Electroencephalography (EEG) is a non-invasive method used to record the electrical activity generated by the brain. It involves placing small electrodes on the scalp to capture the tiny voltage fluctuations created by the synchronized firing of millions of neurons beneath the skull. These raw electrical signals are the foundation of an EEG study. The true utility of the EEG comes from how these signals are organized and presented for interpretation, which is the function of an EEG montage.
What an EEG Montage Represents
A montage is a structured map or configuration that dictates how the differential amplifiers of the EEG machine are connected to the scalp electrodes. Since an amplifier measures the voltage difference between two input sites, a montage defines which pair of electrodes will create a single line of tracing, known as a channel or derivation. The final EEG display is a collection of these channels, arranged in a specific, standardized order to cover the entire scalp.
This organization is dependent on the International 10-20 System, which standardizes electrode placement based on proportional measurements of the head. The 10-20 system ensures that the location of electrodes like Fp1, C3, and O1 is consistent across patients despite variations in head size and shape. The selected montage determines which specific signal differences from these standardized locations will be displayed simultaneously for the clinician to analyze.
Understanding Referential Recordings
Referential montages operate by measuring the electrical potential difference between an active electrode and a designated reference point that is intended to be electrically silent. In this configuration, every active recording electrode, such as F3 or T5, is paired with a single, common reference electrode. Common reference sites include electrodes placed over the earlobes (A1 or A2), the vertex (Cz), or the mastoid bones.
The goal of a referential recording is to provide a broad view of the activity occurring beneath each active electrode, comparing it against a neutral baseline. A common variant is the average reference montage, where the signal from all active electrodes is mathematically averaged and used as the common reference point. This approach is effective for assessing the overall amplitude of brain waves and observing the general distribution of activity. However, if the designated reference electrode is not truly inactive, that unwanted activity is introduced into every single channel, potentially distorting the tracing.
Understanding Bipolar Recordings
Bipolar montages, often called sequential montages, measure the voltage difference between two adjacent, active electrodes. Instead of comparing an active site to a neutral reference, these montages create a chain of channels, where the second electrode of one pair becomes the first electrode of the next pair (e.g., Fp1-F3, F3-C3, C3-P3). This arrangement is particularly useful for localizing focal abnormalities, such as the sharp waves or spikes associated with epilepsy.
The advantage of a bipolar montage is its ability to demonstrate a phenomenon called phase reversal. A phase reversal occurs when a localized electrical event is located directly beneath the common electrode shared by two adjacent channels. When this happens, the shared electrode’s large potential causes the tracing in the channel before it to deflect in one direction, while the tracing in the channel after it deflects in the opposite direction. This opposing deflection pinpoints the exact location of the abnormal activity to the single electrode shared by the two channels.
How Montage Selection Influences Diagnosis
The choice of montage is a deliberate decision made by the technician and the interpreting clinician, as each configuration highlights different aspects of the underlying brain activity. A referential montage excels at revealing generalized or diffuse abnormalities, such as widespread slowing of brain rhythms or reduced amplitude. Because the signal is referenced to a common point, it is easier to gauge the absolute voltage of the activity across different brain regions, which is useful for evaluating the overall background state of the brain.
Conversely, a bipolar montage is necessary for localizing focal pathology, often associated with seizure disorders. The bipolar chain’s ability to clearly show a phase reversal allows the clinician to precisely identify the anatomical source of a discharge or spike. Clinicians frequently switch between viewing both bipolar and referential displays of the same data, utilizing the strengths of each to build a comprehensive picture of both generalized brain function and the exact location of any localized electrical disturbances.