EEG Montages - Part 3: Pros and Cons

Updated: 6 days ago


qEEG electrode cap


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.


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.


Three Montages

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 is based on some complex concepts and calculations. Rather than a simple averaging of the voltage potentials of the electrodes immediately around the electrode of interest, it is rather an attempt to define the electrical field around that electrode. The result is a current source density (CSD) measure of current rather than voltage. In EEG, this measurement is in microamperes. The current is proportional to the potential differences between every two combinations of points or electrodes, one being the center or electrode of interest and the other being one of the perimeter electrodes.


Surface Laplacian Montage

The result of the Laplacian calculations provides an estimate of the electrical field surrounding the electrode of interest and represents the current flowing toward or away from a given electrode. This is a measurement of current flow perpendicular to the cortical surface measured as the rate of change of the potential field gradient around the recording site (Gordon & Rzempoluck, 2004). Because this calculation is based on the average of the surrounding electrodes, common influences are reduced, and focal activity is enhanced (Carvalhaesa & Acacio de Barros, 2014).


There are several methods for calculating the SL, but that discussion is beyond the scope of this post. Please see the cited papers and Nunez and Srinivasan (2006).


The Laplacian montage is not affected by the ear/mastoid references as they aren’t included in the calculation. It also visualizes local detail that is often difficult to see in other montages. The SL suppresses the general effects of global EEG activity and medications/drugs in favor of what is happening immediately below the electrode of interest.


Some drawbacks of the SL include the edge effect, where electrodes at the edges of the measuring field, such as Fp1 and Fp2, F7 and F8, O1 and O2, and so on, only have adjacent electrodes on three sides, and therefore the calculation is less accurate.


The SL also appears to add EEG content to electrodes in some cases. However, this is difficult to demonstrate because every EEG visualization depends on various factors, not the least of which is the referencing system. Finally, EEG activity distributed over the entire scalp would not be seen in this montage; therefore, it is best suited to identifying local activity (Gordon & Rzempoluck, 2004).


However, the Laplacian montage does an excellent job of identifying localized activity and reducing the effects of common influences.


Here is an example of a weighted average Hjorth Laplacian analysis of the electrode at Fz. The electrodes immediately around in radiating circles are weighted by distance in terms of their contribution to the reference.


Weighted average Hjorth montage

Caption: Each sensor is referenced to an average of all other sensors, "weighted" by distance from the sensor.



A more typical Laplacian montage with just the electrodes immediately surrounding the target electrode being used in the reference.


Caption: Each sensor is referenced to the average of the surrounding sensors.



Below are five examples of the same eyes open data viewed in Laplacian, linked ears montage, average reference, longitudinal bipolar, and vertex (Cz) reference. All images show mu rhythm at C3 and C4, but the Laplacian and average reference views show the mu rhythm more clearly. The Laplacian appears to differentiate the mu rhythm from the background most effectively. Note that the Laplacian montage uses a y-scale setting of 500 μA, while all the others use a 50 μV y-scale. Laplacian Montage


Laplacian Montage

Linked Ears Montage

Linked Ears Montage


Average Reference Montage


Average Reference Montage

Longitudinal Bipolar Montage


Longitudinal Bipolar Montage


Vertex (Cz) Reference Montage


Vertex (Cz) Reference Montage


Linked Ears Reference Montage


The linked ears reference montage compares each scalp electrode to the combined signal from both ears or mastoid locations. This is in search of a "neutral" reference and some believe that this is the case. The benefit of the linked ears is better visualization of central/midline sources as well as frontal and prefrontal activity. However, linked ears are well known for adding cardiac activity, EMG activity from neck and jaw muscles, as well as EEG patterns such as alpha, theta, or transient activity to scalp electrodes. The example below shows this clearly in the circled epoch.


Linked Ears Montage


Average Reference Montage

The average reference montage uses an average of all scalp electrodes as the reference. This can help eliminate common sources and generalized EEG patterns in favor of activity that is more specific to each scalp electrode site. At the same time, it can minimize important EEG activity that exists at multiple sites, such as that in the example EEG. It can also contribute scalp EEG patterns to locations where they don’t actually occur.


Average Reference Montage


Longitudinal Bipolar Montage

The longitudinal bipolar (longBP) montage follows the longitudinal sequence of electrode comparisons or derivations. The left-side electrode derivations of the more lateral (lateral frontal, temporal and parietal) locations precede the more medial (parasagittal) derivations, followed either by the center/midline if used or by the right side medial and then lateral derivations. Sometimes the central sequence is at the bottom. See the sequence of numbered derivations shown above. Good for comparing and visualizing differences in the left hemisphere compared to right hemisphere characteristics. See the example below, from the NeuroGuide database representing a sample provided within the software of an individual with a right hemisphere parietal impact injury. Note the marked differences between left and right hemisphere activity. This is a 10-second window and a 50 μV y-scale.


Longitudinal Bipolar Montage


Transverse Bipolar Montage


The transverse bipolar montage follows the suggested anterior-to-posterior orientation, with prefrontal and frontal sequences of electrodes displayed first and the rest following.



Transverse Bipolar Montage


Below is an EEG example from NeuroGuide showing the effects of a right parietal injury. This difference is most clearly seen in derivations involving P4 compared to those involving P3. This montage helps identify relationships that may not be clear in the LongBP montage.




Circular Bipolar Montage


The circular bipolar (CircBP) montage shows electrode pairs following a circular (coronal) orientation, often beginning with Fp1-Fp2, Fp2-F8 or beginning with T3-F7, F7-Fp1 and so on, following the left over right recommendation. This montage also can highlight activity and relationships that escape the other two montages.


Circular Bipolar Montage


The EEG tracing below shows the example EEG in the CircBP montage.


Circular Bipolar Montage


Common Vertex (Cz) Reference Montage


The common vertex (Cz) reference montage, generally using Cz as the common reference for all other electrodes, simply compares each scalp electrode to the same reference. It provides a common voltage in the reference channel. Benefits from this montage include being able to visualize electrodes that are equidistant from the vertex, such as Fp1 and Fp2, F3 and F4, O1 and O2, and so on.


Also, it allows us to compare their characteristics to reveal differences between hemispheres and between frontal and posterior areas. This montage can be useful when identifying interhemispheric amplitude asymmetries and other metrics. In the example EEG below, right and left differences can be seen. However, activity in the general vicinity of Cz can be added to other electrodes in some cases.


Common Vertex Reference Montage


Average Reference Montage


The average reference montage uses an average of all scalp electrodes as the reference. This montage is also useful for identifying local activity, particularly in temporal lobe areas where the ear references may either contribute to or cancel the same activity. If the software can exclude electrodes affected by large EEG sources, then the resulting average excluding these sources will be more neutral. Graphic © learningeeg.com.




It is noted that the average reference skews phase and coherence calculations and is therefore not used for these particular database comparisons where the database was collected using linked ears as the reference. Z-scores can be calculated when the database provides norms specific to the average reference. The average reference montage is often cited as the "best" for viewing the EEG, but, like all referencing systems, the average reference has its own problems. Because it represents an average of the voltage at all electrodes, it contains, within that negative (reference) channel, a fixed voltage that can then affect the resulting EEG tracing on the screen (Nunez & Srinivasan, 2016).


Suppose this contribution contains high amplitude artifacts, persistent EEG patterns such as alpha, theta, delta activity, or any other factor. In that case, these can be added to those electrodes that don’t already have these patterns, which can cause the rejection of these patterns in electrodes that share them. The problem is that the effect is often quite diffuse and, therefore, more difficult to spot.


In the authors' practice, the average reference statistical (z-score) topographic displays (e.g., maps) often correspond quite closely to those derived from the Laplacian montage, whereas those from the linked ears/mastoid montage do not.


Average Reference Montage


Linked Ears Reference Montage


The linked ears reference montage is one of the most commonly used references in neurofeedback. The original NxLink database developed by E. Roy John and the NeuroGuide database developed by Robert Thatcher collected ear electrodes when conducting EEG recordings to make linked ears montage computations. Calculations of phase and subsequent coherence measures derived from the phase calculations in these databases are made using the linked ears montage.


The linked ears montage is generated by comparing each scalp electrode to the average of the two ear/mastoid sensors. The choice of ears or mastoids does not seem significant, but there may be noticeable differences in individual cases. If the equipment used can access/record either or both the ears and the mastoids, the clinician can view the recording as it is occurring while using one and then the other and then choose the best one for that recording session. The presence of pronounced electrical activity in the reference channel and/or individually in each reference sensor would suggest against using that source in favor of the choice with the most minimal contribution to the resulting recording.


The ear reference montage has sometimes been conducted using only one ear or mastoid, and arguments have been made supporting the ipsilateral (same side) versus the contralateral (opposite side) reference. This can be a consideration when using a small number of scalp sensors, but when using all 19 of the 10-20 scalp electrode sites, it is more common to use the two ear/mastoid sensors in a linked approach. Some guidelines suggest that the angle of the mandible may also be used (Acharya et al., 2016).


Angle of the mandible

The linked ears/mastoid is a good montage for viewing EEG activity in the central electrode locations, along the vertex particularly, because they are quite far away from the reference. Therefore there is only minimal common activity subject to rejection. However, it is not as helpful for temporal electrode locations because these often share EEG activity with the references, which is then rejected. Again, the linked ears reference often contributes alpha activity to frontal and central signals due to the retention of anything different in the common mode rejection process. Note the widespread alpha activity seen in the linked ears montage in the images above, showing mu rhythm examples that do not appear in the other montages.


The linked reference montage does simplify the visualization of the EEG tracings (Valentine, 2020). Since there are no bipolar comparisons, there are no phase reversals in the sense of the bipolar montage. However, the field of activity around a large EEG event, such as an epileptiform discharge, can sometimes be identified in this montage. All the negative EEG activity at each electrode site shows a negative (typically up) deflection in the EEG tracing, and positive electrical shifts deflect down. This is, of course, in reference to the linked ears/mastoid or another common reference.


As mentioned, there are other choices for references, including the vertex reference, usually at Cz. The choice of reference is important because it affects what can be seen in the resulting EEG tracing. The vertex Cz location is a good choice if temporal lobe epilepsy is an issue. Using linked ears/mastoids for viewing this activity could result in important data being rejected as the same patterns may show up in the reference as in temporal lobe sensors. Another issue with ear/mastoid electrodes is that they often contain artifacts such as EMG from the jaw or neck muscles as well as ECG (cardiac) artifacts that can then be contributed to all scalp sensors. The problem of reference contamination with alpha activity from the temporal lobes has already been addressed.