|A Guide to Electrode Selection,
Location, and Application for EEG Biofeedback
Thomas F. Collura, Ph.D., P.E.
All of the methods below have the following elements in common:
Basically, there are four types of electrodes
For EEG biofeedback applications, all electrode systems consist of a minimum of one pair of electrodes (an "active" and an "indifferent") to record a channel of EEG, plus a third electrode as the "ground." Generally, the "active" electrode will be located on the head, near the brain area that is being monitored. The "indifferent" electrode can also be on the head, but can also be on an ear, or behind an ear ("mastoid"). The "ground" electrode can be almost anywhere, but a forehead or ear location is preferred.
Therefore, in order to record and do biofeedback on a single channel, it can be as simple as placing a single electrode on the head, and an electrode on or behind each ear, to complete the recording. Such an arrangement can be placed in just a few minutes, and is very common in clinical biofeedback labs.
If a second channel is desired, at least one additional electrode is required; it can share the reference electrode with the first channel. However, if "true" differential two-channel recording is desired, for example for left/right coherence, or to compare two brain locations, then a different reference should be used. The BrainMaster is designed with 5 leads for two channels, thus permitting a separate reference for the two channels.
Physically attaching electrodes
The key to a clean and accurate EEG reading is to make certain that the electrode is in proper contact with the skin. This is very important to maintain at all times throughout the reading, and is very simple to obtain.
Clean the skin with an alcohol swab to remove surface oils. Abrade the skin with fingernail or skin prep gel (Omni-Prep), applied with a Q-tip. Clean area with alcohol swab, prep and clean again. It is imperative to do this twice as the second cleaning insures the skin is free of the gel, so that the adhesive can stick well. Now the skin is ready for the electrode.
Simple method using disposable electrodes
Disposable electrodes are the quickest and simplest way to get a clean and accurate reading. They are limited in the locations they can reach because they will not stick where there is hair. Pay attention that the entire electrode is securely fastened; it may be necessary to pat the electrode firmly and carefully to guarantee this. For example, a "Contour" electrode (PAD-1) can be applied behind the ear on the mastoid bone to provide a good alternative to an ear clip electrode. A "Classic" electrode (CIRC-1) can be used on the forehead or on bald areas.
During the recording, it is wise to inspect the electrodes occasionally to insure that they are not falling off or being pulled loose by the electrode wires. Properly affixed electrodes of this type can provide a secure connection for many hours of undisturbed recording.
Simple method using disc electrodes and 10/20 paste
Prepare the skin as instructed above. Apply generous quantity of 10/20 paste to skin and also to electrode (paste must fill the ball of the electrode cup). Secure electrode to the skin pressing firmly to ensure proper contact.
Earclips are also available using disc electrodes. Either 10/20 paste or electrode gel can be used with them.
Clinical method using disc electrodes with collodion, gauze and electrode gel
This method, generally used in a clinical setting, is time consuming and has specific requirements. The skin is cleaned with alcohol and an electrode is placed on the skin. The hair is parted, if necessary. Next a gauze pad is dipped in collodion and secured over the electrode. Using a hypodermic needle, the electrode gel is injected through the hole in the electrode cup and the skin is abraded with the blunt end of the needle. These electrodes can be re-gelled at any time to provide a long lasting recording session lasting from hours to days. The advantage of this method is that electrodes can be placed virtually anywhere with precise location. It is, however, a costly, uncomfortable, smelly and hazardous method.
Commercial headbands and electrode hats
A variety of products are supplied with headband-type connectors. These include cup and disc-type electrodes, generally of gold or stainless steel, connected to an adjustable elastic band. These devices still require careful skin preparation. Usually they only allow electrodes to be positioned on the forehead or along the "hat band" ring around the head.
For example, the Waverider, Brain Tracer, and Mind Mirror III are supplied with headbands.
Headband electrodes can be used for various EEG recording if one keeps in mind the best electrode placement locations. For example, the Waverider headband could be used to obtain three connections for a "frontal" EEG recording keeping in mind that the forehead derivation will be prone to EMG (muscle) contamination.
A variety of saline-based electrodes are available, that use salt water in place of electrode gel. All require a headband, clips, or other means of attaching them to the head. They can use any electrode material (gold, silver, etc.) and can provide an excellent connection. Some skin preparation is still required. Care must be taken to ensure that the mechanical contact remains firm, and that the electrodes remain wet with saline, to maintain a proper electrical contact.
Maintaining an electrically secure connection
The following tips will help to ensure that a good connection is made, and maintained:
Check electrode impedances if possible. They should be below 20KOhms apiece. If electrode impedances are high or are drifting, add gel or saline, and check to make sure that there is a firm mechanical contact.
Monitor the waveform display, if possible, using visual inspection to insure that a clean EEG signal is being obtained. Tapping or pressing the electrodes should show a clear artifact, demonstrating that the amplifiers are working properly. If there is excessive drift, check the electrodes. If there is high-frequency noise, look for a source of interference such as electrical equipment or stray wires.