An Explanation of Common EEG Montage Configurations

An electroencephalogram, or EEG, is a test that records the brain’s electrical activity. To visualize this activity, clinicians use specific arrangements of electrodes called montages. Each montage represents a different way of connecting the electrodes on the scalp, providing a unique electrical picture of brain function. This allows technicians and doctors to view the brain’s activity from various perspectives to best analyze its patterns.

Mapping the Brain: The 10-20 Electrode System

To ensure EEG recordings are consistent and comparable from person to person, a standardized method for electrode placement is used, known as the International 10-20 system. The name “10-20” comes from the way the distances between electrodes are determined—they are placed at intervals of either 10% or 20% of the total front-to-back or right-to-left distance of the skull. This proportional system ensures that an electrode is in the same relative position on anyone’s head.

The process begins by measuring between key bony landmarks on the head, such as the nasion (the point at the top of the nose) and the inion (the small bump on the back of the head). Electrodes are then identified by a letter and a number.

The letter corresponds to the underlying brain region: F for frontal, P for parietal, T for temporal, and O for occipital. Numbers indicate the hemisphere, with odd numbers for the left side and even for the right, while “z” denotes the midline.

Bipolar vs. Referential: Two Main Montage Approaches

Once electrodes are placed, they are connected into montages, which fall into two main categories: bipolar and referential. A referential montage compares the electrical activity from an active scalp electrode to a single, common reference point. This reference is intended to be electrically quiet and can be an electrode on the earlobe or mastoid bone, or a calculated average of all electrode signals. This method is effective for assessing the true amplitude, or strength, of brainwaves and viewing widespread activity.

In contrast, a bipolar montage measures the voltage difference directly between two adjacent scalp electrodes. These pairs are often linked together in series to form chains that run across the scalp. Because a bipolar montage compares neighboring electrodes, it excels at pinpointing the location of focal brain activity. An abnormality often stands out as a “phase reversal,” where the signal deflects in opposite directions in adjacent channels, pointing to the source between those electrodes.

Common EEG Montage Configurations in Practice

Clinicians use several standard montage configurations. Among bipolar types, the longitudinal bipolar montage is common and often called the “double banana.” This setup consists of electrode chains running from the front to the back of the head, with one outer temporal chain and an inner parasagittal chain on each side. This arrangement is good for comparing symmetrical areas of the brain and identifying asymmetries between the left and right hemispheres. Another bipolar setup is the transverse montage, where electrode chains run from left to right across the head.

For referential montages, a common approach is the common average reference, where the reference point is the mathematical average of all recording electrodes. Another method is a linked-ears reference, where electrodes on both earlobes (A1 and A2) are electrically connected to serve as the reference. Referential montages are preferred for displaying the true voltage of widespread brain patterns, such as those seen during sleep or certain types of seizures.

Why Different Montages Matter for EEG Interpretation

A single montage may not reveal all necessary information, and an abnormality that is unclear in one view may become obvious in another. Using multiple montages allows neurophysiologists to look at the same brain activity from different electrical perspectives, which helps to accurately localize the source of an abnormal signal. For instance, a focal discharge might be hard to see in a referential montage but will stand out as a clear phase reversal in a bipolar chain.

Viewing the data through different montages helps confirm that a suspected abnormality is genuine brain activity and not an artifact caused by muscle movement or electrical interference. By cycling through several standard montages during analysis, an interpreter can build a more complete and accurate picture of brain function.