Electroencephalographic Electrodes, Channels, and Montages and How They Are Chosen

Recommended Montage Conventions

Although electrode channels can be presented in any order when creating a montage, certain conventions are encouraged (see the American Clinical Neurophysiology Society [ ACNS ] Guidelines in References): channels should be placed in a “left over right” configuration (electrode chains from the left side of the head are placed nearer the top of the page, and chains from the right side of the head are placed nearer the bottom of the page). More anterior electrodes should be placed before more posterior electrodes (front-to-back ordering of channels going down the page is encouraged). Interelectrode distances should be consistent (electrode positions should not be unpredictably skipped in electrode chains). Finally, the design should favor the simplest arrangement possible (see Figure 5-1 ).

In the days of paper EEG recording, it was up to the EEG technologist to choose the montage that would best demonstrate the patient’s findings at any particular point in the tracing. If an EEG event happened to be recorded in a montage that was disadvantageous for interpretation, the ink had already dried on the page, and it was not possible to change settings retrospectively. The newer digital EEG technology presents a new set of problems. Although the same EEG page can be viewed in a variety of montages during the course of interpretation, some readers may find that it takes less energy to leave the display in one montage for the whole tracing, usually choosing the montage with which they are most comfortable. Although scanning a whole EEG study in a single montage may allow for faster reading, occasionally an EEG finding may be seen well in one montage but only poorly seen, or perhaps not seen at all, in another. It is therefore best practice for the EEG technologist or the reader to cycle through a minimum set of montages during the course of each study. This will usually include some combination of longitudinal bipolar, transverse bipolar, and referential montages.

Referential Versus Bipolar Montages

Referential and bipolar recording techniques appear to use two distinct strategies for recording, but, as we shall see, there is considerable overlap between the two. A referential montage compares each “point of interest” on the head (which can be referred to as “the electrode of interest”) to a reference point somewhere else on the body, perhaps still on the scalp, which it is hoped will be neutral or “indifferent.” One of the main drawbacks of referential montage recordings is that the reference point often cannot be completely neutral. Here is an example of the left parasagittal chain of five electrodes as it might be displayed in a referential montage:

Fp1–ref

F3–ref

C3–ref

P3–ref

O1–ref

The term ref as used here denotes a reference electrode such as the nose, the chin, the back of the neck, or the earlobes, another electrode on the scalp, or sometimes a combination of scalp electrodes. The specific choice used for the reference electrode is further discussed later in this chapter.

Considering the technique used to display the same chain of five parasagittal electrodes in a bipolar montage, the following channels would be used:

Fp1–F3

F3–C3

C3–P3

P3–O1

The term bipolar derives from the fact that, as can be seen from this chain, each channel represents the voltage difference between two “poles” on the scalp. Bipolar montages are occasionally referred to as differential montages because they display the difference between adjacent electrodes. Although the bipolar technique is a powerful recording technique and a favorite among many readers, bipolar montages also have their drawbacks, as discussed later. Figures 5-2 and 5-3 show a schematic of the difference between the referential and bipolar recording strategies.

Note that, even the so-called referential montage is, in reality, “bipolar.” Every channel must represent a comparison of two electrode positions—there are always two “poles” being compared. This is one of the criticisms of the term bipolar montage; it seems to imply that the counterpart term, referential montage, is not bipolar in some way. Indeed, there is no such thing as a “monopolar” montage, and technically all montages are, at their root, “bipolar” because every channel displayed on the EEG page, be it part of a bipolar or referential montage, is a voltage comparison between at least two points.

Theoretical Strategies: How Can EEG Activity Best Be Recorded From a Single Point?

It is useful to consider the theoretical pros and cons of each recording technique starting by asking the elemental question: what would be the best way to demonstrate the electrocerebral activity at any given point on the head? Consider the example of the left frontopolar area, where the Fp1 electrode records at a point on the forehead above the left eye. If we were assigned the task of deciding how best to record the electrical activity occurring under the Fp1 electrode, momentarily leaving aside what is going on elsewhere in the brain, what would be the best way to do this?

Recalling that our basic tool for creating an EEG channel is to print the amplified output of a voltmeter for which the needle sweeps one way or the other depending on the voltage difference between two points, there must always be two separate amplifier inputs, Input 1 and Input 2, to make a comparison. We decide to attach the “electrode of interest,” Fp1, to one pole of our voltmeter (Input 1), but what would be the most advantageous choice of electrode(s) to attach to Input 2, the comparison point? This comparison electrode is called the reference electrode, and serves as the reference point against which the electrode of interest is measured. As described in Chapter 4 , the voltage measured from the reference will be subtracted from the voltage measured at Fp1, and the resulting difference is the output for the channel that we will label as Fp1-ref.

It is clear that the choice of the reference electrode could have as big an impact on the appearance of the resulting output trace as the Fp1 electrode. At first glance, the channel label “Fp1-ref” may seem to imply that its output represents the voltage at Fp1. Note, however, that the “active electrode,” Fp1, does not enjoy any privileged position by being attached to Input 1 as opposed to Input 2; the two voltages will simply be subtracted one from the other and displayed. For that reason, if the chosen reference electrode were the earlobe and there happened to be more electrical activity present in the earlobe than in Fp1, the output of an Fp1-earlobe channel would be dominated by the electrical activity in the earlobe—not the desired result. Clearly, the choice of the reference electrode in referential montages is important.

Choice of a Reference Electrode

Considering different possible locations to place the reference electrode, because we are looking for a location with a “neutral” voltage for comparison, one possible choice would be an electrical ground. Possible grounds include the grounding prong from a three-prong wall outlet or a metallic cold water pipe that runs into the ground. In reality, the choice of attaching Input 2 to an electrical ground is not feasible. The resulting tracing would be full of noise, and we would have little success in our goal of displaying the very low-voltage EEG activity coming from the Fp1 electrode for two reasons.

First, electrical grounds, particularly those in large buildings such as hospitals, are not really electrically neutral. A variety of electrical devices throughout the building are attached to the same ground, and these devices spill electrical noise into the ground. Therefore, an Fp1-ground channel could include a terrific amount of noise from all of these distant sources and the signal of interest, the microvolt-level electrical activity emanating from the brain from the region of the Fp1 electrode, could be overwhelmed by all of this electrical noise.

The second and perhaps less intuitive problem is that the Fp1 electrode itself has a large array of electrical activity in it, but not all of that activity is brain wave activity. In addition to the very low-voltage activity that emanates from the left frontal pole of the brain (the activity that we really want to see), there is also a large amount of ambient electrical noise at that scalp location. Some of this noise comes from the human body, including electrical activity given off by the frontalis muscle of the forehead, electrical activity from the heart (the electrocardiogram), electrical noise from movement of the eyes, and even noise from swallowing movements. Even more troublesome, however, is the fact that, especially indoors, the environments we live in are full of electrical activity. Relatively large amounts of electrical noise flow through our bodies at all times, especially in electrically active environments such as hospital rooms or EEG laboratories where EEG recordings are usually performed. As previously described, contamination of our bodies with 60-Hz signals from electrical outlets is especially common. Comparison of Fp1 to a completely indifferent electrode (such as an ideally quiet ground) would do nothing to attenuate (or “subtract out”) the large amount of electrical noise in the vicinity of the Fp1 electrode in favor of the true brain wave signal in that area. Therefore, paradoxically, we do want some noise in the reference electrode. Ideally, we would like the reference electrode to have the exact noise that is present in the Fp1 electrode so that our differential amplifiers will subtract it out.

Considering these goals, choice of an ideal reference electrode becomes an interesting challenge. Instead of using electrical ground as the reference electrode, consider the possibility of using an electrode placed on the foot. The choice of the foot electrode would be a vast improvement over using the ground because a recording electrode on the foot would detect a lot of the same electrical “body noise” as an electrode on the head (and also would lack the ground noise). Because the signals from Fp1 and the foot would be subtracted from one another, electrical noise in the body common to both locations would cancel out and only the difference would appear in the resulting tracing. The chance that the brain wave signal we seek under Fp1 would rise above the fray and be recognizable to us is now much higher.

Next, consider the idea of using an electrode position adjacent to Fp1 (such as Fp2, the electrode over the right frontal pole) as the reference electrode. Subtracting Fp2 from Fp1 yields a signal that appears much cleaner and will be of lower voltage compared with the Fp1-foot pairing. At first glance, this Fp1-Fp2 pairing would seem to be the most satisfactory because the amount of noise in the channel is minimized—any eletrical noise in Fp1 is probably also present in Fp2 and that common noise should cancel out. However, there is also a significant potential disadvantage that may not be obvious at first glance. If there is brain wave activity common to both the Fp1 and Fp2 electrodes, the subtraction of the Fp2 signal from Fp1 may actually cancel out some of that common brain wave activity, the activity we actually seek to record. The only signal displayed will be the difference between the two locations that may only represent a portion of the total brain wave activity at Fp1, the display of which was the original goal of picking an ideal reference. The idea that there could be a lot of brain wave activity in common between Fp1 and Fp2 is plausible because there could be, for example, a frontal slow wave shared more or less equally by Fp1 and Fp2. The common mode rejection amplifiers used in our EEG instruments would cancel some or all of this common brain wave activity when the Fp2 activity is subtracted from Fp1. We would then have a tracing that appears to be “electrically clean” at the expense of losing some of the EEG activity we are looking for.

Finally, consider the extreme example of moving the reference electrode so close to Fp1 that it is nearly in the same position as Fp1. In such an example, the signal in Fp1 and the reference are so similar that the difference output will become nearly flat. Some generalizations can then be made regarding the distance between the electrodes attached to Input 1 and Input 2 of a channel’s amplifier. When two electrodes are moved closer to one another, there is greater commonality between what each detects in terms of both the noise signal and the brain wave signal. Therefore, as two electrodes become closer to one another, a channel comparing them will flatten. Conversely, as interelectrode distances increase, there is a bigger difference in both the noise signal and the brain wave signal and the channel"s output signal tend to increase.

All things being equal, larger interelectrode distances are associated with higher voltages, and smaller interelectrode distances are associated with lower voltages. This effect explains the recommendation that electrodes should not be skipped in bipolar montage chains (or stated another way, that interelectrode distances should be held constant). A channel pair with an inconsistently large interelectrode distance will tend to produce a higher voltage channel, possibly giving the erroneous impression that there is more electrical activity at that location than at adjacent locations. Variation in interelectrode distances may occur in two ways: a montage design that skips an electrode or a significant mismeasurement of electrode position during electrode application (see Figure 5-4 ).

What would the ideal reference electrode location be?

The ideal reference electrode includes all of the electrical noise in the “electrode of interest” but none of the electrocerebral activity.

It will come as no surprise that no such ideal reference electrode exists. The goal is to find some compromise location for the reference electrode that will tend to cancel out noise fairly well but will not cancel out too much brain wave activity. The choice of a good reference electrode position represents a balance between wanting the electrode to be close to the “electrode of interest” so that it will share as much noise signal with it as possible and wanting it to be distant enough so that there is not too much cerebral activity in common between the two electrodes (to avoid cancelling out EEG information). Commonly used reference points include the earlobes or mastoid areas, the nose, the CS2 location representing the skin overlying the spinous process of the seventh cervical vertebra ( vertebra prominens or the most prominent bony bump at the base of the neck), or more rarely the Cz electrode. More complex reference electrodes may be used as well, including techniques in which the arithmetic mean of some or all of the scalp electrodes is used as a “virtual” reference electrode (discussed later in more detail). Each of these techniques is associated with its own advantages and disadvantages, which will also vary according to clinical circumstance.

Each potential reference location is prone to certain problems and advantages—no single reference electrode position is ideal for all patients at all times. For instance, the earlobes often share a portion of the electrocerebral activity present in the adjacent midtemporal areas, causing a cancellation effect. This makes an earlobe reference a poor choice for a patient with a high voltage midtemporal discharge. Also, the left earlobe in particular is often contaminated with EKG artifact because of the left-sided position of the heart. In some, the nose reference may be contaminated by a surprising amount of muscle artifact, whereas in others it can be fairly clean. The cervical area is contaminated by muscle artifact or EKG signal in some patients and may also be disturbed by movement when patients are lying on their backs. The midline vertex electrode, Cz, is usually free of muscle artifact because of its location at the vertex of the scalp but contains a large amount of electrocerebral activity, especially during sleep. In different individuals, any of these reference locations could generate satisfactory or unsatisfactory recordings at different times.

In the case of designing a referential montage, after the reference electrode has been chosen, there is little more to decide than the order to arrange the channels on the page. There are two general strategies for channel arrangement. One is to arrange the channels in left–right pairs so that each position on the brain can be compared with its homologous counterpart in the opposite hemisphere. This approach leads to a “left–right–left–right. . .” arrangement of channels (e.g. Fp1 followed by Fp2, F7 followed by F8, F3 followed by F4, and so on). The alternative strategy for ordering the channels conforms to the “left-over-right” and “front-to-back” conventions mentioned above in which channels are “clumped” according to their location. Although each of the two systems offers specific advantages, neither is clearly superior to the other as each EEG electrode’s channel will appear somewhere on the page with either system, only in a different order.

The main difference between the two systems, one of which alternates left- and right-sided electrodes and the other of which groups left-sided electrodes in one area and right-sided electrodes in another, is the comparative ease of visual scanning for asymmetries. Choice of one of these arrangements over the other is essentially a matter of preference. Figures 5-5 and 5-6 show examples of dramatic left-sided slowing displayed with each of these two common strategies. The latter system, which groups electrodes together by location, is seen to have certain advantages. Asymmetries that occur between hemispheres are not usually localized to a single electrode but to a regional group of electrodes. For instance, if a temporal slow wave is present, it may be recorded in multiple electrodes such as F7, T7, and P7. When an abnormality such as a spike or a slow wave is present in multiple adjacent channels on a page, it tends to be easier to identify visually when the spatial clustering system is used. In the alternating left–right setup, the abnormality will only be seen in alternating channels. Another advantage of the clustering system is the ease of visualizing the topography of the discharge on the scalp; with this system a region of the page corresponds to a region of the brain starting with the fact that the left hemisphere is represented on the top of the page and the right hemisphere is on the bottom. Grouping the channels by region, especially when gaps are used between electrode groups as in the example illustrated here, makes it simple to know where a given channel is on the head at a glance, even without reading the channel labels. In contrast, when reading in a montage in which the left and right channels are alternated, it can be difficult to know the location of each electrode without consulting the channel labels. Proponents of the left–right alternating system like the fact that homologous channels are adjacent to one another, making individual interchannel comparisons easier. Considering these reasons together, the author prefers the clustering system (left over right and front to back), which should allow for easier and more accurate scanning and spatial visualization for most readers. However, the choice of either of these two arrangements is really a matter of laboratory or reader preference.

Bipolar Montages

In practice, most electroencephalographers do the bulk of their EEG interpretation using bipolar montages rather than referential montages, despite the potential disadvantages of the former. Bipolar montages are generally preferred because they produce “cleaner” tracings due to the proximity of the electrode pairs, which leads to more efficient noise cancellation. The clean appearance of the tracings and ease of reading does come at a potential price, however: the risk that the proximity of the electrode pairs has led to cancellation or an understated appearance of some of the brain wave activity, perhaps unbeknownst to the reader. When recording in a referential montage, if noise is present in the reference electrode, that noise may potentially obliterate all the channels at once. Given the different strengths and weaknesses of each technique, it is best practice to interpret any given EEG study using some combination of the two techniques.