EEG montages combine derivations, which are pairs of electrodes. We will explore differential amplifiers and common mode rejection to understand better how montages compare EEG activity. 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. A montage, also known as an array, combines derivations to record EEG activity (Thomas, 2007). Below is a referential monopolar montage that places one active electrode (A) on the scalp and a "neutral" reference (R) and ground (G) on the ear or mastoid.
All montages compare EEG activity between one or more pairs of electrode sites.
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Overview
Every neurofeedback clinician needs to understand the benefits and limitations of the EEG sensor comparisons comprising EEG montages. To understand the how and why of montage development and use, we must first understand the differential amplifier and its use of common mode rejection.
Differential Amplifiers
A differential amplifier is an electronic amplifier that boosts the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and one output in which the output is ideally proportional to the difference between the two voltages:
Vout = A(V+in – V-in), where A is the gain of the amplifier.
In plain English, the voltage out equals the voltage from the active (+) electrode minus the voltage of the reference (-) electrode times the gain of the amplifier – how much the amplifier multiplies the voltage. Differential amplifier graphic © Hand Robot/Shutterstock.com.
We work with two separate signals. The electrodes' active (+) and reference (-) designations are assigned within the amplifier. It is not due to sensor construction or location.
This terminology derives from early analog amplifiers like the Grass Model 6 below. Clinicians plugged an electrode into a + or - port (input 1 or 2) or assigned a + or - value using a mechanical switch.
Modern digital amplifiers often use a recording reference as the negative electrode, which they compare to each electrode.
In the next section, we will try to reduce the confusion that arises from incorrect labeling and terminology. We encourage you to read Collura's (2014) Technical Foundations of Neurofeedback and Libenson's (2009) Practical Approach to Electroencephalography for more in-depth coverage.
Positive, Negative, and Ground Inputs
Each sensor input, whether labeled active (+) or reference (-), is simply an input to the differential amplifier. It compares these two inputs to the system reference, usually the ground.
In the graphic below, the active (+) is red, the reference (-) is black, and the ground electrode (Gnd/Ref) is white. Color coding varies across manufacturers.
The voltages of the active and reference inputs are based on the ground.
The ground/system reference is the return pathway back to the amplifier. It is not an earth ground, an electrical and physical connection to the earth. The graphic shows cables connected to a copper grounding bar driven into the earth © rachenstocker/Shutterstock.com.
Sensor placement for the ground/reference depends somewhat on the number of electrode sites. An ear ground/reference is often used for one or two scalp sensors. In contrast, a system ground/reference placed on the scalp, often at FCz between the Fz and Cz electrodes, is typically used for multi-channel recordings.
Therefore, each comparison between a pair of sites where the electrodes are placed requires three electrodes, a positive, a negative, and a ground/reference. The ground/reference is the same for all pairs of positive and negative electrode comparisons.
A channel is the collection of three electrodes, the electronics that compare them, and the resulting output.
A Differential Amplifier in Action
The differential amplifier calculates the difference between the voltages from the ground/reference and the positive electrode and between the ground/reference and negative electrode voltages:
voltage difference 1: ground/reference - positive electrode voltage difference 2: ground/reference - negative electrode
Next, it subtracts the two voltage differences and multiplies this value by a large number, such as 100,000. It multiplies the remainder by a small number, such as 1. The result is the common mode rejection ratio (CMRR) of 100,000 to 1 or 100 dB on a logarithmic scale.
Why is this done? In the early days of amplifiers, there was a single input and return pathway back to the amplifier. This was known as a single-ended amplifier. Such amplifiers are strongly affected by electromagnetic forces (EMF), particularly the EMF signal associated with alternating currents (AC).
Electrical currents represent alternating currents changing directions at a set frequency. In North and South America, AC changes polarity 60 times every second (60 Hertz). Elsewhere, AC reverses 50 times per second (50 Hertz). Wherever you travel, electrical waveform oscillates 50 or 60 times per second. Power station graphic © silvaborn/Shutterstock.com.
This is why AC artifacts are called 50 or 60 Hz, electrical mains, or simply mains artifacts. It is easily seen in most EEG amplifiers unless all factors are in the correct balance, and it becomes the primary signal in single-ended amplifiers. The 60-Hz artifact graphic below © John S. Anderson. Note the closely packed waves in the line tracings that show the fast 60-Hz signal. Such signals are sometimes called "fuzzy" or "noisy." Also, note the high amplitude peak in the spectral display on the right end of each tracing.
A BioGraph ® Infiniti display of a 60-Hz artifact is shown below in red. Note the cyclical voltage fluctuations and 60-Hz peak in the power spectral display.
Electrical engineers primarily developed differential amplifiers to control mains artifacts. The assumption (mostly correct) is that each input described above will show a substantial electrical artifact. When one is subtracted from the other, anything that is the same (common) in the two signals will be eliminated (multiplied by 1), and what is left will be amplified (multiplied by 100,000).
Because the oscillating mains artifact is pervasive in both signals, the signal resulting from common mode rejection (CMR) should have only minimal amounts of this artifact remaining, and therefore, the "real" EEG activity will be visible.
In the recording below, note the red-circled similar signals from the Fp1-LE and Fp1-O2 leads and from the O2-LE and Fp1-O2 leads that represent signals that have been added to the Fp1-O2 tracing due to reference contamination. The individual signals are present in Fp1-LE and O2-LE only. Since they do not occur in each other's tracing, anything that is different is retained when they are compared to each other. Rather than being subtracted during common mode rejection, the differences now appear in the Fp1-O2 derivation. LE stands for the linked ear. LE-LE (at the bottom) shows no voltage due to the complete subtraction of identical voltages from the exact anatomical location.
Although mains artifact is CMR's primary target, other waveforms might appear identical. For example, the ECG (cardiac electrical activity), EMG (muscle action potentials), or common EEG signals seen at multiple locations.
The frequency range for ECG artifact is 0.05-80 Hz, which contaminates the delta through beta bands. Since multiple electrodes detect this artifact, it can create the appearance of greater coherence than is present. Graphic © eegatlas-online.com.
Some waveforms represent real EEG. The differential amplifier retains them when they have different phase angles and/or voltages measured at their locations.
However, the differential amplifier often subtracts highly rhythmic alpha signals in active (+) and reference (-) electrodes because they are in phase and/or their voltages look very similar.
Brain activity is more similar when electrodes are close together, causing the rejection of real EEG in adjacent electrodes. EEG voltages are less similar when electrodes are farther apart. Compare Fp1-Fp2 with Fp1-O2 below.
Common mode rejection is sensitive to phase (the degree to which the peaks and valleys of two waveforms coincide) and amplitude (signal power). Phase is the similarity in timing of the waves at two locations. When two signals are 180 degrees out of phase, the top signal peaks when the bottom signal reaches its trough. Phase graphic © petrroudny43/Shutterstock.com.
A derivation using these two signals would result in both signals being retained (added together). A derivation from in-phase signals would result in both being rejected and hence a flat line.
Amplitude is the signal voltage or power measured in microvolts or picowatts. Amplitude graphic © petrroudny43/Shutterstock.com.
If the two signals oscillate simultaneously (in phase) and with similar amplitudes, the alpha activity will be removed from the displayed active electrode tracing.
This is also true when a signal is present in the reference (-) but not in the active (+) electrode. Then, the resulting signal on the computer screen, supposedly coming from the active (+) electrode, will include the difference signal contributed by the reference (-) electrode. This reference contamination often occurs in the linked-ear or linked-mastoid montage but is also an integral component of the various bipolar montages, such as the longitudinal bipolar montage. This feature can help us identify locally occurring transients, epileptiform activity, or other isolated waveforms. This will be covered more fully in the next installment.
Because there is often substantial alpha activity in temporal and lateral parietal electrodes T7, T8, P7, and P8, which are close to the ears and mastoids, such alpha activity is often present in the reference (-) electrode signal. The International 10-10 system is pictured below.
We can try to eliminate this by adding and subtracting the individual ear or mastoid signals and then using the resulting signal for the negative channel. However, this isn't always successful. Any electrodes, such as frontal sensors that do not have substantial alpha activity, will show the alpha activity from the reference (-) channel.
Conclusion
To understand the how and why of montage development and use, we must first understand the differential amplifier and its use of common mode rejection. Differential amplifiers help to control powerful artifacts, like ECG and mains artifacts, that can contaminate the EEG signal.
Various montages help identify EEG signal sources. All montages have benefits and limitations. There is no free lunch in neurofeedback. The next neurofeedback blog will explore your montage options.
Glossary
alpha rhythm: 8-12-Hz activity that depends on the interaction between rhythmic burst firing by a subset of thalamocortical (TC) neurons linked by gap junctions and rhythmic inhibition by widely distributed reticular nucleus neurons. Researchers have correlated the alpha rhythm with "relaxed wakefulness." Alpha is the dominant rhythm in adults and is located posteriorly. The alpha rhythm may be divided into alpha 1 (8-10 Hz) and alpha 2 (10-12 Hz). alternating current (AC): an electric current that periodically reverses its direction.
amplitude: the strength of the EEG signal measured in microvolts or picowatts.
artifact: false signals like 50/60Hz noise produced by line current. channel: the collection of three electrodes, the electronics that compare them, and the resulting output.
common-mode rejection ratio (CMRR): the degree by which a differential amplifier boosts signal (differential gain) and artifact (common-mode gain).
derivation: assigning two electrodes to an amplifier's inputs 1 and 2. Montages combine derivations.
differential amplifier (balanced amplifier): a device that boosts the difference between two inputs: the active (input 1) and reference (input 2).
ear ground/reference: used for one or two scalp sensors. earth ground: an electrical and physical connection to the earth.
EEG artifacts: noncerebral electrical activity in an EEG recording can be divided into physiological and exogenous artifacts. electrocardiogram (ECG) artifact: contamination of the EEG signal by cardiac electrical activity.
electromagnetic force: the physical interaction between electrically charged particles.
frequency (Hz): the number of complete cycles that an AC signal completes in a second, usually expressed in hertz.
gain: an amplifier's ability to increase the magnitude of an input signal to create a higher output voltage; the ratio of output/input voltages.
ground electrode: a sensor placed on an earlobe, mastoid bone, or scalp that is grounded to the amplifier.
ground/system reference: an electrode that provides a return pathway back to the amplifier.
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.
linked-ear (LinkEar) montage: EEG recording configuration that compares individual electrode potentials to voltages detected at two linked earlobe references (-). This montage is vulnerable to reference contamination.
linked-mastoid montage: EEG recording configuration that compares individual electrode potentials to voltages detected at two linked mastoid references (-). This montage is vulnerable to reference contamination.
mains (50/60Hz) artifact: contamination of the EEG signal by 50/60Hz activity.
mastoid bone (or process): bony prominence behind the ear.
microvolt (μV): the unit of amplitude (signal strength) that is one-millionth of a volt.
montage: EEG recording configuration that groups electrodes (combines derivations) to monitor EEG activity.
negative electrode: reference/ground electrode.
phase: the degree to which the peaks and valleys of two waveforms coincide.
picowatt: billionths of a watt.
positive electrode: active electrode.
power (W): the rate at which energy is transferred, which is proportional to the product of current and voltage. Power is measured in watts.
reference contamination: the difference signal from the reference (-) electrode appears in the active (+) electrode voltage. This often occurs in the linked-ear or linked-mastoid montage.
reference electrode: the electrode placed over a less-electrically active site like the mastoid bone behind the ear. synchrony: the coordinated firing of pools of neurons due to pacemakers and mutual coordination. system ground/reference: an electrode placed on the scalp, often at FCz between the Fz and Cz electrodes, is typically used for multi-channel recordings.
volt (V): unit of electrical potential difference (electromotive force) that moves electrons in a circuit.
voltage (E): the amount of electrical potential difference (electromotive force).
watt (W): a power unit that expresses signal strength in the qEEG.
References
Andreassi, J. L. (2000). Psychophysiology: Human behavior and physiological response. Lawrence Erlbaum and Associates, Inc.
Collura, T. F. (2014). Technical foundations of neurofeedback. Taylor & Francis. Fisch, B. J. (1999). Fisch and Spehlmann's EEG primer (3rd ed.). Elsevier.
Floyd, T. L. (1987). Electronics fundamentals: Circuits, devices, and applications. Merrill Publishing Company. Kubala, T. (2009). Electricity 1: Devices, circuits, and materials (9th ed.). Cengage Learning.
Laplante, P. A. (2005). Comprehensive Dictionary of Electrical Engineering (2nd ed.). CRC Press. Libenson, M. H. (2009). Practical approach to electroencephalography. Saunders Elsevier. Nilsson, J. W., & Riedel, S. (2018). Electric circuits (11th ed.). Pearson Education (US).
Thatcher, R. W. (2012). Coherence, phase differences, phase shift, and phase lock in EEG/ERP analyses. Developmental Neuropsychology, 37(6), 476-496. https://doi.org/10.1080/87565641.2011.619241 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.
Wadman, W. J., & Lopes da Silva, F. H. (2011). In D. L. Schomer & F. H. Lopes da Silva (Eds.). Niedermeyer's electroencephalography: Basic principles, clinical applications, and related fields (6th ed.). Lippincott Williams & Wilkins.
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