Standard electrode positions

Many standard electrode positions are based on the distances between Nasion and Inion and between left and right preauricular points. The Nasion is the point between the forehead and the nose. The Inion is the lowest point of the skull from the back of the head and is normally indicated by a prominent bump. The bump is more pronounced in males compared to females. A preauricular point is a point of the posterior root of the zygomatic arch lying immediately in front of the upper end of the tragus. The approximate position of the Inion and the preauricular points is indicated in Figure 1 (see also FieldTrip and BrainStorm documentation at the following links:
http://www.fieldtriptoolbox.org/faq/how_are_the_lpa_and_rpa_points_defined/ 
https://neuroimage.usc.edu/brainstorm/CoordinateSystems).

 

Figure 1: Approximate position of the Inion and preauricular points (figure downloaded from https://neuroimage.usc.edu/brainstorm/CoordinateSystems).
Figure 1: Approximate position of the Inion and preauricular points (figure downloaded from https://neuroimage.usc.edu/brainstorm/CoordinateSystems).

 

The "10-20" system and the "10-10" system are internationally recognized methods to describe the location of scalp electrodes for EEG recordings. For instance, in the 10-10 system the "10" and "10" refer to the fact that the actual distances between adjacent electrodes are 10% of the total front-back or right-left distance of the skull. 

The naming convention of the electrode positions is related to the underlying brain areas. More specifically, each electrode site has a letter to identify the underlying brain lobe and a number to identify the underlying hemisphere location. The letters F, T, C, P and O stand for Frontal, Temporal, Central, Parietal and Occipital lobes, respectively. However, there exists no central lobe. The "C" letter is used for identification purposes only. Odd numbers refer to electrode positions on the left hemisphere and even numbers refer to those on the right hemisphere. A lower case "z" (zero) refers to an electrode placed on the midline (for more details on standard electrode systems, click on the following link: https://robertoostenveld.nl/electrode/).

 

Figure 2: Layout of extended 10-20 system (figure downloaded from https://www.researchgate.net/publication/280046005_Electroencephalography_EEG-Based_Brain-Computer_Interfaces/figures?lo=1).
Figure 2: Layout of extended 10-20 system (figure downloaded from https://www.researchgate.net/publication/280046005_Electroencephalography_EEG-Based_Brain-Computer_Interfaces/figures?lo=1).

 

 

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Mounting the EEG cap

Prior to mounting the EEG cap, remember to measure the distance between the Nasion and Inion and the distance between the left and right preauricular points of the participant. By definition, the vertex Cz is placed on the cap at the half of both distances. Therefore, try your best to align the midpoint of the two distances with the position of the electrode Cz. Once the alignment is satisfactory, the EEG cap can be considered in the correct position.

 

Figure 3: Nasion-Inion distance in the 10-20 system (figure downloaded from https://slideplayer.com/slide/5662211/).
Figure 3: Nasion-Inion distance in the 10-20 system (figure downloaded from https://slideplayer.com/slide/5662211/).

 

Finally, fix the EEG cap with the chin strap to avoid the cap getting out of place. 

Note: In any EEG cap with predefined positions, it is possible to observe a certain amount of misalignment between the theoretical 10-20 positions and the position indicated on the cap. The amount of the deviation could depend on head shape, head size, and cap size. To keep the deviations as small as possible, it is important to achieve a good alignment between the electrode position Cz and the midpoint of the Nasion-Inion and left-right distances on the participant’s head.

 

 

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Skin preparation and gel application

Passive electrodes

For passive electrodes, preparation of the skin is highly recommended to achieve a good signal quality. After mounting the EEG cap on the participant’s head, use the wooden end of a cotton swab to push aside the hair until the skin is visible in the opening of each electrode. Put a small amount of abrasive gel (e.g., Abralyt or NuPrep) on the tip of the cotton swab and insert it through the electrode opening. Then rub the skin by moving the cotton swab to the left and to the right with slight pressure against the skin (see Figure 4). More pressure will only result in reddening of the skin. This procedure is aimed at stripping away the very top layer of skin. As a result, impedance values between electrodes and skin will tend to decrease, thus improving conductivity.
 

Figure 4: Skin preparation and gel application
Figure 4: Skin preparation and gel application

 

Then insert Abralyt gel with a syringe into the electrode. Make sure that the entire electrode is filled with gel (see Figure 4). 

Check the impedances of all electrodes using the integrated impedance check. It is worth noting that impedances improve after a few minutes by themselves. Thus, it is convenient to start filling all electrodes and only thereafter check impedance values. Assure perfect skin preparation for the ground electrode to obtain reliable impedances. The impedances should be below 5 kOhm. If the impedance of an electrode is too high, it is advisable to repeat the abrasive skin preparation.

Attention: Skin preparation may cause an unpleasing feeling to the participant but should never cause pain or skin lesion!

Note: For electro-oculogram (EOG) derivation or for mastoid reference, electrodes can be used with adhesive washers directly on the skin. Prior to attach the adhesive washer at the appropriate position, it is useful to clean the skin with a degreasing agent (e.g., Isopropyl alcohol). Attach the electrode using the adhesive washer and then prepare the skin with abrasive gel. Finally, fill the electrode as described above.

 

Active electrodes

When using active electrodes, skin preparation with abrasive gel is not required. However, an optimal contact between skin and gel or gel and electrode has to be assured for proper signal quality. Use a syringe with a blunt cannula to apply the gel. Insert the tip of the blunted needle into the ready-mounted electrode and make sure to reach the skin (see Figure 5). While filling the electrode with gel, move the syringe around a bit with the needle on the skin. Make sure that gel is distributed on the skin and that the gap between the skin and the electrode surface is properly filled. 

For further details on Brain Products active electrodos (actiCAP slim), see the official video tutorial at the following link: https://www.youtube.com/watch?v=CiophiSq610.

 

Figure 5: Gel application
Figure 5: Gel application

 

Note: Active electrodes can be used with adhesive washers directly on the skin as well. Just attach the electrode and fill it as described above. 

General remarks

  • In case a signal shows high noise, drift or other disturbance, check the proper filling of the electrode and assure that the gel reaches the skin. Be aware of the fact that the ground electrode needs to be mounted with special care. A very high impedance at the ground electrode may reduce the signal quality. 
  • Using too much gel may cause shortcuts between electrodes.
  • Although uncommon, keep in mind that the gel/paste used to lower the impedance values of the electrodes may irritate the participant’s skin.

 

 

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Removal of the EEG cap and cleaning

After using the EEG cap, carefully disconnect the cables from the amplifier and the chin strap from the cap. Clean the cap together with the electrodes immediately after use. Follow the instructions accurately to avoid damage of electrodes and connectors: 

  • protect connectors from contamination with gel, water or disinfectant;
  • soak the cap with electrodes in warm water for 10 minutes only in case the gel got dry from long-term use; you may use some liquid soap for washing the cap;
  • clean the electrodes with a smooth toothbrush under running warm water. Avoid heavy mechanical treatment of the electrode surface, especially for sintered Ag/AgCl electrodes;
  • a water pick (oral irrigator) may be used at low pressure for gentle cleaning of active electrodes;
  • rinse the cap and the electrodes with clean water; 
  • remove the water with a towel (gently dub the cap inside and outside);
  • put the cap to an appropriate place for drying; avoid stretching the textile during the process of drying; 
  • make sure that electrodes, cables and caps are completely dry before storing.

Note: Both passive and active electrodes have a limited lifetime and need to be replaced after a certain period of time. Careful treatment prolongs lifespam. 

Note: Wash the chin straps only if necessary as they may lose tension from frequent washing.
 

 

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