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About Electrodes

How many electrodes do I need?

For two channel EEG recording you will need 5 electrodes: An active and a reference electrode for each channel and 1 ground electrode. (People using Thought Technology equipment need 2 ground electrodes or a jumper.) For four channel and eight channel recording you need 4 or 8 active electrodes, a ground electrode and 2 or more reference electrodes (jumpers can make one electrode serve as reference for two or more actives).

Where do I place the electrodes?

In “monopolar” recording, the more usual scenario, one electrode (the “active”) is placed on the scalp or forehead, a place where one would expect a strong EEG signal, and another (the “reference”) on a relatively neutral spot such as the earlobe or the bone behind the ear. (See the sidebar for a discussion of other electrode placements.) What is recorded is the difference in the signal between the two points. The ground may be placed anywhere. It creates a safe electrical path off the body, to protect the amplifier.

By recording the difference between the active and reference electrode, we eliminate from our reading the background electrical activity in the environment that is affecting both electrodes. If we used only an active electrode, without a reference, we would be recording very large electrical signals having nothing to do with the brain. The small EEG signal would be lost.

What are the different kinds of electrodes?

There are basically two electrode choices: disposable or cup (disc) electrodes. Disposables are more expensive to use than cup electrodes, because the disposable components have to be replaced frequently. On the other hand, they are convenient to use, and they eliminate concerns about damaging and having to replace an expensive electrode.

Cup electrodes consist of a lead with a cup shaped disk permanently attached. The cup is filled with conductive paste.

The manner of attaching the lead wire to the electrode is important. The upper electrode in the picture is molded, the lower is a cheaper stamped electrode. We recommend only molded electrodes, because the connection between cup and lead wire is more stable.

Once the metal plating chips on a plated electrode, the electrodes are useless, so be sure to get high quality electrodes. We sell solid silver molded electrodes.
Quality gold plated (we recommend Grass electrodes) and tin also work well.

It is difficult to attach reference cup electrodes behind the ear, and so special ear clip electrodes are attached to the ear lobes. If you are doing monopolar recording, for 2 channels you would need one pair of ear clips and 3 standard electrodes (2 actives and a ground).

Disposable electrodes come in two parts: 1) A wire or “lead,” one end of which plugs into your EEG equipment and the other end of which is a snap-on connector. 2) A small disposable electrode snaps into the connector and is attached to the trainee's head.

The Intellectrodes are of this type. These “active” elexctrodes contain a circuit board inside the lead head which eliminates electrical impedance. We also sell relatively inexpensive leads, without the active technology, that accept disposables.

You will want two kinds of disposable snap-ins: adhesive pre-gelled and non-adhesive. The adhesive ones make skin placement very easy and are especially useful for putting reference electrodes behind the ear. The non-adhesive ones are used where there is a lot of hair.

With either the cup or disposable electrodes, it is useful to have a jumper, which can be used to allow one electrode to act as reference for two actives, or to allow the combined signal from two electrodes to act as reference for two actives (the “linked ear" reference).

What supplies do I need to attach the electrodes?

There are two important aspects to attaching the electrodes: 1) You need a good electrical connection, which requires skin preparation and a conductive medium. 2) You need a stable connection which does not slip or wiggle. You may want to purchase our video on applying electrodes.

The Intellectrodes do not require skin preparation. With other electrodes, you apply a mild abrasive lotion (we sell NuPrep) before applying the electrode.

The conductive medium can be 1) conductive paste (10/20 paste shown applied at the left), 2) the gel that comes on the adhesive disposables, or 3) saline solution.

The adhesive disposables are particularly nice because they create no mess, but they cannot be used where there is hair. Conductive paste can create mess in the hair (it cleans up with warm water) but there are techniques for applying it that minimize this. Again see our video. Saline solution is an excellent conducter and does not create a mess in the hair. You soak a pad of some kind in the solution and wrap the pad around the electrode. You do have to be sure the solution does not run across the scalp.

When 10/20 paste is used it serves not only as a conducter but also attaches the electrode to the skin or scalp. It can tend to melt in very warm conditions. The adhesive disposables are, of course, applied with the adhesive. The saline wrapped electrodes require a saline headset to hold them in place. We sell two different saline headset systems, one is shown at the right.

Even with the best hookups, your EEG recordings can still be affected by electrical interference from the environment. A Gauss meter can detect these. The Gauss Master/Dr.Gauss is effective and inexpensive

MONTAGES

The pattern of electrode placements on the head is called the montage.

As we have said, each channel consists of two electrodes, usually one is considered the active and the other the reference. If the reference is placed on the same side of the head as the active, this is called an ipsilateral reference. Ipsi=same, lateral=side. If the reference is placed on the opposite side of the head, this is called a contralateral reference.

In a two (or more) channel montage, there are several relationships the references can have with each other. The references may be independent of each other, i.e., each one plugs directly into its own receptacle on the amplifier. Or the two references may be linked in what is called the linked ear reference. Both are plugged into a jumper and the jumper is plugged into both reference receptacles on the amplifier. In this way, the combined signal from both references becomes the reference for the two actives.

Another way to give both actives the same reference signal is to use a common reference. A single reference electrode is plugged into a jumper which is then plugged into the two reference receptacles.

In monopolar recording the active electrode is over an active brain site and the reference electrode is over an inactive site. In the bipolar montage, both electrodes are over active brain sites, and it does not make sense to refer to them as active and reference. We are still recording (and training up or down) the difference between the two electrodes.

Usually the bipolar montage will be interhemispheric, i.e., one electrode will be place on the right and one on the left hemisphere. And usually these will be mirror sites, the same location on each hemisphere.