EEG Electrode Placement and Montages 2 — Questions and Answers
Question 1: In the International 10-20 system, what do the numbers 10 and 20 represent?
- The number of electrodes used
- The percentage of distance between skull landmarks for electrode placement (Correct answer)
- The impedance levels required in kilohms
- The amplifier gain settings
Correct answer: The percentage of distance between skull landmarks for electrode placement
The 10-20 system places electrodes at 10% and 20% intervals of the measured distances between standardized skull landmarks (nasion, inion, preauricular points).
The International 10-20 system, developed by Jasper in 1958, uses four skull landmarks: nasion, inion, and left and right preauricular points. Distances between these landmarks are measured, and electrodes are placed at 10% and 20% intervals along these measured distances. For example, Fp1 is at 10% of the nasion-to-inion distance above the nasion. The outer ring of electrodes (Fp, O positions) sits at 10% intervals, while the inner electrodes are at 20% intervals. This proportional system ensures consistent electrode placement regardless of head size, making recordings comparable across individuals and sessions.
Question 2: Which montage type displays the voltage difference between each electrode and a common reference point?
- Bipolar longitudinal montage
- Bipolar transverse montage
- Referential montage (Correct answer)
- Laplacian montage
Correct answer: Referential montage
A referential montage shows the potential at each electrode compared to a single common reference (such as linked ears, Cz, or an average reference), allowing direct comparison of amplitudes across channels.
Referential montages pair each scalp electrode with a common reference electrode. Common references include linked mastoids (A1+A2), Cz, or an average reference (computed average of all electrodes). The advantage is that true amplitude at each electrode can be directly compared across channels. The disadvantage is that if the reference electrode is contaminated by artifact or active signal, it affects every channel. Bipolar montages show the difference between adjacent electrode pairs and are better for localizing phase reversals. Choosing between montages is essential for accurate interpretation — most technologists record with a referential setup and reformat digitally.
Question 3: In a bipolar longitudinal (anterior-posterior) montage, a phase reversal at C3 indicates:
- An artifact at the C3 electrode
- Maximum negativity or positivity at C3 (Correct answer)
- The C3 electrode has high impedance
- Activity originating from the contralateral hemisphere
Correct answer: Maximum negativity or positivity at C3
A phase reversal occurs where adjacent channels sharing an electrode deflect in opposite directions, indicating the electrode at the reversal point has the maximum voltage for that event.
In bipolar montages, a phase reversal is the fundamental localization tool. When a negative potential is maximal at C3, the channel with C3 as input 2 (e.g., F3-C3) deflects downward while the channel with C3 as input 1 (e.g., C3-P3) deflects upward, creating opposite deflections that 'reverse' at C3. This indicates C3 is the point of maximum voltage. A negative phase reversal (pen deflections pointing toward each other) indicates maximum negativity, while positive phase reversal (pens diverging) indicates maximum positivity. Understanding phase reversal is essential for localizing epileptiform discharges and focal slowing.
Question 4: The electrode labeled 'Fz' in the 10-20 system is located at:
- The left frontal region
- The midline frontal position (Correct answer)
- The right frontopolar region
- The left frontotemporal junction
Correct answer: The midline frontal position
The 'z' designation indicates a midline position. Fz (frontal zero) is the midline frontal electrode positioned at 30% of the nasion-to-inion distance from the nasion.
In the 10-20 system naming convention, the first letter indicates the brain region (F=frontal, C=central, P=parietal, O=occipital, T=temporal), and the number or letter 'z' indicates laterality. 'z' (zero) denotes the midline. Odd numbers are left hemisphere, even numbers are right hemisphere. Fz sits at 30% of the nasion-to-inion distance from the nasion, on the midline. The midline electrodes (Fz, Cz, Pz, Oz) are crucial for identifying vertex sharp waves, generalized discharges, and serving as reference points. Fz is often used for sleep staging as vertex waves are well-represented there.
Question 5: What is the maximum recommended electrode impedance for routine EEG recording according to ACNS guidelines?
- 1 kOhm
- 5 kOhm (Correct answer)
- 10 kOhm
- 20 kOhm
Correct answer: 5 kOhm
The American Clinical Neurophysiology Society recommends electrode impedances be below 5 kOhm and balanced across all electrodes to optimize signal quality and common-mode rejection.
The ACNS recommends keeping electrode impedances below 5 kOhm for routine EEG. Low, balanced impedances are critical because: (1) they reduce thermal noise that degrades signal quality, (2) they maximize the differential amplifier's common-mode rejection ratio (CMRR) for rejecting 60 Hz and other environmental noise, and (3) impedance imbalances between electrodes create differential noise that cannot be filtered. Skin preparation with mild abrasion and proper electrode gel application are essential. Impedances should be checked at the beginning and periodically during the recording. Modern digital systems are more tolerant of slightly higher impedances, but the 5 kOhm standard remains the benchmark.
Question 6: An average reference montage calculates each channel's display by:
- Subtracting the voltage of the nearest neighbor electrode
- Subtracting the computed average of all electrode voltages from each electrode (Correct answer)
- Comparing left hemisphere electrodes to right hemisphere
- Using a weighted sum based on electrode distance
Correct answer: Subtracting the computed average of all electrode voltages from each electrode
The average reference uses the arithmetic mean of all active electrodes as the reference point. Each channel displays the difference between one electrode and this common average, providing a relatively neutral reference.
The average reference montage computes the mean voltage across all scalp electrodes at each time point and subtracts this average from each individual electrode's recording. This creates a theoretically neutral reference because, by Kirchhoff's law, the sum of all potentials on a closed surface approaches zero. In practice, the limited number of electrodes (21 in standard 10-20) means the average is an approximation. A focal high-amplitude event can contaminate the average, creating artificial activity across all channels. Excluding the electrode with the maximum signal or using a weighted average can mitigate this. Average reference is valuable for amplitude comparison and dipole analysis.
EEG Electrode Placement and Montages 2
In the International 10-20 system, what do the numbers 10 and 20 represent?