EEG Montages - Part 3: Pros and Cons

We will review the remaining montages not previously discussed in the two previous posts, best practices, the strengths and weaknesses of popular montages, montage selection strategy, and optimal display settings. You will gain more from this post if you read the two previous installments.

Click on our narrator icon to listen to this post.

Best Practices from the American Clinical Neurophysiology Society Guideline 3 (2016)

The Committee reaffirms the statements pertaining to montages set forth previously in the Guidelines of the American Clinical Neurophysiology Society (ACNS) and that are paraphrased as follows:

(a) that no less than 16 channels of simultaneous recording be used, and that a larger number of channels be encouraged,

(b) that the full 21 electrode placements of the 10-20 system be used,

(c) that both bipolar and referential montages be used for clinical interpretation,

(d) that the electrode derivations of each channel be clearly identified at the beginning of each montage,

(e) that the pattern of electrode connections be made as simple as possible, and that montages should be easily comprehended,

(f) that the electrode pairs (bipolar) preferentially should run in straight (unbroken) lines and the interelectrode distances kept equal,

(g) that tracings from the more anterior electrodes be placed above those from the more posterior electrodes on the recording page, and

(h) that it is very desirable to have some of the montages comparable for all EEG laboratories.

2.2 The Committee recommends a “left above right” order of derivations, i.e., on the recording page, left-sided leads should be placed above right-sided leads for either alternating pairs of derivations or blocks of derivations. This recommendation coincides with the prevailing practice of most EEG laboratories, at least in North America and in many other areas.

Perspective

The entire EEG field is rife with semantic disagreements. We have made the point that all montages as well as all sensor comparisons are referential

Additional Montages

We briefly touched on the average reference montage. Additionally, several montages are also in common use. One is the linked ears montage. This is one of the montages sometimes referred to as “referential” montages to distinguish them from the sequential bipolar montages. The difference is that each scalp electrode is assigned the positive (+ or active) condition and a single common reference is used for the negative (- or reference) condition in the common mode rejection comparison. In most cases, these montages could be called common reference montages rather than simply referential, which would help differentiate them from other approaches since all montages are essentially referential.

The linked ears montage is one of these common reference montages because each scalp electrode is compared to the sum of the two ear or mastoid electrodes. Another common reference is the common vertex reference – generally using the Cz electrode as the reference for all other scalp electrodes. The other frequently used common reference is the common average reference, where all scalp electrodes are averaged. This montage uses this result for the reference for each individual scalp electrode.

The image below, adapted from Lopez et al. (2017), shows examples of these three montages. Unlike this graphic, in most cases, the midline electrodes are also included in these calculations.

Caption: three common referential montages include: (A) the Common Vertex Reference (Cz), (B) the Linked Ears Reference (LE), and (C) the Average Reference (AR).

In the linked ears/mastoid reference, there is generally a calculation involving adding and subtracting the signals from the ears before the combined signal is used as the reference. Older systems used a physical connection (called a “jumper”) between the two reference electrodes, often causing a current flow between the two electrodes and distorting the recording results. Often a resistor was added to this jumper to inhibit this effect. The digital processing of each signal independently and the resultant mathematical derivation are not susceptible to such distortion.

Another referencing system in common use is the Laplacian montage. This is a montage approach that was not available with older analog systems. It is sometimes called a local average montage since it uses a subset of electrodes surrounding the electrode of interest to create a local average value to which the center electrode can be compared. This is generally thought to enhance the ability to visualize locally occurring events in the EEG while suppressing effects that are common to the area. This may include the suppression of drug or medication effects, but this suppression is not the total elimination of these effects, and this claim must be viewed with caution. Gordon and Rzempoluck (2004) suggest that this approach can enhance the visualization of focal discharges and improve localization.

A Closer Look

We will examine the surface Laplacian (SL), linked ears reference, average reference, longitudinal bipolar, transverse bipolar, circular bipolar, common vertex (Cz) reference, average reference, and linked ears reference montages.

Surface Laplacian (SL) Montage

The SL