The 10-20 system & electrodes

Why do I need this step?

When you read that the ERP component P3 is largest at Pz, or that blinks show up strongly at Fp1 and Fp2, you need to know where these electrodes are right away.

Two words you will see often mean different things: the electrode is the metal piece on the head, and the channel is the signal recorded from it. EEGLAB calls each signal a “channel”.

How do I do it in EEGLAB?

There are no EEGLAB steps in this lesson. You will work with electrode locations in the GUI later, in the channel locations lesson, from the Edit › then Channel locations menu.

Once you have a dataset with locations, you can see the electrode map with names from Plot › then Channel locations › then By name.

How do I write it as code?

This script is optional. Run it after installing EEGLAB in station 1 to see the electrode map of the dataset that will follow you through most of the path (eeglab_data.set). It does not need to open the dataset itself, only the location file that comes with it.

% Plot the 32-channel montage of the EEGLAB sample dataset with labels
eeglab nogui;                                         % load EEGLAB functions only
eeglabRoot = fileparts(which('eeglab'));
locs = readlocs(fullfile(eeglabRoot, 'sample_data', 'eeglab_chan32.locs'));

figure;
topoplot([], locs, 'style', 'blank', 'electrodes', 'labelpoint');
title('eeglab\_data.set montage');

disp({locs.labels});                                  % list all channel names

• readlocs reads the location file, and topoplot draws the head from above with the nose up.
• 'style', 'blank' draws only the electrodes without a color map, and 'electrodes', 'labelpoint' writes each electrode name next to its dot.
• You will see 32 names. Thirty are 10-10 names, and two are not: EOG1 and EOG2.

Which value should I choose?

There is no value to choose here. But this table is all you need to read any name.

First: the letter tells you the region

Second: the number or z tells you the side

Third: the system tells you the electrode density

How the 10-20 proportions work: on the midline from nasion to inion, Fpz sits at 10% of the length, then Fz, Cz, Pz and Oz each 20% further, leaving 10% to the inion. The same way along the line between the ears: T7, then C3, then Cz, then C4, then T8.

How do I check it worked?

Find each electrode on the drawing in “Before and after” without looking at the tables, then read the answer.

• Where is P4? Answer: over the parietal lobe, in the right half of the head (even number).
• Where is FC5? Answer: between the frontal lobe and the central area, in the left half, fairly far from the midline.
• An old paper reports a peak at T6. What is its modern name? Answer: P8.
• Which is closer to the midline: F3 or F7? Answer: F3, because its number is smaller.
• If you run the script after installing EEGLAB: the plot shows 32 dots and Cz is exactly in the center. The two dots farthest outside the circle are EOG1 and EOG2, and edge channels such as T7, O1 and Oz sit just outside it, because they are below the head’s mid-line (EEGLAB tutorial).

What mistakes do people make?

• Swapping the sides. Odd is left and even is right, from the participant’s point of view, not yours as you face them. In EEGLAB plots the head is drawn from above with the nose up, so the left of the plot is the participant’s left.
• Mixing old and new names. The 10-10 system replaced T3 and T4 with T7 and T8, and T5 and T6 with P7 and P8, because the old names did not follow the rule (ACNS guideline, 2016). The old names still appear in many books, papers and devices. If you read T5, it means P7.
• Reading z as the number 2. The lowercase z means the midline. Cz is exactly in the middle of the head, while C2 is an electrode right of the middle.
• Thinking a name marks a fixed point in millimeters. Names are proportions of each participant’s head, so the actual location differs a little between heads and caps.
• Looking for a 10-10 name for every channel. Some channels are not on the scalp at all. In eeglab_data.set the channels EOG1 and EOG2 record eye movement, and EEGLAB plots them farthest outside the head circle because they are below the head’s mid-plane (EEGLAB tutorial). In other data you will find M1 and M2 on the mastoid bones behind the ears, or A1 and A2 on the earlobes, often used as reference.

Going further

• EEGLAB Tutorial: https://eeglab.org/tutorials/04_Import/Channel_Locations.html (Why are electrodes plotted outside of the head limits?)
• ACNS Guideline 2: Guidelines for Standard Electrode Position Nomenclature (Acharya et al., 2016): https://acns.org/UserFiles/file/EEGGuideline2Electrodenomenclature_final_v1.pdf (The 10-10 system; modified combinatorial nomenclature)
• Luck, Applied Event-Related Potential Data Analysis (1st edition): https://socialsci.libretexts.org/Bookshelves/Psychology/Biological_Psychology/Applied_Event-Related_Potential_Data_Analysis_(Luck)/12%3A_Appendix_1%3A_A_Very_Brief_Introduction_to_EEG_and_ERPs (Appendix 1 > The EEG)
• Oostenveld & Praamstra (2001), The five percent electrode system for high-resolution EEG and ERP measurements. Clinical Neurophysiology 112(4): https://doi.org/10.1016/S1388-2457(00)00527-7 (Whole article)