EEG Montages: Part 2 - Selecting the Best Comparison
Updated: Mar 24
A montage groups electrodes together (combines derivations) to record EEG activity (Thomas, 2007).
Clinicians monitor EEG activity using the classical International 10-20 System for standardized electrode placement or the modified 10-10 system known as the Modified Combinatorial Nomenclature System. They often record from several sites and measure the amplitude (signal power) of EEG signals within frequency bands (like alpha and theta) to provide a complete picture of brain activity.
Software-based montage reformatting allows clinicians to reanalyze session data by referencing an electrode to other sites or combinations of sites. This system also allows for the computation of multiple variables associated with communication and network functions within the central nervous system (CNS).
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The Quantitative EEG
The quantitative EEG (qEEG) measures EEG amplitudes within selected frequency bands. A full-cap 21-channel EEG recording (19 scalp sensors plus two reference sensors) and resulting qEEG analysis may be valuable in designing treatment protocols for complicated cases like Asperger's or traumatic brain injury. EEG topography displays the qEEG on a cortical surface map to show the spatial distribution of EEG activity.
The movie below is a 19-channel BioTrace+ /NeXus-32 display of theta activity © John S. Anderson. Brighter colors represent higher theta amplitudes. Frequency histograms are displayed for each channel.
EEG contamination by physiological and exogenous artifacts requires that clinicians take extensive precautions, examine the raw EEG record, and remove contaminated epochs through artifacting. Impedance tests and behavioral tests help ensure the fidelity of EEG recording.
Finally, clinicians interpret EEG recordings with an understanding of normal values and recognition of the effects of eye closure, age, diurnal influences, alertness and drowsiness, medication, and relaxation on these readings. Graphic © Medical-R/Shutterstock.com.
International 10-20 and 10-10 Systems
The International 10-20 system is a standardized procedure for electrode placement on 19 scalp and reference and ground sites. Electrodes measure electrical activity from a surrounding area the size of an American quarter. The site recorded may be distant from the EEG generator due to neural pathways.
The 10-20 system assigns recording electrodes a letter and subscript. The letters represent the underlying region and include Fp (frontopolar or prefrontal), F (frontal), C (central), P (parietal), O (occipital), and A (auricular). A subscript of z represents a midline (central axis from nasion to inion) placement.
Numerical subscripts range from 1-8 and increase with distance from the midline. The 10-20 system assigns odd-numbered recording electrodes on the left and even-numbered electrodes on the right side of the head. Two reference electrodes are usually placed on the earlobe.
The 10-10 System's Modified Combinatorial Nomenclature
The American Clinical Neurophysiology Society published guidelines for expanding the 10-20 system to 75 electrode sites. While more complex, this system also allows us to define the placement sites for our electrodes precisely.
The expansion of the 10-20 system allows clinicians to define the sites midway between two 10-20 sites commonly used in clinical practice, better localize epileptiform activity, increase EEG spatial resolution, and improve detection of localized evoked potentials. The modified combinatorial system replaces inconsistent designations (T3/T4 and T5/T6) with consistent ones (T7/T8 and P7/P8). Black circles depict these replacement sites with white lettering in the diagram below.