DERIVATIONS AND MONTAGES

The selection of electrodes for inputs 1 and 2 for any one amplifier channel (see CHANNELS AND AMPLIFIERS) is referred to as the derivations. The combination of multiple derivations is referred to as a montage. Different montages provide different views of the electrical activity in brain.

Usually, two kinds of montages are used: common electrode reference (or referential) montage and bipolar montage.

COMMON ELECTRODE REFERENCE (REFERENTIAL) MONTAGE
The common electrode reference montage consists of a series of derivations in which the same electrode is used in input 2 of each amplifier. In other words, electrodes in their various placements over the scalp are all referred to one single electrode, called the reference electrode, and montage is known as the referential montage.





The reference electrodes may be cephalic or non-cephalic. Usually the reference electrodes are placed over left and right ear lobules. All ipsilateral scalp electrodes are referred to these reference electrodes.


Referential montage is useful for assessing inter-hemispheric amplitude asymmetries.

BIPOLAR MONTAGE
Here, the potential difference is measured b/w pairs of neighboring electrodes going serially in an anterior-posterior or transverse plane.Thus, they are longitudinal bipolar or transverse bipolar montages.

Bipolar montages are best suited for identification of highly localized cerebral activity.



References:

1. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
2. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Williams and Wilkins, 4th edition