a button onto an empty stack, then you access the scripts for these elements by right-clicking on them and choosing edit script.
SHORTCUTS TO THE EDITOR: When you have selected the edit tool (the arrow with the crosshairs in it) you can double-click on objects and the property menu for that object will appear. Click on the play button and choose edit script. Also, if you are on a mac you can hold down option-command and click on an object to automatically open the script editor.
18.104.22.168.1 Getting started with Scripting
To get a hang of the basic syntax of LIVECODE drag a text field and a button onto the card. Open the script associated with the button. You will see an editor appear with the following text:
on mouseUp end mouseUp
These statements are the bookends of the scripts that can be written for the button. They are the opening and closing clauses. The statement “on mouseUp” is waiting for the button to receive a mouse-click. When you click a mouse, you first press the button down, then you release the button. When the button is released it will receive a “mouseUp” message. When this message is received the button will execute the script that is contained between the on and end mouseUp messages.
Type the following between the on and end statements so that it looks like this:
on mouseUp put "hello world" into field 1 end mouseUp
Once you have changed the contents of the script, you must press the **apply** button located in the left hand corner. Now the button is ready to run. Change the cursor from the pointer arrow to the browse arrow (just the big arrow), and click the button. If you have made a text field, then when you press the button you should see the message hello world appear inside the text field. You have made your first program in LIVECODE.
8.3.3 LIVECODE syntax
The LIVECODE language is written mostly in English and is relatively easy to follow. The following introduces you to important central concepts in the vocabulary. LIVECODE comes with its own dictionary which is very helpful. Also, if you can’t find the answer in the dictionary, you might try googling.
- put: the word put is used to move information between variables.
put 1 into x put 2 into x put "hello world" into field 1
Take a look at the first example. In this example there are two livecode intrinsics, these are words that are reserved for use by LIVECODE. The intrinsics are put and into. You will also see the double dashes “–”. These allow inline commenting for your code. Anything that appears after the double dashes (# also works) will not be interpreted as code. Commenting is useful to keep track and make notes as you code. It is also helpful for others when they are trying to make sense of your code. It is good to practice commenting as you code.
into: replaces the contents of a variables. Replaces is very important, into first wipes the variable, then inserts the new information into the variable
after: appends new information to a variable. After is different from into, after does not erase information from a variable, it puts new information after the old information in the variable.
put 1 into x put 1 after x put 2 into x
The words put, into, and after are central to the language. They take a general form:
put something into somevariable put something after somevariable
The something, can be any kind of information, numbers, letters, words, whole paragraphs. The somevariable is simply a variable that you create to hold the information, think of it like a container with a name. You choose the name by writing a string of letters following the command into or after. Here’s a few more examples
put field 1 into field 2 put word 1 of field 1 into field 2 put item 1 of x into xx put the last word of field 1 into field 2 put char 1 of word 5 of field 1 into field 2 put char 1 of word 5 of field 1 into char 6 of word 3 of field 2
These examples show some more LIVECODE intrinsics like field, char, word, and item. These will be discussed shortly.
8.3.4 Manipulating words and letters (strings)
We know by now that variables are containers for information. The programmer can give a variable any name, and this variable name is used to retrieve the stored information. The information that is stored in a variable can be numbers or letters. Depending on how the data is stored, and the kind of data that is stored, the data can be addressed or found in different ways. LIVECODE is particularly useful for dealing with information from written language. To illustrate, let’s say field 1 has the following sentence:
The quick brown fox jumped over the lazy dog
Now let’s look at some example scripts that we could write in the button to manipulate the words in the sentence. The examples assume that you have dragged a second text field (field 2) onto the stack.
put field 1 into field 2 put word 1 of field 1 into field 2 put word 3 of field 1 into field 2 put char 1 of word 3 of field 1 into field 2 put the last word of field 1 into field 2
We see a hierarchical structure. Fields are the highest level, they contain all of the words and letters. Words are the next highest level and are defined by series of characters separated by a space. Chars are the lowest level, they refer to individual characters.
8.3.5 Manipulating numbers (items)
LIVECODE uses the above syntax to parse any series of characters that are separated by a space. It doesn’t matter if the characters are letter or numbers, LIVECODE would treat them the same way. Importantly, there are other ways to store data in variables. Data is often stored in small parcels that are kept separate by a delimiter. Think of storing some data in Microsoft Excel, each piece of data is put in separate row or column. Excel is using a delimiter to keep the individual pieces of data separate. In LIVECODE the comma “,” is the default item-delimiter. The comma is used to keep pieces of data separate. For example, if we wanted to keep the numbers 1 through 10 separate, we could put them into a variable in the following way:
put "1,2,3,4,5,6,7,8,9,10" into x
Now x contains the numbers 1 through 10, with each separated by a comma. This means that variable x is item-delimited. Whenever a variable is item-delimited, the individual items can be addressed just like individual words can be addressed in the above example.
put item 3 of x into field 1 – the third item happens to be a 3 so a three is placed in field 1
IMPORTANT for later: When you store numbers using an item-delimiter then you will be able to perform math functions on those numbers.
put sum(x) into field 1
8.3.6 Advanced data storage (arrays)
LIVECODE supports more powerful ways to store data. One way is to use arrays. You can think of an array as a variable that can contain variables, each of which can contain more variables and so on. Please read the livecode user guide to learn more about using arrays. Arrays will also be covered in more detail in the data-analysis section.
8.3.7 Logic statements (IF THEN)
Logic statements are a fundamental component of programming. They are used to specify the conditions under which operations occur. The most common logic statement is the IF THEN statement. Let’s take an example
put random(2) into x if x is 1 then put "heads" into field 1 else put "tails" into field 1 end if
The first statement uses a function called random. Random generates a random number between 1 and the number that is put in between parentheses. In this case, random(2) will either generate a 1 or a 2, and then put this value into x. The next lines show the IF THEN statement. The statement looks at the value of x and then prints different results to field 1 depending on the value of x. If x is 1 then the word heads is printed to field 1. Notice the word else, which refers to any condition that does not meet the first condition. If x is anything else besides 1 then the word tails will be put into x. Here are some more examples
If x == 1 then put true into testme end if if x > 5 then put true into testme end if if x < 21 then put false into testme end if if x >= 5 then if x <= 10 then put true into testme end if end if
This last example shows us that IF THEN statements can be nested within each other, so that multiple conditions can be evaluated. Look at the full user guide, or the dictionary for more information on syntax when using IF THEN statements.
22.214.171.124.1 Other logic statements (SWITCH)
There are times when you may wish to evaluate several different conditions. You may find yourself nesting many IF THEN statements together, and the final result could be clumsy and difficult to follow. One option is to use a SWITCH statement. Learn more about SWITCH and CASE by using the dictionary or the user guide.
Another central concept in programming is the repeat loop. There are many tasks that involve doing operations over and over again. These tasks are often automatized with repeat loops. Consider the problem of adding the numbers from 1 to 10. You could try this using the add function without a repeat loop:
add 1 to x add 2 to x add 3 to x add 4 to x and so on
If you kept typing you would eventually add up all of the numbers from 1 to 10. It would be easier to use a repeat loop. There are several forms of the repeat in LIVECODE, the examples illustrates two of them.
Repeat 10 times add 1 to x add x to totalsum end repeat
This repeat loop runs 10 times. Each time a 1 is added to x, then the contents of x is added to the variable totalsum. On the first loop, x will become 1, and a 1 will be added to total sum. On the second loop, 1 will be added to x, so x will become 2, and then x will be added to total sum, which will now equal 3. At the end of the loop, the contents of the variable totalsum will be the sum of all the numbers from 1 to 10.
repeat with n = 1 to 10 add n to totalsum end repeat
This repeat loop also runs 10 times. The syntax is different. Here, the repeat loop runs in integer steps from the first to last integer, in this case from 1 to 10. For each step the variable n will be replaced with a 1, next a 2, next a 3, and so on. Inside this loop we simply add the value of n, which is changing across iterations of the loop to the variable totalsum.
Notice, just like the IF THEN statements, the repeat statements are closed with “end repeat”. This tells the computer which lines should be looped , and which should not. Repeat loops can also be nested within other.
Repeat loops can be used with words too. Let’s take another look at this sentence “The quick brown fox jumped over the lazy dog”. We could put this sentence into a variable:
put "The quick brown fox jumped over the lazy dog" into x
Now x contains the entire sentence. Remember item-delimiters? Currently each word in the sentence is separated by a space, let’s use a repeat loop to separate each word by a “,” instead. This will allow us to illustrate a different form of the repeat loop.
Repeat for each word k in x put k & "," after x2 end repeat
This repeat loop does what it says. It goes through each word (remember words are separated by spaces). In this case, the loop goes through each word in the variable x (the one that contains the sentence). As the loop moves forward it puts each word into the variable k (this variable name is arbitrary, we could use any string of letters to name the variable). To unroll this, each step of the way the variable k will contain the next word in the sentence:
Step 1, k = The
Step 2, k = quick
Step 3, k = brown
Inside the repeat loop we see only one line of code: put k & “,” after x2
The & character is a special character used by LIVECODE, it means AND in the sense of appending (put this and that somewhere). We are telling LIVECODE to put the contents of k (which on step 1 is the word The), AND “,” after the variable x2. When you use quotation marks “” in LIVECODE, this tells LIVECODE to treat whatever is inside the quotes as text. So, the statement is telling LIVECODE to do the following to x2.
Step 1, x2 = The,
Step 2, x2 = The, quick,
Step 3, x2 = The, quick, brown,
When you are writing code you have the option of writing functions. A benefit of writing functions is that they are portable, and they can help you solve the same problem in the future. In this section we will cover the syntax for writing a function, and describe why they are useful.
Let’s imagine you want to find the sum of the numbers in an itemized variable x.
put "1,2,3,4,5,6,7,8,9,10" into x
Now, x contains the numbers 1 through 10. We could write some code to add these numbers together for us.
repeat for each item j in x add j to thesum end repeat put thesum into field 1
This code will add all of the numbers and put the result into field 1. We didn’t need to write this script. LIVECODE already has a function called sum, that will do the same operation.
put sum(x) into field 1
This line of code would accomplish the same as above. The sum function is general, it will add together any item delimited set of numbers. Although LIVECODE already has a built in (intrinsic) sum function, it is instructive to write our own.
function mysum x repeat for each item j in x add j to temp end repeat return temp end mysum
Once you have written the function, then you call it using the function name and brackets ().
put mysum(x) into field 1
The code for the function can be placed inside the stack script, and this would allow the function to be used in all of the card and other element scripts. You could also place the function inside a card or a button. Here is an example of placing the function inside a a button script
on mouseUp put "1,2,3,4,5,6,7,8,9,10" into someNumbers put mysum(someNumbers) into field 1 end mouseUp function mysum x repeat for each item j in x add j to temp end repeat return temp end mysum
Note that the code for the function is placed underneath the end of the mouseUp message. When you click apply to the changes made in the button script, LIVECODE will compile the function and add it to the button so that it can be used.
8.4 Practice coding
If you are new to programming entirely, then acquiring basic competency usually means a personal investment in time and effort to learning and applying the syntax to programming problems. It is helpful to have examples of working code, and it is helpful to have a series of simple to complex problems that you can attempt to solve on your own. The process of learning how to creatively construct algorithms for solving different problems is very important, and will come in handy when designing completely new experiments or running completely new kinds of analyses. Solving problems on your will also develop your ability to debug your own code.
There are lots of different websites that list ranges of problems that can be solved by most any programming language. Check out http://projecteuler.net) as an example. Here you will be presented with problems, usually involving math, that you can try to solve with LIVECODE scripts. An example problem might be something like, what is the sum of all the numbers from 1 to 100? How would you use LIVECODE to solve this problem?
Below I have created three lists intended to help establish basic familiarity with LIVECODE. The first is a list of common LIVECODE features, this can act as a checklist of things you should know how to do in LIVECODE. The second is a list of common commands. You should know how to use each of these commands, and be able to produce examples of working code that utilizes each command. Finally, there is a list of simple to intermediate programming problems that you should be able to solve in LIVECODE.
8.4.1 LIVECODE features checklist
This is a checklist of basic to intermediate LIVECODE operations. You should be confident that you can perform each of these steps, and solve each of these problems.
8.4.2 Using the interface
126.96.36.199.1 Stacks and Cards
Create a new stack.
Resize the stack window
Save the stack to a folder on your computer
Give the stack a name using the property inspector
Add new cards to the stack
Give new cards unique names using the property inspector
Understand the difference between using the two arrow buttons
188.8.131.52.2 Text fields
Add a text field to a card
Give the text field a unique name
Resize the text field
Type text directly into the text field
Add text to the text field using the property inspector
Change the font of the text using the property inspector
Change the color of the text using the property inspector
Change the background color of the text field
Change the size of the text
Change the formatting of the text to left, right, or center justification
Hide or show the border of the text field
Lock the text so a user is prevented from making changes to the text field
Hide or show a scroll bar to the text field
184.108.40.206.4 Other Elements
Add an image element to a card
Use the property inspector to load an image from your hard drive into the image element so that it displays on the card
Name the image element
Change the size of the image element
Add a graphic element (rectangle, circle, etc.)
Name the graphic element
Change the fill color of the graphic element
Change the line width of the graphic element
Change the line color of the graphic element
220.127.116.11.5 Other basic livecode operations
Open the message box
Open the livecode dictionary
Open the property inspector for any livecode element
Open the application browser
Copy and paste livecode elements (text fields, buttons etc.) inside a card
Copy and paste a card inside a stack
Open the script editor for any livecode element
Open the script editor for a card
Open the script editor for a stack
8.4.3 LIVECODE commands
For each of these basic livecode commands you should be able to give two examples of how the command can be used in a script, and explain the meaning of the command. Here is an example using the command put.
Put: The command put is used to place content into a variable. Examples of content could be a number, a string of characters, of the contents of another variable
put 1 into x put x into y put "hello world" into field 1
on mouseDown – end mouseDown
on openCard – end openCard
on openStack – end openStack
on preOpenStack – end preOpenStack
on keyDown – end keyDown
repeat x times – end repeat
repeat with n =x to x – end repeat
repeat for each line n in field 1 – end repeat
repeat for each word n in field 1 – end repeat
repeat for each char n in field 1 – end repeat
repeat for each item n in x – end repeat
repeat forever – end repeat
repeat while – end repeat
8.4.4 Simple programming problems
For each of these programming problems, give example code that solves the problem. Create a livecode stack and solve each problem with a new button labeled with the number for each problem.
create a variable, give it a name of your choice, then put any number into the variable.
put some text into a variable
Add two numbers together and put the result into a variable
Subtract two numbers and put the result into a variable
Multiple two numbers and put the result into a variable
Put a number into a variable called x, and another number into a variable called y. Add the x and y variables together and put the result into a new variable
Put a number into a variable x. Then write another line of code that replaces the contents of x with a new number
Put a number into a variable x. Then write another line of code that appends a new number after the existing contents of x.
Put the following sentence into a variable “This is a short sentence.”. Then, put the 4th word of the variable into a new variable.
Using the above sentence, put the number of words in the variable into a new variable
Using the above sentence, put the first letter of the 4th word into a new variable
From 11, create a logical test to determine whether the first letter is the letter “s”.
Create a variable that contains the numbers 1 to 3 separated by commas
Create a variable that contains the numbers 1 to 3 separated by spaces
Create a variable that contains the numbers 1 to 3 separated by tabs
Create a variable that contains the numbers 1 to 3 separated by new lines
Use a repeat loop to create a variable that contains the numbers 1 to 100 separated by commas
Using the above variable, use the sum function to put the sum of the above variable into a new variable
From 13, use the average function to put the average of the above variable into a new variable
From 13, use the stdDev function to put the standard deviation of the above variable into a new variable
From 13, use the max function to find the largest number and put it into a new variable
From 13, use the min function to find the smallest number and put it into a new variable
Use a repeat loop to add up all of the numbers from 1 to 100
Use a repeat loop, if then statements, and the mod function to list all of the odd numbers from 1 to 100.
Copy a paragraph (e.g., something from Wikipedia) into a text field. Write a repeat loop that finds all of the words beginning with the letter “a”, and lists (separated by new lines) them in a new variable.
Using the same paragraph, write a repeat loop that finds all of the 3 letter words, and lists them in a new variable
From 24, write a repeat loop that counts all of the 3 letter words and puts the final count into a new variable
From 24, write a repeat loop that finds all of the unique 3 letter words. The final list should not contain any repeated words.
From 24, write a repeat loop that lists the number of letters in each word in the paragraph. E.g., if the first sentence is “Luke I am your father”, the answer should be 4,1,2,4,6. (for this problem include punctuation as part of the letter count. Extra challenge, can you write code that does not count the punctuation).
From 27, Your code from above should produce a list of numbers representing the length of each word in the paragraph. Using this list, count and list the number of occurrences of each word length.
8.5 Logic for programming experiments
8.5.1 Experiments are designed to create data-files
Experiments are designed with the aim of producing data. At this level, experiments are machines designed to produce data-files. It is important to keep this in mind when programming experiments for several reasons. First, the format of the data file changes how the data will ultimately be analysed. Good formatting decisions will facilitate the process of data-analysis after the data has been collected. Second, like any product, data files need to accomplish the job for which they were designed. The most important aspect of creating data files is to ensure that all of the important aspects of the experiment are preserved in the data. Any researcher should be able to look at your data file and reconstruct the series of events that occured in the experiment. This includes the order of trials, and the events that happened on each trials, including the stimulus that was presented, the experimental condition(s) for the stimulus, and the responses (times and accuracy) made on each trial.
When programming experiments from scratch, the programmer is responsible for deciding how the data file will be formatted, and is responsible for ensuring that the data file stores all of the necessary information. There is room for flexibility here. The data could potentially be stored in many different formats to achieve the same goals of storing all of the necessary information. This tutorial outlines one tried and true method that can be used to create data files for most any multi-trial design, using long-format data files.
8.5.2 Long-format data files
The basic strategy is to produce a table in a text file where each line corresponds to the data for each trial in an experiment. Each line will contain words and numbers separated by a tab (or other delimiter of your choice) that represent the order, events, conditions, and responses on each trial. It is sensible to have the first row correspond to the first trial, the second for the 2nd trial, and so one for the remaining trials.
The following is an example of a data-file for 12 trials in a simple Stroop experiment.
A data file like the above stores all of the necessary information to reconstruct the events of the experiment. We can see the order of trials. For each trial we can see what word was presented, and what color it was presented in. We can see when the stimulus appeared on the screen (OnsetTime), and when the response was made. We can also see the response key that was pressed on each trial, and whether or not the response was correct or incorrect. Many experiment builder programs (like PsychoPy) format their data-files in a similar manner. As well, data in this format will facilitate data-analysis later on.
One thing to note about the data is the column for congruency, which represents the names for each of the levels of the congruency variable. Usually it is a very good idea to include columns that describe the conditions of the experiment, as this makes data-analysis easier later on. Sometimes it is not strictly necessary to include the condition names because they can be inferred from other information in the table. For example, congruent conditions can be inferred by determining whether the word and color involve the same or different words. However, as a general rule, it is always a good idea to include all of the condition information in the data file, even if the conditions can be inferred at a later stage.
8.5.3 The Data-file is the Design
When you are new to experimental design, you might think that something like a data file comes along only near the end of a project. First, the experiment needs to be designed, then programmed, then subjects need to be run, and only then do data file appears. However, the creation of data files is central to the very beginning of programming an experiment. Of course, the data files will be empty because you will not have run any subjects. Yet, the data file will represent and define the experimental design at its most granular level, and it can be used to tell the computer how to run each trial of the experiment. Consider the following empty data file. It is empty because it contains the information that would be presented on each trial, but it is missing all of the response information:
This data file implies that certain kinds of events would happen on each trial. On trial 1, the word red would be presented in red ink. On trial 2, the word blue would be presented in red ink, and so on. A complete data file is a record of what did happen in an experiment, and an empty data file is a record of what should happen in an experiment. In order to use this list to control the experiment, all that is needed is a script to read each line, interpret the events that are supposed to occur, and then present the events and collect response information the subject. In this way there are two major components to programming an experiment:
1. Creating the list of trials (making the empty data-file) 2. Interpreting the list to run each trial
8.5.4 Creating the list of trials
Most behavioral experiments follow a similar logic. On each trial some stimulus is presented, and a response is collected. Usually, there are many kinds of stimuli, and each belong to various conditions. The list of trials for an experiment specifies which stimulus (and its conditions) occur on which trial. Depending on the purposes of the experiment, it is usually desirable to control the order with which stimuli are presented. Sometimes the order might be deliberately randomized, or deliberately sequenced. There are other decisions such as determining the frequency with which particular stimuli, or conditions occur across the trials.
Consider the problem of running a simple Stroop experiment with 4 colors, Red, Blue, Green, and Yellow. These four colors can be combined to produce 4 congruent items (blue in blue, red in red, green in green, and yellow in yellow), and 12 inocongruent items (red in blue, red in green, etc.). So far we have 2 conditions, and 16 items. Typically, Stroop experiments are conducted with an equal proportion of congruent and incongruent trials. How can this be accomplished? There are 12 incongruent items and only 4 congruent items. If we present all 12 incongruent items, then the congruent items need to be presented 3 times each (4 x 3 = 12). So, at a minimum, we need to have 24 trials (12 congruent and 12 incongruent). And, if we want to run more trials, and keep the number of congruent and incongruent trials balanced, then we would run trials in multiples of 24. Let’s run 96 trials (24*4).
So, how to make a trial list with 96 trials that conforms to the above demands? One simple strategy would be to write the list as a table, just like the one above by hand. Once the first 24 items are all written down, then they could be copy and pasted below to make the list 96 trials long. Sometimes this simple approaches are the fastest and easiest. Let’s explore how to create the list using scripts in LIVECODE.
The following scripts are assumed to be placed in a button, and the results are placed into a text field
on mouseUp put empty into field 1 put "red blue green yellow" into StroopWords put "red blue green yellow" into StroopColors repeat for each word n in StroopWords repeat for each word j in StroopColors if n is j then put "congruent" into conditionLabel else put "incongruent" into conditionLabel end if put n & tab & j & tab & conditionLabel & return after field 1 end repeat end repeat end mouseUp
The text field should now contain lines with the following 16 lines, each representing 4 of the possible congruent items, and 12 of the possible incongruent items.
red red congruent red blue incongruent red green incongruent red yellow incongruent blue red incongruent blue blue congruent blue green incongruent blue yellow incongruent green red incongruent green blue incongruent green green congruent green yellow incongruent yellow red incongruent yellow blue incongruent yellow green incongruent yellow yellow congruent
Although this is the full list of possible stimuli, it is not the full list that balances the number of congruent and incongruent items. Remember, we need to make sure that each congruent item appears three times each. Consider how the modified code accomplishes this task:
on mouseUp put empty into field 1 put "red blue green yellow" into StroopWords put "red blue green yellow" into StroopColors repeat for each word n in StroopWords repeat for each word j in StroopColors if n is j then put "congruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 else put "incongruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 end if end repeat end repeat end mouseUp
Here is the result, as you can see all 24 trials are present.
red red congruent red red congruent red red congruent red blue incongruent red green incongruent red yellow incongruent blue red incongruent blue blue congruent blue blue congruent blue blue congruent blue green incongruent blue yellow incongruent green red incongruent green blue incongruent green green congruent green green congruent green green congruent green yellow incongruent yellow red incongruent yellow blue incongruent yellow green incongruent yellow yellow congruent yellow yellow congruent yellow yellow congruent
It would be easy to change the script to make 96 trials. Just add another repeat loop to repeat the process 4 total times.
on mouseUp put empty into field 1 put "red blue green yellow" into StroopWords put "red blue green yellow" into StroopColors repeat 4 times repeat for each word n in StroopWords repeat for each word j in StroopColors if n is j then put "congruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 else put "incongruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 end if end repeat end repeat end repeat end mouseUp
At this point, you should have a list of 96 trials that satisfy the constraints of the design from above. A next step is to randomize the order of these trials so that participants are unable to predict the order of items and responses. Lines in a text file can easily be randomized using the sort function. For example:
sort lines of field 1 by random(the number of lines in field 1)
The above line will randomize the order of the lines in field 1. Assuming that field 1 contains the 96 Stroop trials, this list will now be randomized. Each line refers to each trial. If you want to be more specific, and add trial numbers, consider the following code:
on mouseUp put empty into field 1 put "red blue green yellow" into StroopWords put "red blue green yellow" into StroopColors repeat 4 times repeat for each word n in StroopWords repeat for each word j in StroopColors if n is j then put "congruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 put n & tab & j & tab & conditionLabel & return after field 1 else put "incongruent" into conditionLabel put n & tab & j & tab & conditionLabel & return after field 1 end if end repeat end repeat end repeat sort lines of field 1 by random(the number of lines in field 1) put field 1 into tempVar repeat for each line k in tempVar add 1 to lineCounter put lineCounter & tab & k & return after outputWithTrialNumbers end repeat put outputWithTrialNumbers into field 1 end mouseUp
Here is the result from the first 24 lines of the output from this script:
1 blue green incongruent 2 blue blue congruent 3 yellow red incongruent 4 blue red incongruent 5 green green congruent 6 red red congruent 7 red red congruent 8 red green incongruent 9 yellow green incongruent 10 yellow yellow congruent 11 green green congruent 12 blue blue congruent 13 red red congruent 14 blue yellow incongruent 15 yellow yellow congruent 16 blue blue congruent 17 yellow blue incongruent 18 green blue incongruent 19 blue blue congruent 20 green yellow incongruent 21 red yellow incongruent 22 yellow yellow congruent 23 blue blue congruent 24 yellow yellow congruent
The full 96 trials have now been randomized, and the first part of creating the Stroop experiment is “finished”. There can often be many more considerations at this present stage. Note that this list of 24 trials only contains 10 incongruent trials, and 14 congruent trials. This is due to the fact that all 96 trials were first compiled, and then randomized. This means that the 12 congruent and 12 incongruent trials will not necessarilly be distributed evenly within each bin of 24 trials. Issues like this arise when programming longer experiments. For example, imagine an experiment with 8 blocks of 96 trials. It wouldn’t make sense to create the list of trials in the same way that we did here. Instead, each block of 96 trials should be separately randomized to ensure that each block is balanced with the same number of items and conditions as each other block. In the present example, an experimenter may want to make other changes to the trial list, for example to vary the relative proportion of congruent vs. incongruent items, or to change the names of the words, or colors and so on.
8.5.5 Interpeting the list to run each trial
The next bits of code will show how to interpret the contents of each line of the trial list to run each trial of the experiment. The code assumes that you have a card with the following elements.
A text field named trials that contains the list of 96 trials
An empty text field named StroopStimDisplay placed in the center of the card (this will display the Stroop item)
An empty text field named dataVar (to store the data on each trial)
A button named begin placed in the bottom center of card (to initiate the experiment)
8.5.6 Presenting a stimulus
The following code will be placed in the button Begin. The logic of the code is as follows. Each press of the button should initiate the next trial. The button will increment a counter (that counts 1 to 96 for each trial). The value of the counter will serve as an index into the appropriate line from the trials list (1 for line 1, 2 for line 2 etc.). The 2nd word in each line will be presented as the word on each trial, and the 3rd word will be used to set the font color of the word.
on mouseUp global TrialCount add 1 to TrialCount put line TrialCount of field trials into trialTemp if TrialCount <97 then put word 2 of trialTemp into wordVar put word 3 of trialTemp into colorVar put wordVar into field StroopStimDisplay set the foregroundcolor of field StroopStimDisplay to colorVar show field StroopStimDisplay put the milliseconds into onsetTime put trialTemp & tab & onsetTime & tab after field dataVar else put "Finished" into field StroopStimDisplay show field StroopStimDisplay end if end mouseUp
Now, everytime the button is clicked, a new Stroop item should appear in the field StroopStimDisplay. As well, the trial information for each trial should be stored into the field dataVar.
8.5.7 Collecting a response
After each trial is presented on screen, the participant should be able to enter a response to identify the ink color of the presented stimulus. Response collection will be handled by the card script. The goal of response collection could be to
Save the identity of the response key that was pressed
Save the time when the response occured
Trigger the next trial automatically so the button doesn’t need to be pressed manually to start the next trial
The following code goes in the card script:
on keydown whichkey put the milliseconds into RT put RT & tab & whichkey & return after field dataVar hide field StroopStimDisplay send mouseUp to button begin in 1 second end keydown
With the code in the button and the card in place, the basic elements of interpreting the trial list to present an item and collect and record a response are in place. The code will run for 96 trials, and then a message saying “finished” will appear in the field StroopStimDisplay.
8.6 Look and feel
The above code shows the basic logic that goes into coding a multi-trial Stroop experiment. There are many additions that could be made to make the experiment look better and run more smoothly. The size of the font in the field StroopStimDisplay can be made larger (using the property inspector). The look of the text field can be changed. A basic text field usually starts with a visible border. This can be turned off so that the word appears on its own without a border. The data field can be hidden. The trials field can be hidden.
For most experiments, including the demo Stroop experiment in the repository, it is usally desirable to have several cards in your stack. The first card can act as an opening screen, perhaps with a title for the experiment, and a text field for entering subject numbers or counterbalancing codes. There might be a button that is pressed to begin the experiment and take the subject to a second card that displays instructions. This button can also be used to insert the code that creates the trial list. At this point, it would also be important to initialize aspects of the experiment. For example, the button could have code that empties the contents of data field, sets the global counter to zero, and hides or shows important elements on diffent cards. The card containing the instructions could also have a button that sends the subject to a third card containing the code from above that implements the Stroop experiment. Finally, when the trials are over, the participant could automatically be sent to a final card that displays a message like “Thank you for your participation, please find the experimenter”.
Many experiment builder programs like e-prime or PsychoPy present trials on a completely blank computer screen. This can be accomplished in LIVECODE by hiding things the like menubar. Check out what the following commands do:
hide menubar show menubar set the decorations of this stack to default set the decorations of this stack to empty set the backdrop to black set the backdrop to none
In combination with setting the background colors of the card and fields, it is possible to create a completely blank screen for presenting stimuli, if so desired.
8.6.1 Preventing unwanted actions
The example code shown so far does implement a basic Stroop experiment, however the code allows for some unwanted behavior. We expect that subjects would not press buttons until a stimulus is presented on the screen, and then only press one button (say r to identify a red stimulus) and not many buttons. Unfortunately, even the best subjects will press buttons in error and at inopportune moments. The current version of the code is vulnerable to this kind of button pressing. For example, try pressing the “r” button three times in rapid succession. You should see three trials automatically be presented in rapid succession on the screen. To prevent this kind of behavior, we need to build in logical switches or toggles into the code that allow responses to be collected only during certain times. Fortunately, this is very easy. Consider a variable called toggleResponse. If the contents of the variable is 0 then we do not allow response collection, but if the contents is 1, then we do allow it. Consider, the following code implementing the toggle.
button begin script
on mouseUp global TrialCount global toggleResponse -- new toggle declared as a global variable put 0 into toggleResponse -- set to 0, this could be set elsewhere and early perhaps when the stack is opened add 1 to TrialCount put line TrialCount of field trials into trialTemp if TrialCount <97 then put word 2 of trialTemp into wordVar put word 3 of trialTemp into colorVar put wordVar into field StroopStimDisplay set the foregroundcolor of field StroopStimDisplay to colorVar show field StroopStimDisplay put 1 into toggleResponse -- turn on response Collection put the milliseconds into onsetTime put trialTemp & tab & onsetTime & tab after field dataVar else put "Finished" into field StroopStimDisplay show field StroopStimDisplay end if end mouseUp
on keydown whichkey global toggleResponse -- declare toggle as a global variable put the milliseconds into RT if toggleResponse is 1 then -- only do if toggle is on put 0 into toggleResponse --turn toggle off to stop multiple keystrokes put RT & tab & whichkey & return after field dataVar hide field StroopStimDisplay send mouseUp to button begin in 1 second end if end keydown
The toggling logic implemented above will only allow response collection to occur after a Stroop stimulus has been presented. It also only collects one response before turning off, so it prevents multiple key presses from triggering multiple trials in rapid succession. It is worth noting that this kind of toggle is not always desired. Sometimes it is desirable to collect premature responses that occur before the presentation of a stimulus. In this case a different toggle logic would need to be implemented. This shows a great example of the challenges of programming experiments from scratch (having to deal with these issues), as well as the power of programming from scratch (it is possible to do what you want, but you have to figure out how to do it).
8.7 Saving the data
At the end of the experiment the field containing the data should have 96 lines.
One option for saving the data-file would be to
Make the data field visible
Copy and paste the data into a text file
Save the text-file to a folder on your computer
Another option is to use LIVECODE to create a file and save the contents to that file. Here is a general strategy for saving text to files in LIVECODE.
Create a folder for your experiment, then save your experiment stack into this folder.
Inside your experiment folder you should now see the LIVECODE stack. Create a new folder inside the current folder called data.
Your experiment folder should now contain the LIVECODE stack, and a folder called data
Create a new button on your card, give it a name like savedata (or incorporate this code into your script in another way)
The code for the button script should read as follows (note: this code assumes that your data is in a field called dataVar):
on mouseUp put the filename of this stack into fname set the itemDelimiter to to "\" delete the last item of fname put "\data\" after fname put field dataVar into url("file:" & fname & "data.txt") end mouseUp
If your data field has text inside it, then clicking this button should create a new field called data.txt inside the data folder that you created on your hard-drive. This kind of code can be changed to add more functionality. For example, instead of using “data” as the filename, a subject number could be used. Note that in the demo version there is a field on the first card for entering a subject number, the contents of this field could be used to define the filename for the data file.
If you have created the Stroop experiment and run yourself in it, then you should have created a datafile with 96 lines that looks something like the shortened version below:
1 blue blue congruent 1389995871068 1389995871785 b 2 blue blue congruent 1389995872788 1389995873384 b 3 yellow yellow congruent 1389995874388 1389995875191 y 4 red blue incongruent 1389995876195 1389995877182 b 5 red blue incongruent 1389995878186 1389995878886 b 6 yellow yellow congruent 1389995879889 1389995880629 y 7 yellow yellow congruent 1389995881632 1389995882259 y 8 blue blue congruent 1389995883262 1389995883811 b 9 green red incongruent 1389995884815 1389995885514 r 10 green blue incongruent 1389995886517 1389995887321 b 11 blue blue congruent 1389995888325 1389995888801 b 12 green green congruent 1389995889804 1389995890536 g 13 blue yellow incongruent 1389995891540 1389995892263 y 14 red red congruent 1389995893266 1389995893830 r 15 yellow yellow congruent 1389995894833 1389995895365 y
The next step might be to get the mean reaction times for congruent vs incongruent trials. This involves finding all of the reaction times for each trial, then separating them into congruent and incongruent types, and finally computing the average for each type.
Here is a straightforward approach using a repeat loop. This assumes you have opened a new stack for accomplishing the analysis. Field 1 could contain the data, and field 2 could be reserved for the output of the analysis.
repeat for each line n in field 1 put word 6 of n - word 5 of n into RT -- computes reaction time if word 4 of is congruent then put RT & "," after CongruentVar else put RT & "," after IncongruentVar end if end repeat put "Congruent" & tab & "Incongruent" & return after field 2 put average(CongruentVar) & tab & average(IncongruentVar) after field 2