MATLAB basics for EEGLAB

What does this show?

Your layout may differ, because MATLAB lets you move the parts around. What matters is finding all three by name.

Why do I need this step?

The data itself lives in MATLAB as a variable called EEG. To read the number of channels or the sampling rate from it, you need only four ideas: the variable, the matrix, the struct and the path.

This lesson is not a MATLAB course. It is the minimum you need to follow the path. If you want more, the official EEGLAB documentation recommends the free MATLAB Onramp course from MathWorks.

How do I do it in EEGLAB?

Here the “GUI” is the MATLAB window itself, not EEGLAB.

1. Open MATLAB and find the three parts: Command Window, Workspace and Current Folder.
2. In the Command Window type x = 5 and press Enter. MATLAB prints the result, and the variable x appears in the Workspace.
3. Type y = 7; with a semicolon at the end. Nothing is printed, but y appears in the Workspace. The semicolon only stops the printing.
4. Double-click x in the Workspace. The Variables editor opens and shows its value.
5. Open a new script with the New Script button in the Home tab. Paste the script from the next section into it.
6. Save it as demo_basics.m in the folder shown in the Current Folder.
7. Run it with the Run button in the Editor tab, or type its name without .m in the Command Window: demo_basics.

How do I write it as code?

The script builds a small recording by hand: 3 channels for 10 seconds at 128 samples per second, with random numbers instead of a real signal. It is called demo so it does not get mixed up with the real EEG variable you will meet in station 1.

% A toy recording: 3 channels, 10 seconds at 128 Hz
demo.srate  = 128;                              % sampling rate (samples per second)
demo.nbchan = 3;                                % number of channels
demo.pnts   = 10 * demo.srate;                  % samples per channel
demo.labels = {'Fz', 'Cz', 'Pz'};               % channel names (a cell array of text)
demo.data   = randn(demo.nbchan, demo.pnts);    % random numbers, channels x samples

% Read from the struct
size(demo.data)                                 % rows = channels, columns = samples
demo.srate                                      % one field
demo.labels{2}                                  % text of the second channel name

% Indexing: channel 2, samples 1 to 5
firstSamples = demo.data(2, 1:5)

% Time of each sample in seconds (the first sample is at time 0)
t = (0:demo.pnts-1) / demo.srate;
t(end)

% Plot one channel
figure;
plot(t, demo.data(2, :));
xlabel('Time (s)'); ylabel('Amplitude');
title(['Channel ' demo.labels{2}]);

% Where MATLAB finds things
whos demo                                       % size and type of a variable
which plot                                      % location of a function on the path

What each part does:

• struct: the variable demo is a box with fields, each with a name. You reach a field with a dot: demo.srate. The EEG variable in EEGLAB is a struct with the same idea, just more fields.
• Matrix: demo.data is a table of numbers. Each row is a channel and each column is a time sample. This is the same order as EEG.data in EEGLAB: channels first, then samples.
• Indexing: demo.data(2, 1:5) means row 2, columns 1 to 5. A colon alone : means “all columns”. Numbering in MATLAB starts at 1, not 0.
• Time: the first sample is at time zero, so we divide 0 to (number of samples minus 1) by the sampling rate. The last sample is at 9.9922 seconds, not 10.
• Path: MATLAB finds functions and scripts in the current folder and in a list of folders called the path. The which command tells you where any name will come from. The addpath command adds a folder to the path for this session only. You will use it in the installation lesson in station 1 to add the EEGLAB folder and save it.

Which value should I choose?

There is no value to choose here, but some similar-looking symbols mean different things. This table is enough to read any line in the EEGLAB lessons.

How do I check it worked?

• size(demo.data) prints 3 then 1280.
• demo.srate prints 128, and demo.labels{2} prints 'Cz'.
• t(end) prints 9.9922.
• A figure window shows a line wiggling around zero for 10 seconds, titled Channel Cz.
• which plot prints a path inside the MATLAB installation folder, not inside your own folder.

What mistakes do people make?

• Counting from zero. The first element in MATLAB is number 1. Typing demo.data(0, 1) gives an error.
• Forgetting the semicolon with a large matrix. A line such as x = EEG.data without a semicolon prints millions of numbers and freezes the Command Window. If this happens, press Ctrl+C to stop it.
• Mixing upper and lower case. EEG and eeg are different variables. EEGLAB always uses EEG in capitals.
• Round brackets with a cell array. demo.labels(2) gives a cell containing the text, not the text itself. For the text, use demo.labels{2}.
• A script file named after an existing function. If you name your script plot.m or eeglab.m, MATLAB runs your file instead of the original function. Check with which if a function behaves strangely.

Going further

• EEGLAB Tutorial: https://eeglab.org/tutorials/misc/tutorial_matlab.html (Reference Topics > MATLAB tutorial)
• MATLAB Onramp (MathWorks): https://matlabacademy.mathworks.com/ (MATLAB Onramp)
• MATLAB Documentation: https://www.mathworks.com/help/matlab/ref/struct.html (struct)
• MATLAB Documentation: https://www.mathworks.com/help/matlab/matlab_env/what-is-the-matlab-search-path.html (What Is the MATLAB Search Path?)