Channel locations

What does this show?

Why do I need this step?

EEGLAB finds the positions by matching each channel name with a template file of standard names, such as FC5 or Cz. The match works only if the names are written the standard way.

In S001R03.edf the names end with extra dots: Fc5., Cz.., T10.. The dots pad every name to four characters. A name such as Cz.. does not match Cz in the template, so EEGLAB finds no position for it. This is a real problem you will meet with files from other systems, so we fix it here first.

How do I do it in EEGLAB?

1. See the problem

1. Open the dataset S001R03.set you saved in the previous lesson.
2. In the main window, the Channel locations line shows No (labels only).
3. In the Command Window, type {EEG.chanlocs(1:3).labels}. The result is 'Fc5.' 'Fc3.' 'Fc1.': each name ends with a dot.

2. Remove the dots from the names

You can change one name by hand in Edit › then Channel locations, in the Channel label (“label”) field. But there are 64 names, so it is faster to remove the dots from all of them with four lines in the Command Window:

for iChan = 1:EEG.nbchan
    EEG.chanlocs(iChan).labels = strrep(EEG.chanlocs(iChan).labels, '.', '');
end
[ALLEEG, EEG, CURRENTSET] = eeg_store(ALLEEG, EEG, CURRENTSET); eeglab redraw;

Type {EEG.chanlocs(1:3).labels} again. The result is now 'Fc5' 'Fc3' 'Fc1'.

3. Look up the positions

1. Open Edit › then Channel locations.
2. Because the channels have no positions yet, EEGLAB first opens a window titled Look up channel locations?.
3. Keep the default choice in the list: use MNI coordinate file for BEM dipfit model. The file under it is standard_1005.elc.
4. Leave Import file instead and erase all channels unticked.
5. Click OK. The Command Window prints 64 channel with the same label found and imported.
6. The channel editing window opens, showing each channel with its coordinates. Click OK at the bottom to close it. The changes take effect only after you close this window.

4. Check the positions on the head

Open Plot › then Channel locations › then By name. A head seen from above appears, nose at the top, with each electrode’s name in its place.

5. Save

Open File › then Resave current dataset(s). This writes the fixed names and the positions into the same file S001R03.set in your lessons folder.

How do I write it as code?

% Start EEGLAB and load the file from the previous lesson
[ALLEEG, EEG, CURRENTSET, ALLCOM] = eeglab;
dataFolder = 'C:\Users\me\Documents\eeg-course';   % change it
EEG = pop_loadset('filename', 'S001R03.set', 'filepath', dataFolder);
[ALLEEG, EEG, CURRENTSET] = eeg_store(ALLEEG, EEG, 0);

% Remove the trailing dots from the names (Fc5. -> Fc5, Cz.. -> Cz)
for iChan = 1:EEG.nbchan
    EEG.chanlocs(iChan).labels = strrep(EEG.chanlocs(iChan).labels, '.', '');
end

% Look up the positions in the standard MNI template
% (case does not matter: Fc5 matches FC5)
EEG = pop_chanedit(EEG, 'lookup', 'standard_1005.elc');
EEG = eeg_checkset(EEG);

% Plot the electrodes by name (same as Plot > Channel locations > By name)
figure; topoplot([], EEG.chanlocs, 'style', 'blank', ...
    'electrodes', 'labelpoint', 'chaninfo', EEG.chaninfo);

% Save over the lesson file and show the result in the main window
EEG = pop_saveset(EEG, 'savemode', 'resave');
[ALLEEG, EEG, CURRENTSET] = eeg_store(ALLEEG, EEG, CURRENTSET);
eeglab redraw

• strrep(text, '.', '') replaces every dot in the text with nothing.
• 'lookup', 'standard_1005.elc' is the same choice as the default in the lookup window. EEGLAB finds this file inside the dipfit plugin, which ships with EEGLAB.

Which value should I choose?

The main choice is where the positions come from:

Upper or lower case does not matter. EEGLAB compares names in lower case, so Fc5 finds FC5 in the template. Extra characters such as dots or spaces do matter.

How do I check it worked?

1. The Channel locations line in the main window shows Yes.
2. The Command Window printed 64 channel with the same label found and imported.
3. Plot › then Channel locations › then By name shows 64 electrodes: Cz in the center, Fpz at the front, Oz at the back.
4. all(~cellfun(@isempty, {EEG.chanlocs.X})) gives 1: every channel has coordinates.

Keep S001R03.set. The next lesson (events) continues on it.

What mistakes do people make?

• Looking up before removing the dots. No name matches, and no warning appears. The Channel locations line stays No (labels only). Remove the dots, then look up again.
• Not closing the channel editing window with OK. The changes take effect only after you close it.
• Thinking the electrodes outside the circle are a mistake. The circle marks the horizontal plane through the center of the head. Electrodes below that plane, around the lower edge of the cap (such as T7, T8, T9, T10 and Iz), are drawn on or outside it. This is expected.
• Ticking Import file instead and erase all channels. This replaces your channels with all the template’s channels (hundreds of names). Leave it unticked.
• Assuming the file order follows the head. In S001R03.edf the first channel is Fc5, not Fp1. Always look a channel up by its name, not by its number.

How do I write it in my paper?

A template: edit the data, settings and versions to match what you ran.

Channel labels were cleaned of trailing periods and matched to the standard 10-5 template in MNI space (standard_1005.elc) with pop_chanedit (EEGLAB 2026.1).

Going further

• EEGLAB Tutorial: https://eeglab.org/tutorials/04_Import/Channel_Locations.html (c. Channel Locations: Command-line channel location look up example; Why are electrodes plotted outside of the head limits?)
• EEGLAB preprocessing #2: Events and channel locations: https://www.youtube.com/watch?v=2CPmmPG5Beo (Full video)
• EEG Motor Movement/Imagery Dataset (PhysioNet): https://physionet.org/content/eegmmidb/1.0.0/ (Montage)