Referential is a broad category of montage, in which each electrode’s signal is compared to a separate reference point. Common types include 1) average, where each electrode’s voltage is compared to the average voltage across all electrodes, or 2) auricular, in which each electrode’s voltage is compared to the relatively electro-silent earlobe. In referential montages, each channel focuses on the output from only a single electrode (as compared to bipolar where each channel has a pair of electrodes being compared to each other).

Bipolar is a broad category of montage; in which each electrode’s signal is compared to the signal of an adjacent electrode (and thus each channel’s information is a combination of two electrodes), forming chains of electrode pairings. The most common is the anterior-to-posterior bipolar montage, commonly called the double banana (because it makes the shape of two bananas on the head), in which you have an outer temporal chain and an inner parasagittal chain on either side. Other common types of bipolar montages include 1) transverse, which makes chains across the head to better compare side to side activity, and 2) circumferential, which makes a circular chain around the edge of the head to avoid the end of chain effect for occipital and frontopolar discharges.

A montage describes how the electrodes are all compared and connected to one another by the computer, and determines how each electrode’s signal appears on the overall EEG tracing. There are two main types: bipolar and referential. In bipolar montages, each electrode’s signal is compared to the signal of an adjacent electrode (and thus each channel’s information is a combination of two electrodes).

In referential montages, each electrode’s signal is compared to a separate reference point (commonly the earlobe, which is not very electrically active, or the average of all the electrodes’ signals combined), and thus each channel focuses on the output from only a single electrode. With modern digital EEG, montages can be freely changed even after recording is done.

A channel on EEG is a single line of the tracing. The contents of a channel can vary based on how the electrodes are compared via the montage. In bipolar montages, each channel is comparing two adjacent electrodes to one another, such as Fp1-F7. In referential montages, each channel is comparing one electrode to a separate reference point, such as the average of the total electrode potential or the electrically silent earlobe, such as Fp1-Avg.

Conductance describes how easily current flows through a circuit (it’s the opposite of electrical resistance). Regarding EEG, conductance is important because higher conductance makes the signal easier to detect; to this end, we place conductance paste containing sodium chloride between the skull and the electrodes, to facilitate the detection of voltage changes on the brain by the electrode.

An electrode is the disc-shaped metal item placed on the patient’s head to collect the raw electrical signal from the brain; most commonly, we use silver or silver chloride for the material, less often stainless steel. A conductive paste is used to optimize the detection of electricity on the scalp in the electrode, and the electrode is attached to a wire that runs to the EEG computer.