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LESSON 126

Electrode placement and the 10-20

We explain where electrodes are placed for auditory brainstem response (ABR) recording according to the 10-20 system, the roles of the active, reference and ground electrodes, and the common montage options. We also cover skin preparation, the goal of low and balanced impedance below 5 kΩ, and electrode placement in infants.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn where and how electrodes are placed to record the auditory brainstem response, or ABR.
Step by step, we will cover the 10-20 system, the roles of the active, reference and ground electrodes, and impedance checks.
The auditory nerve and brainstem that generate the ABR lie several centimeters deep to the electrodes.
So the ABR is a far-field recording; by the time it reaches the scalp, the response is usually below one microvolt.
At the same time, the brain's ongoing electrical activity, called electroencephalography or EEG, also enters the recording.
The averaging we saw in the previous lesson separates this small response from EEG and muscle activity.
But for averaging to work, the signal must first be picked up from the right points and cleanly.
Electrode sites determine which waves stand out and how noisy the recording will be.
The electrodes pick up small voltage changes on the scalp.
The amplifier boosts the difference between two electrodes about one hundred thousand times.
The filter removes very slow and very fast changes that the ABR does not need.
Then the short recording windows after each stimulus are summed and averaged.
Because the electrode is the first link in this chain, an error here cannot be fully corrected at any later step.
Electrode sites are standardized by the international 10-20 system, described by Jasper in 1958.
Measurement starts from the distance between the nasion at the root of the nose and the inion, the bump at the back of the head.
This distance is divided into ten percent and twenty percent segments; the system is named after these ratios.
Fpz lies ten percent above the nasion, and Fz lies a further twenty percent higher.
The exact midpoint between nasion and inion, where it crosses the line joining the two preauricular points, is Cz, the vertex.
Because ratios are used, the system gives matching points on a small infant head and on an adult head alike.
In clinical ABR, a full tape measurement is not always done, but knowing these points well is the basis of consistent recording.
Letters show the region: F stands for frontal, the forehead region, and C for central, the region at the top of the head.
The small letter z means zero and tells us the point lies exactly on the midline.
Odd numbers indicate the left side of the head, and even numbers the right side.
The letter M stands for mastoid; M1 is the left mastoid and M2 the right mastoid.
The earlobe is marked with the letter A; some centers use the A1 and A2 points instead of the mastoids.
To avoid mixing up right and left, just remember: odd numbers are left, even numbers are right.
In every ABR channel, three electrodes work together.
The active electrode, the noninverting or positive input, is usually placed at Cz or Fz.
The reference electrode, the inverting or negative input, is usually on the mastoid or earlobe on the stimulated side.
The ground electrode does not measure; it is the common point of the amplifier and usually sits on the forehead, at Fpz.
Some devices and sources name these three inputs differently; what matters is knowing which input goes where.
Although it is called the reference, the mastoid electrode is very close to the part of the auditory nerve near the cochlea.
This electrode picks up the nerve activity that forms wave I up close and with opposite polarity.
Because the amplifier takes the difference between the two electrodes, this inverted contribution makes wave I look larger.
In other words, the classic ABR waveform arises from the difference between the vertex and the mastoid.
If the reference electrode is connected to the wrong side, wave I shrinks and the recording may be misinterpreted.
So when the right ear is tested, the reference should be on the right mastoid, and for the left ear on the left mastoid.
The ABR amplifier is differential; that is, it amplifies the voltage difference between two electrodes.
Signals that reach both electrodes equally, such as interference from mains power, largely cancel out in this subtraction.
This property is called common mode rejection, and it makes ABR possible even in electrically noisy rooms.
However, common mode rejection works well only when the electrode impedances are close to each other.
If one electrode has a much higher impedance than the others, interference does not reach both inputs equally and leaks in.
The choice of points between which the active and reference electrodes are connected is called the montage.
In the most common, ipsilateral montage, the channel runs between Cz and the mastoid on the stimulated side.
In the contralateral montage, Cz is paired with the mastoid on the nonstimulated side.
A horizontal montage between the two mastoids can make wave I even more prominent.
In infants, the active electrode is often placed at Fz or the high forehead instead of Cz; this area is easier to prepare.
Many clinics record both an ipsilateral and a contralateral channel at the same time.
In the ipsilateral channel, wave I is prominent, because the reference electrode is close to the stimulated cochlea.
In the contralateral channel, wave I is usually very small or not seen at all.
On the other hand, waves IV and V often separate better in the contralateral channel, which helps in marking wave V.
The difference between channels also helps show which cochlea responds in bone-conduction ABR, covered in a later lesson.
Electrode impedance can be thought of as resistance to current flow between the skin and the electrode.
Many sources recommend that each electrode's impedance be below 5 kΩ.
In this example measurement, Cz, M1 and Fpz appear below this limit.
M2, however, is above the limit at about 8 kΩ; this electrode should be prepared again.
Values close to each other also matter; the difference between electrodes should generally not exceed about 2 kΩ.
Impedance is measured before recording and checked again if noise increases during the recording.
First the electrode sites are located; on the scalp, the hair is carefully parted.
The skin is gently rubbed with an abrasive gel to remove oil and dead cells.
Then a disposable electrode, or an electrode coated with conductive paste, is applied.
Impedance is measured; an electrode that reads high is removed and the same site is prepared again.
Finally, the electrodes are secured with tape and the cables are arranged so they do not pull.
A clinical tip: braid the electrode leads together instead of leaving them loose.
Braided leads make electrical interference in the room reach both inputs in a similar way and strengthen common mode rejection.
Keep the earphone or bone vibrator cable as far as possible from the electrode leads.
If only one electrode has high impedance, do not remove all the electrodes; preparing that site again is enough.
In infants, the active electrode is often placed at Fz or the high forehead.
Pressing on the soft spot, the anterior fontanelle, is avoided.
Because infant skin is delicate, abrasion is very light, and small electrodes suitable for infants are used.
The electrodes are applied before the baby falls asleep, while the baby is in the parent's lap.
This way, the long, quiet recording time needed for threshold estimation is not lost to electrode problems.
If conductive gel spreads between two electrodes, they become bridged and the recording comes out almost flat.
Connecting the reference electrode to the wrong side can make wave I disappear and lead to misinterpretation.
A loose electrode or a broken lead causes sudden jumps and high noise in the recording.
If the earphone cable is too close to the electrode leads, stimulus artifact increases.
If a recording looks unexpected, check the electrodes and cables before moving on to interpretation.
The 10-20 system standardizes electrode points on the head using ten and twenty percent ratios.
The active electrode is usually at Cz or Fz, the reference on the stimulated-side mastoid, and the ground on the forehead.
Impedance should be below 5 kΩ at each electrode and balanced across electrodes.
In infants, Fz or the high forehead is used, and preparation is gentle and done before sleep.
Test yourself: when the right ear is tested, where does the reference electrode go in the ipsilateral channel?
The correct answer is B: the right mastoid, M2.
Even numbers indicate the right side of the head, and the reference sits on the mastoid on the stimulated side.
In the next lesson, we will learn the generators and normal latencies of the ABR waves we record with these electrodes.
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Prerequisites

LESSON 125 Introduction to Auditory Evoked Potentials

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