EEG Montages: Part 2 - Selecting the Best Comparison

Updated: Aug 10

A montage groups electrodes together (combines derivations) to record EEG activity (Thomas, 2007).

Clinicians monitor EEG activity using the classical International 10-20 System for standardized electrode placement or the modified 10-10 system known as the Modified Combinatorial Nomenclature System. They often record from several sites and measure the amplitude (signal power) of EEG signals within frequency bands (like alpha and theta) to provide a complete picture of brain activity.

Software-based montage reformatting allows clinicians to reanalyze session data by referencing an electrode to other sites or combinations of sites. This system also allows for the computation of multiple variables associated with communication and network functions within the central nervous system (CNS).

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The Quantitative EEG

The quantitative EEG (qEEG) measures EEG amplitudes within selected frequency bands. A full-cap 21-channel EEG recording (19 scalp sensors plus two reference sensors) and resulting qEEG analysis may be valuable in designing treatment protocols for complicated cases like Asperger's or traumatic brain injury. EEG topography displays the qEEG on a cortical surface map to show the spatial distribution of EEG activity.

The movie below is a 19-channel BioTrace+ /NeXus-32 display of theta activity © John S. Anderson. Brighter colors represent higher theta amplitudes. Frequency histograms are displayed for each channel.

EEG contamination by physiological and exogenous artifacts requires that clinicians take extensive precautions, examine the raw EEG record, and remove contaminated epochs through artifacting. Impedance tests and behavioral tests help ensure the fidelity of EEG recording.

Finally, clinicians interpret EEG recordings with an understanding of normal values and recognition of the effects of eye closure, age, diurnal influences, alertness and drowsiness, medication, and relaxation on these readings. Graphic © Medical-R/Shutterstock.com.

International 10-20 and 10-10 Systems

The International 10-20 system is a standardized procedure for electrode placement on 19 scalp and reference and ground sites. Electrodes measure electrical activity from a surrounding area the size of an American quarter. The site recorded may be distant from the EEG generator due to neural pathways.

The 10-20 system assigns recording electrodes a letter and subscript. The letters represent the underlying region and include Fp (frontopolar or prefrontal), F (frontal), C (central), P (parietal), O (occipital), and A (auricular). A subscript of z represents a midline (central axis from nasion to inion) placement.

Numerical subscripts range from 1-8 and increase with distance from the midline. The 10-20 system assigns odd-numbered recording electrodes on the left and even-numbered electrodes on the right side of the head. Two reference electrodes are usually placed on the earlobe.

The 10-10 System's Modified Combinatorial Nomenclature

The American Clinical Neurophysiology Society published guidelines for expanding the 10-20 system to 75 electrode sites. While more complex, this system also allows us to define the placement sites for our electrodes precisely.

The expansion of the 10-20 system allows clinicians to define the sites midway between two 10-20 sites commonly used in clinical practice, better localize epileptiform activity, increase EEG spatial resolution, and improve detection of localized evoked potentials. The modified combinatorial system replaces inconsistent designations (T3/T4 and T5/T6) with consistent ones (T7/T8 and P7/P8). Black circles depict these replacement sites with white lettering in the diagram below.

The modified combinatorial system, called the 10-10 system, locates electrodes at every 10% along medial-lateral contours and adds new contours. Each electrode site intersects a medial-to-lateral coronal line (designated by letters) and a longitudinal sagittal line (designated by numerical subscripts).

Although some practitioners conduct assessment and training with a single channel, assessments and training methods that use more than one channel have become more available. For example, the cost of a full 19-channel EEG assessment has decreased substantially. Such an assessment can provide not only EEG amplitude data from all sites in the 10-20 system but additionally makes calculations of metrics such as coherence and phase that give information on how well the 10-20 sites communicate with each other.

As noted in our Montage 1 post, a channel is an EEG amplifier output resulting from scalp electrical activity from three electrode/sensor connections to the scalp. These sensors are commonly known as the active, reference, and ground electrodes. They are more appropriately called positive +, negative -, and reference.

Clinicians place them on the head in the following manner: an active or positive electrode is placed over a known EEG generator like Cz. A reference or negative electrode may be located on the scalp, earlobe, or mastoid (bony prominence behind the ear). A ground/reference electrode may also be placed on an earlobe or mastoid (Thompson & Thompson, 2015).

Active and reference sensors are identical balanced inputs and interchangeable. However, some neurofeedback data acquisition systems require the designation of a specific sensor as a reference, as in a linked-ears reference.

A derivation assigns two electrodes to an amplifier's inputs 1 and 2. For example, Fp1 to O2 means that Fp1 is placed in input 1 and O2 in input 2.

Montages combine derivations. All montages compare EEG activity between one or more pairs of electrode sites. Modern amplifiers record all input sensors in reference to a common sensor, often Cz. All montage (sensor comparison) changes are performed in the software. Amplifiers no longer require manual switching of electrodes between inputs.

Montage Options and Their Consequences

Referential (Monopolar) Montage

A referential (monopolar) montage places one active (+) electrode (A) on the scalp and a "neutral" reference (-) electrode (R) and ground (G) on the ear or mastoid. It should be noted that all montages are referential as described in detail in Montages 1.

A referential montage assumes that the EEG activity seen on the computer screen represents the active (+) site for two reasons. First, we assume that the reference (-) site is neutral (i.e., producing no EEG activity). Second, a differential amplifier's common-mode rejection subtracts signals produced by noise and artifacts shared by both active and reference sites. In the photograph below, the blue cable would be used for the active electrode, the yellow cable with an ear clip for reference, and the black cable with an ear clip for the ground.

However, this montage is vulnerable to artifacts from the contraction of facial muscles (Demos, 2019). The ear reference is also known to produce reference contamination, where EEG signals from this electrode are contributed or added to other electrodes via the mechanism of the differential amplifier, where anything different between the active and reference sensors is retained. This commonly results in alpha activity produced by posterior alpha sources close to the ear to be contributed to other scalp sensor locations.

Sequential (Bipolar) Montage

A sequential (bipolar) montage presents a sequence of comparisons of positive (+) and negative (-) electrodes (often called active and reference) that are attached to sites on the scalp and therefore considers the reference electrode to be a second active electrode. The ground (G) electrode is attached to the scalp, to an earlobe, or over the mastoid process.

The sequential montage detects the difference in EEG between the positive and the negative electrodes, as the referential montage does. However, now the signal for the channel represents the difference between two sources of EEG activity. In cases when 19-channels are used, this montage is usually presented with electrode pairs shown in sequence. Note that only the black cable for the ground has an ear clip in the photograph below.

When used as only a single channel, this montage detects localized EEG activity poorly because it shows only the difference between the A (+) and R (-) signals. However, when used as part of a multi-channel assessment, it helps isolate EEG events related to epilepsy or other localized causes. This montage can also reduce artifacts when the A and R electrodes are relatively close.

Professionals frequently use sequential montages in neurofeedback to train the difference between EEG activity at the A and R electrodes. However, when neurofeedback training produces a change, it remains uncertain whether it is because of a change in EEG at the A electrode, the R electrode, or both.

Recognizing and Correcting Signals of Noncerebral Origin

EEG artifacts, consisting of noncerebral electrical activity, can be divided into physiological and exogenous artifacts. Physiological artifacts include electromyographic, electro-ocular (eye blink and eye movement), cardiac (pulse), sweat (skin impedance), drowsiness, and evoked potential. Exogenous artifacts include movement, 60 Hz and field effect, and electrode (impedance, bridging, and electrode pop) artifacts.

Using a variety of montages can help us identify the source of the artifact and possibly address it so that a clean recording may be obtained. A more detailed discussion with examples is included in Neurofeedback Tutor.

Recognizing Normal EEG Patterns

The healthy adult EEG is a cerebral symphony comprised of multiple, complex electrical discharges recorded from the scalp surface. The deconstruction of these complex signals into the typical EEG patterns known as theta, alpha, sensorimotor rhythm, beta, and gamma activity is also covered in great detail in Neurofeedback Tutor. Here we will simply discuss how montages can help us identify normal and abnormal patterns and help us distinguish between true EEG activity and artifact.

Bipolar Montage Options

Because a particular grouping of electrodes in a montage or array results in specific display characteristics, it is useful to understand them visually. We will examine the longitudinal, transverse, and circular bipolar montages.

Longitudinal Bipolar Montage

The longitudinal bipolar montage is also known as the “double banana” because the shapes resemble two bananas.

There are various approaches to this montage with different choices for which electrode combination to start with, but the image above shows a typical approach. The Fp1 electrode is compared to F7, then F7 is compared to T3, T3 to T5, and T5 to O1. Starting again from the front with Fp1 compared to F3 and so on for sides anterior to posterior and sometimes midline comparisons.

The benefit of this montage is that adjacent electrode sites can be compared in a sequence, and EEG features that appear in one combination can be compared to the next anterior or posterior combinations to see if the features appear there. Phase reversals, where the waves reverse polarity, and one shows a negative [up] direction while the other shows a positive [down] direction, can point to the source as three derivations are compared in a sequence as shown below.

Note that the wave patterns in the montage derivations shown below have opposite polarity. They are 180 degrees out of phase from one another. This indicates that the source of that particular wave is the circular dashed lines near the right ear. The first derivation shows that input 1 is more positive than input 2. The second channel shows little activity because the input 1 and 2 electrodes are showing essentially the same waveform and same voltage. The third channel is reversed from the first channel because input 1 is more negative than input 2.

The middle channel shows the effect of common mode rejection (canceling what is the same) and shows the source of the activity is between the electrodes in that channel, while the other two derivations point to the source.

Other bipolar montages can allow us to check the accuracy of the assumptions derived from the longitudinal bipolar montage. The transverse bipolar montage and the circular bipolar montage are two examples shown below.

Transverse Bipolar Montage

Circular (Circumferential) Bipolar Montage

Sometimes the phase reversals are in adjacent derivations. Below are three examples of the same data showing the source at O1.

Longitudinal Bipolar Montage Phase Reversal

Transverse Bipolar Montage

Circular Bipolar Montage Phase Reversal

The last comparison we will make shows the same recording in the average reference montage. The average reference montage uses an average of all scalp electrodes as the reference for each individual electrode. The ear or mastoid reference electrodes are excluded from this average. Graphic © learningeeg.com.

The average reference montage can identify focal activity, particularly from the temporal lobes, where ear references might contribute to or cancel the same activity. The results can be improved by excluding affected electrodes. You can identify them by starting with a bipolar montage if this is allowed by your software.

Sometimes the average reference montage contributes activity from one or more sources to all sensor locations, as seen on the right side of the recording shown below. This montage shows the broad distribution of alpha activity. In contrast, a bipolar montage for the same data shows the alpha activity as more localized. The circled section on the left shows the same data highlighted in the bipolar views.

Summary

Neurofeedback providers should select montages to answer specific diagnostic questions. This requires an understanding of underlying neurophysiology and the strengths and limitations of each montage with respect to source localization and vulnerability to artifacts.

Glossary

50/60 Hz: external artifacts transmitted by nearby electrical sources.

active electrode: an electrode placed over a site that is a known EEG generator like Cz.

amplitude: the energy or power contained within the EEG signal measured in microvolts or picowatts.

artifact: false signals like 50/60Hz noise produced by line current.

average reference montage: EEG recording configuration using the average of all scalp electrodes as the reference for each individual electrode. The ear or mastoid reference electrodes are excluded from this average.

bipolar transverse montage: EEG recording configuration chaining adjacent electrodes from left to right (Fp1 to Fp2, F7 to F8, A1 to A2, T5 to T6, and O1 to O2).

channel: an EEG amplifier output resulting from scalp electrical activity from three electrode/sensor connections to the scalp. circular (circumferential) bipolar montage: EEG recording configuration involving the counterclockwise chaining of electrodes around the head's circumference, starting at Fp1 and ending at Fp2.

common mode rejection: a differential amplifier's ability to suppress signals common to its + and - inputs.

derivation: assigning two electrodes to an amplifier's inputs 1 and 2.

differential amplifier (balanced amplifier): a device that boosts the difference between two inputs: the active (input 1) and reference (input 2). EEG topography: displaying the qEEG on a cortical surface map to show the spatial distribution of EEG activity.

electrode: a specialized conductor that converts biological signals like the EEG into currents of electrons.

exogenous artifacts: noncerebral electrical activity generated by movement, 50/60 Hz and field effect, bridging, and electrode (electrode “pop" and impedance) artifacts.

field: EEG signal weakening with increasing electrode distance from its source.

International 10-10 system: a modified combinatorial system for electrode placement that expands the 10-20 system to 75 electrode sites to increase EEG spatial resolution and improve the localization of electrical potentials.

International 10-20 system: a standardized procedure for placing 21 recording and 1 ground electrode on adults on adults to provide a total of 19 channels. This system is used for typical 19-channel qEEG recordings, using 19 "active" electrodes, "reference" electrodes at A1 and A2, and a ground electrode.

longitudinal bipolar montage or double banana: EEG recording configuration involving the anterior-to-posterior chaining of adjacent electrodes in two lines on each side (Fp1 to O1 and Fp2 to O2) and connecting the midline electrodes (Fz to Pz).

mastoid bone: the bony prominence behind the ear.

montage: EEG recording configuration that groups electrodes (combines derivations) to monitor EEG activity.

phase: the degree to which the peaks and valleys of two waveforms coincide.

phase reversal: reverse polarity observed in voltages from contiguous electrodes that can signal spike epileptogenic foci.

Quantitative EEG (qEEG): the statistical description and analysis of EEG features based on the digitization of analog EEG activity obtained using at least a 19-channel montage.

reference electrode: an electrode placed on the scalp, earlobe, or mastoid.

referential (monopolar) montage: EEG recording configuration with an active (+) electrode (A) on the scalp and a "neutral" reference (-) electrode (R) and ground (G) on the ear or mastoid.

sequential (bipolar) montage: EEG recording configuration using a sequence of comparisons of positive (+) and negative (-) electrodes (often called active and reference) that are attached to sites on the scalp. A sequential montage considers the reference electrode to be a second active electrode. The ground (G) electrode is attached to the scalp, to an earlobe, or over the mastoid process.

vertex (Cz): the intersection of imaginary lines drawn from the nasion to inion and between the two preauricular points in the International 10-10 and 10-20 systems.

References

Collura, T. F. (2014). Technical foundations of neurofeedback. Taylor & Francis.

Demos, J. N. (2019). Getting started with neurofeedback (2nd ed.). W. W. Norton & Company.

Libenson, M. H. (2010). Practical approach to electroencephalography. Saunders Elsevier.

Thomas, C. (2007). What is a montage? EEG instrumentation. American Society of Electroneurodiagnostic Technologists, Inc.

Thompson, M., & Thompson, L. (2015). The biofeedback book: An introduction to basic concepts in applied psychophysiology (2nd ed.). Association for Applied Psychophysiology and Biofeedback.