EEG Montages: Part 1 - Differential Amplifiers

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. The graphic below from Kwak et al. (2017) shows eight EEG channels (PO7, PO3, PO, PO4, PO8, O1, Oz, and O2) with a reference (FCz) and ground (Fpz).