The EEGLAB window & menus

Why do I need this step?

Before any processing, you need to know what you opened: how many channels, at what sampling rate, and for how many minutes. These numbers change with each step, and watching them is the fastest way to catch a mistake.

In this lesson we open the official EEGLAB sample, eeglab_data.set. It ships with EEGLAB in the sample_data folder, so there is nothing to download.

How do I do it in EEGLAB?

1. Meet the seven menus

Start eeglab. The window bar has seven menus, roughly in the order you use them:

Most menus are greyed out for now, because they need an open dataset.

2. Open the dataset

1. Open File › then Load existing dataset.
2. A file selection window opens. Go into the EEGLAB folder, then the sample_data folder.
3. Select eeglab_data.set and click Open.

The main window fills with the dataset summary, and the menus become available.

3. Browse the signal

1. Open Plot › then Channel data (scroll).
2. A scrolling window opens: each horizontal line is a channel, and the horizontal axis is time in seconds.
3. Move with the arrow buttons at the bottom: the single arrow (> or <) moves one second, and the double arrow (>> or <<) moves almost a full window.
4. Zoom the signal in or out with the + and - buttons. The number next to them is the spacing between channels in microvolts.
5. The colored vertical lines are the events, with each event’s name above it (square or rt).
6. To change the time window length or the number of channels shown, use Settings › then Time range to display inside the scrolling window.

How do I write it as code?

% Start EEGLAB and get its main variables
[ALLEEG, EEG, CURRENTSET, ALLCOM] = eeglab;

% Load the sample dataset that ships with EEGLAB
sampleFolder = fullfile(fileparts(which('eeglab')), 'sample_data');
EEG = pop_loadset('filename', 'eeglab_data.set', 'filepath', sampleFolder);

% Register it in EEGLAB and refresh the main window
[ALLEEG, EEG, CURRENTSET] = pop_newset(ALLEEG, EEG, 0, 'gui', 'off');
eeglab redraw;

% Scroll through the channel data
pop_eegplot(EEG, 1, 1, 1);

• fileparts(which('eeglab')) returns the EEGLAB folder wherever you installed it, so the script works on any computer.
• pop_newset adds the dataset to EEGLAB’s list of datasets ('gui', 'off' skips the naming window), and eeglab redraw refreshes the main window. EEGLAB uses the same function when you open a dataset from File > Load existing dataset.

Which value should I choose?

There is no value to choose here. What matters is reading the main window correctly. This is what each line means, and its value in the tutorial dataset:

Recording length = number of samples ÷ sampling rate = 30504 ÷ 128 = 238.3 seconds, about 4 minutes. This is roughly the number in Epoch end (sec).

How do I check it worked?

1. The first line of the main window starts with #1: followed by the dataset name Continuous EEG Data.
2. The values match the table above: 32 channels, 128 Hz, 30504 samples, and 154 events.
3. The scrolling window shows 32 channels with their names, and the square and rt event lines appear in it.
4. From the Command Window: type EEG.setname. It should print 'Continuous EEG Data'.

What mistakes do people make?

• Menus are grey and do not respond. You have not opened a dataset yet, or the open dataset does not suit that menu. Open a dataset first.
• Reading Frames per epoch as time. It is a sample count, not seconds. Divide it by the sampling rate to get seconds.
• Picking the .fdt file in the open window. Always open the .set file. The .fdt file holds the signal only, and EEGLAB reads it automatically with the .set file (details in the .set and .fdt lesson, an optional lesson).
• Saving over the sample file. Do not use File › then Resave current dataset(s) after changing eeglab_data.set. Save any modified copy under a new name in your own folder with File › then Save current dataset as, so the original sample stays intact for the other lessons.
• Closing the main window by mistake. The data stays in memory. Type eeglab redraw in the Command Window to bring the window back with the same data.

Going further

• EEGLAB Tutorial: https://eeglab.org/tutorials/02_Quickstart/quickstart.html (2. Quickstart)
• EEGLAB Tutorial data: https://eeglab.org/tutorials/tutorial_data.html (Tutorial data)