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

CHANNELS AND AMPLIFIERS

EEG is defined as “the difference in voltage recorded b/w two electrodes and plotted over time, when at least one of them is placed over the scalp (see definition of EEG).” One pair of electrodes usually make up a channel. Each horizontal tracing corresponds to an electrode pair placed on a particular area of the patient's scalp.






In usual EEG recordings, 16 channels are recorded in parallel - appearing as 16 traces (as shown in figure). However number can go up to 40 depending on the indication.

Each channel of EEG recording is produced by the output of one differential amplifier. This type of amplifier (also known as balanced amplifier), is constructed to amplify only the difference in the potential b/w its two inputs, identical voltages appearing at the two inputs are not amplified and create a flat line output.

This is one way of reducing contamination of the physiological signal by electrical noise, (for example, 60 Hz noise from line voltage devices) because this noise tends to be the same at all electrode positions and cancels out when a difference in potential is formed. On the other hand, physiologic signals are usually different at different electrode positions.

References:

1. Electricity and Electronics in Clinical Neurophysiology : Clinical Neurophysiology by Jasper R. Daube (F. A. DAVIS COMPANY)
2. Technological basis of EEG Recording : Electroencephalography - Basic Principles, Clinical Applications and Related Fields by Ernst Niedermeyer (WILLIAMS AND WILKINS)

10-20 SYSTEM OF ELECTRODE PLACEMENT

A committee of the International Federation of Societies for Electroencephalography and Clinical Neurophysiology recommended a specific system of electrode placement - now known as the international 10-20 system. Specific measurements from bony landmarks (inion, nasion and preauricular point) are used to generate a system of lines, which run across the head and intersect at intervals of 10 or 20% of their total length (hence called 10-20 system).

The standard set of electrodes for adults consists of 21 recording electrodes and one ground electrode. Measurements are made as follows:





The distance b/w nasion and inion is measured along the midline (Let us assume it is 40 cm). Along this line, the frontopolar point, Fpz (see electrode placement for description), is marked at 10% above the nasion (i.e. 4 cm above the nasion). Frontal (Fz), central (Cz), parietal (Pz) and occipital (Oz) points are marked at intervals of 20% of the entire distance (i.e. 8 cm, 16 cm, 24 cm and 32 cm respectively, above Fpz) leaving 10% for the interval b/w Oz and inion. The midline points Fpz and Oz routinely do not receive any electrode.




The distance b/w two pre-auricular points across Cz is measured. Along this line, the transverse position for the central points C3 and C4 and the temporal points T3 and T4 are marked 20 and 40% respectively from the midline.

The circumference of the head is measured form the occipital point (Oz) thru’ temporal points T3 and T4 and the frontopolar point (Fpz). The longitudinal measurement for Fp1 is located on that circumference, 5% of the total length of the circumference to the left of Fpz. The longitudinal measurements for F7, T3, T5, O1, O2, T6, T4, F8 and Fp2 are at the distance of 10% of the circumference

F3 electrode is placed at the point of intersection of two lines drawn by joining Fp1 to C3 and F7 to Fz. Similarly F4 is placed on the right side.
P3 electrode is placed at the point of intersection of two lines drawn by joining O1 to C3 and T5 to Pz. Similarly P4 electrode is placed on the right side.

Electrodes placed on or near both ears in positions are called A1 and A2 or on the mandibular angles in positions called M1 and M2.

NEUROLOGY MCQs

Reference:

1. Jasper, HH. Report of committee on methods of clinical exam in EEG. Electroencephalogr. Clin. Neurophysiol. 10:370-375
2. Jasper HH. The ten-twenty electrode system of the International Federation. In: Internal Federation of Societies for Electroencephalography and Clinical Neurophysiology: Recommendations for the practice of clinical electroencephalography Amsterdam: Elsevier, 1983:3-10
   

P.S. Figures not to scale.

NOMENCLATURE OF ELECTRODE POSITION

A committee of the International Federation of Societies for Electroencephalography and Clinical Neurophysiology recommended a specific system of electrode placement for use in all laboratories under standard conditions. According to their recommendations, the recording electrodes are named with a letter and a subscript.

The letter is an abbreviation of the underlying region: Fp (frontopolar), F (frontal), C (central), P (parietal), O (occipital) and A (auricular). The subscript is letter z, indicating zero or midline sagittal placement, or a number, indicating lateral placement. Odd numbers refer to electrodes on the left; even numbers refer to electrodes on the right side of the head. Therefore, F3 would be left frontal, Fz would be midline frontal and F4 would be right frontal position.

The inferior frontal electrodes F7 and F8 are often called ‘anterior temporal’ electrodes because they are placed closed to (and record activity from) the anterior temporal area.

References:

1. Jasper, H. Report of committee on methods of clinical exam in EEG. Electroencephalogr. Clin. Neurophysiol. 10:370-375


2. Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography. J. Clin. Neurophysiol. 11 (1) 2-5, Raven Press Ltd. New York