High-density EEG electrode placement

Some time ago we wrote an article on electrode placement for high-resolution EEG measurement (referred to as the 5% article). After its apearance I have noticed that there is a demand for a concise and methodological overview of electrode placement systems. With this page I want to share some of my knowledge on this subject. This page contains non-technical comments on the different standards for electrode placement. Furthermore I have included the electrode positions as measured and published by different researchers and the electrode locations according to the extended 10-20 standard on a spherical and on a realistical head model.

The official 10-20 standard

At the first International EEG congress, held in London in 1947, it was recognised that a standard method of placement of electrodes used in electroencephalography (EEG) was needed. Possible methods to standardise electrode placement were studied by H.H. Jasper, which resulted in the definition of the 10-20 electrode system (Jasper 1958). Since then, the 10-20 electrode system has become the de-facto standard for both clinical EEG and for the study of event related potentials (ERPs) in non-clinical settings. However, the advancement of multi-channel EEG hardware systems and of topographic methods to study evoked and event-related potentials, necessitated the standardisation of a larger number of channels. Therefore an extension to the original 10-20 system was proposed which increases the number of electrodes from 21 up to 74 (Chatrian et al. 1985). This extended 10-20 system of electrode placement, also known as the “10% system” or the “10-10 system” has been accepted and is currently endorsed as the standard of the American Electroencephalographic Society (AES 1994, Klem et al. 1999) and the International Federation of Societies for Electroencephalography and Clinical Neurophysioly (Nuwer 1998).

The 10% or the 10-10 system

The original 10-20 standard consisted only of 21 electrodes. Although this is still considered to be enough for most clinical use, ERP research requires a higher density of electrodes. This does not mean that it is always neccessary to measure a lot of channels at the same time, but sometimes it desirable to place more electrodes in a certain area of interest, e.g. the area overlying primary sensory and motor areas where originally only C3 and C4 are located.

In 1985, Chatrian suggested a logical extension of the 10-20 system. The original sytem places electrodes at distances of 10% and 20% along certain contours over the scalp. Chatrian simply extended this by placing electrodes at every 10% along the medial-lateral contours, and by introducing new contours in between the existing ones. As a matter of fact, the original proposed system by Jasper already anticipated upon the extra electrodes, by leaving the electrode names for the “1” and “5” electrodes free (e.g. C3 was added in between C1 and C5).

The additional electrode positions of the 10% system have been adopted by the standardising comitees, and the official 10-20 system now includes these positions. To discriminate it with the original 10-20 system, the standard is called the extended 10-20 system. It should be noted that the extended 10-20 system uses another nomenclature than the original 10% system, see the section on the nomenclature for more details.

The 5% or the 10-5 system

This is our proposed extension of the 10-20 system to accomodate the large number of electrodes that are currently being used in EEG recordings in cognitive neuropscience. The 5% electrode system contains the standard locations of the original 10-20 system and those of the 10-10 system. Our proposal is a logical extension of the 10-10 system, enabling the use of up to over 300 electrode locations. As the system uses proportional distances of 5% of the total length along contours between skull landmarks, compared to the 20% and 10% distances used in the 10-20 and 10-10 systems, respectively, we call it the 5% system or the 10-5 system.

You can find all its details in the 5% article. Contact me if you want a reprint.

Nomenclature for electrode locations

The nomenclature for the electrode locations, that is the name of each location, may look quite complicated for a newbie but follows a couple of simple rules:

  • electrode names consist of a single or multiple letters, combined with a numer
  • electrodes on the left are numbered odd, electrodes on the right are numbered even
  • electrodes on the center (midline) are appended with the letter z. The z stands for zero (that is, not even and not odd). It is indicated by “z” instead of the number “0” to avoid confusion with the letter “O”.
  • electrodes near the midline (the zero-line) have the smallest numbers, and they increase towards the side
  • the letter indicates the location on the head (actually the cortical lobe above which the electrode lies):
    • Fp: frontal pole
    • F: frontal
    • C: central
    • T: temporal
    • P: parietal
    • O: occipital
  • combinations of two letters indicates intermediate locations, e.g.
    • FC: in between frontal and central electrode locations
    • PO: in between parietal and occipital electrode locations

Combining these rules gives straight forward labels for all electrode positions. However, no rule is complete without some exceptions, and this also goes for the 10-20 standard.

The contour in between the frontal pole (Fp) and the frontal (F) electrodes is called “AF” (anterio-frontal). The electrodes overlying the temporal lobe are indicated with a T. In the original 21 channel 10-20 standard, the electrodes in the central contour running approximately from the vertex towards the left ear are labeled Cz-C3-T3. The untermediate locations C1 and C5 were added in the extended system, and location T3 was renamed to T7 (is also could have been called C7). Similarly, electrode T4 (old) has been renamed to T8. The parietal-temporal electrodes T5 and T6 (old) have been renamed to P7 and P8. Electrode T7 and T8 would correspond to C7 and C8. The fronto-central (FC) electrode row and the parieto-central (CP) electrode row use the letter T for the electrodes overlying the temporal lobe (e.g. FT7 and TP7). You only need to remember that “T” in the official extended 10-20 system always can be read as “C”. The electrode names T3, T4, T5 and T6 are still commonly used in clinical EEG with 19 or 21 channels, but should not be used any more in experimental ERP studies.

Although the extra electrode locations proposed in the 10% have been adopted right away, the nomenclature of the electrode locations has led to some debate. Chatrians original proposal used a single primes ‘ to indicate positions posterior of a known location. The electrode between C4 and P4 was called C4′. Likewise, electrode locations anterior of a known location were appended with a double prime ”, so that the electrode between C4 and F4 becomes C4”. This system with single and double primes was only one of the alternative naming schemes for the intermediate locations. Other naming schemes introduced the additional letters B, D, E and H to designate intermediate coronal rows, or appended the letters a (for anterior) and p (for posterior) to the beginning or end of the electrode label. See Nuwer (1987) for a review of the suggestions that did not make it into the standard.

The combination of two letters to indicate the electrode location is very intuitive, as it links the electrode label to the anatomical location on the scalp. It is similar to the naming of geographical directions, derived from the orientations of a compass. For example, halfway between North and West (at 315 degrees) lies the direction North-West. The direction halfway North and North-West (at 337.5 degrees) is commonly labelled North-North-West. In the 5% electrode system, we have proposed to use the same method for labelling the intermediate positions on the head in the antero-posterior direction. In this way, the locations on the contour between the C-contour and the CP-contour for example get labelled “CCP”. This naming scheme for the coronal contours gives from anterior to posterior locations the following names: AF, AFF, F, FFC, FC, FCC, C, CCP, CP, CPP, P, PPO, PO. The contour halfway between Fp (frontal pole) locations and the AF contour would be called AFp (which we prefer over FpA). Likewise, the contour between the O locations and the PO contour would be called POO.

Downloads

Using a spherical descripotion of the head, I have computed all electrode positions of the 10-5 electrode system.The 10-20 and 10-10 electrode system all follow the same convention, and are included in this set.

The first version (“sphere1”) assumes Fpz, Oz, T7 and T9 to be on the poles of the sphere. The location of Nz, Iz, LPA and RPA is subsequently determined according to the following: Oz is at the equator, Cz is at the north-pole. The difference between them is 90 degrees or pi/2 along the surface of the unit sphere. The distance between each of Cz-CPz-Pz-POz-Oz is equal and is 22.5 degree (note that Pz is at 45 degrees from the north-pole). The distance between Iz and Oz is equal to the other distances, therefore Iz is at 22.5 below the equator. Using this, the coordinates of Iz can be computed using the sin() and cos() of 22.5 degrees. The other “fiducial” points have similar coordinates. The Z value for the whole fiducial points contour is -sin(22.5)=-0.3827.

In the second version (“sphere2”) I have tried to mimic the electrode locations that are used in BESA as close as possible. Compared to the “sphere1” version described above, the electrodes are all shifted to slightly lower locations.

I have also made a computation of all the electrode locations on a reallistical head surface, based on the distances along the (triangulated) surface of the head. The head surface used was constructed from the canonical MRI that is included in the SPM2 package, and locations are expressed in MNI coordinates.

You can download ascii files with the labels and positions (expressed in carthesian coordinates) here:

  • sphere1 for plotting
  • sphere2 for source analysis using a spherical headmodel
  • realistic for realistic plotting and modelling (BEM)
  • according to BESA, this has carthesian coordinates of 81 electrodes on a unit sphere

A selection of 131 channels of the 10-5 system that is usefull for a 128 channel EEG system can be downloaded here.

References

Official standards and suggestions therefore:

Discussions about nomenclature:

Manufacturers of electrode caps:

Technical and experimental notes:

Related organisations:

  • The American Electroencephalographic Society
  • The International Federation of Societies for Electroencephalography and Clinical Neurophysiology

Copyright (C) 2002, Robert Oostenveld