Recording Evoked Potentials


Written by Travis M. Moore
Last edited 18-Jun-2020


Making Contact

We now understand that electrophysiological tests measure large groups of neurons firing in synchrony. We also understand just how neurons fire and the difficulties the electrical signal has reaching the surface of the skin (e.g., volume conduction). Now it is time to put electrode to skin and make first contact with the brain.

As with any type of electrical current, biological signals can only travel through good conductors - metals like gold, silver and platinum. Some very common electrodes you will find in the audiology clinic are Ag/AgCl electrodes. If you don't remember your periodic table of elements, that's silver/silver chloride. Suffice it to say that Ag/AgCl electrodes are so widely used because they are stable over longer periods of time when recording. Silver/silver chloride electrodes are often disposable, meaning after you have completed all your testing, they just go in the trash. Figure 1 shows a typical Ag/AgCl electrode from the front and back.

Ag/AgCl electrode
FIG. 1. © chinamedica

The left side of Figure 1 shows the back of the electrode (the part you see after placing it on the skin), and the right side shows the part that touches the skin. Notice that the actual electrode is the tiny grey circle seen in the image to the right. The rest, in fact the majority, is just an adhesive sticker to hold the Ag/AgCl pellet in place. The clear area in the right image typically contains a sponge filled with an electrolyte gel that is an excellent conductor. The gel is what makes contact with the skin, and conducts the voltage at the scalp to the Ag/AgCl bead. Take a look at the image on the left. Directly opposite the grey disk is a metal snap. This is where you attach the leads (wires) that carry the voltage to the computer. Figure 2 shows some leads snapped onto some circular electrodes.

Ag/AgCl electrode with leads
FIG. 2. Ag/AgCl electrodes with leads attached.

Preparing the Skin

If you recall our discussion of Ohm's Law in a different module (not online yet), you know that one way to increase the voltage is to decrease the resistance. Of all the structures the electrical current travels through from neuron to scalp, the only place we can affect the resistance is the topmost layer of skin. Still, preparing the skin makes quite a difference in the voltages we can measure. The outer layer of skin is made up of dead keratin cells, which are poor conductors, and need to be removed. This is actually a painless process, and one that you likely experience regularly: exfoliation. First we wipe the area we plan to scrub with an alcohol pad. Then we dip a cotton swab applicator into a small amount of hospital-grade exfoliant (commonly NuPrep; Figure 3), and vigorously rub the sites where the electrodes will be placed. It's absolutely essential you experience this first-hand, both as the scrubber and the "scrubbee" so you get an idea of what exactly vigorous rubbing should be. You need to remove the dead skin cells, and exfoliating someone else is typically a new experience. You have to rub harder than you think, but you obviously want to avoid hurting your patient and rubbing the skin raw. Wipe the remaining exfoliant away with a gauze pad, and you're ready to place your electrode.

Ag/AgCl electrode with leads
FIG. 3. NuPrep

You will have to peel off the backing of the adhesive part of the electrode, line up the actual Ag/AgCl pellet (the grey disk in Figure 1), then place the electrode. It is important to avoid pressing on the pellet area with the electrolyte. There is no adhesive there, just foam that will expel the electrolyte if pressed. So make certain you only press down on the adhesive sticker when securing the electrode to the skin. It is possible that you'll have to use some medical tape to hold the electrode in place, especially if the area is covered in hair (like most of the scalp) or particularly angular (like the chin).

That's it! You are now ready to record neuronal activity from the scalp! It's really quite amazing when you think about it, but this setup allows us to peer into the brain.

International 10-20 System

So now you turn to your patient with an electrode in your hand and you place it... where? How would you even describe to someone where you placed an electrode if asked? Early investigators also faced this conundrum, and in 1949 proposed what's known today as the international 10-20 system1. It is so named because all of the standard electrode positions are either 10% or 20% away from each other, based on two measurements of the head: the total distance from front-to-back or side-to-side of the head (sagittal and coronal planes, respectively). You could place all of your electrodes by hand by physically measuring these distances with a flexible, soft measuring tape (the kind tailors use). Figures 1 through 3 show the primary electrode sites from different angles. Figure 3 shows a top-down view of the flattened surface of the head so you can see all the electrode sites at once.

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FIG. 1. © Recommendations for the Practice of Clinical Neurophysiology
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FIG. 2. © Recommendations for the Practice of Clinical Neurophysiology
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FIG. 3. © Wikimedia Commons

Audiologists routinely measure the head in the sagittal plane (front-to-back) in the clinic for common tests, such as the auditory brainstem response (ABR). The front-to-back head measurement starts at the the nasion, goes over the top of the head, and ends at the inion. Figure 1 shows where both of these points are located on the head. The nasion is the impression just below the forehead where the nose begins. The inion is the bony protrusion you can feel in the back of the head just above the neck. The 10-20 system states that you should measure the distance between nasion and inion on your patient, then calculate what 10% and 20% of the distance is in order to space the electrodes properly.

The coronal (side-to-side) measurement is made from one preauricular point to the other. You should recognize the word "auricular" as the adjectival form of the noun "auricle," referring to the outer ear. So preauricular just means "in front of the ear." Therefore, the 10-20 system states you should measure from a point just in front of one tragus, up over the head, to just in front of the other tragus. You would then calculate 10% and 20% of the total length to place the electrodes.

Notice that you have to make this measurement on every patient, so the placement of electrodes changes to fit the size of each patient's head. This way we can position electrodes in the same relative area for large, medium and small heads. For example, using a fixed number to place electrodes, say 4 cm from the inion, would result in electrodes positioned over different brain regions in a child compared to an adult (4 cm is a lot longer on an infant's head!). Measuring 10% and 20% of the head measurements we make allows the electrode sites to scale with head size.

Now, before you start to dread plotting all these points, keep in mind that researchers have done the heavy lifting for you. They have figured out which electrodes are best for recording different potentials, so we really only have to make a few measurements to find the locations most useful for whatever waveform we want to record. Furthermore, it just so happens that the only location used in the audiology clinic that requires measurement is the center of the head (Cz). And, because it's the center, all you do is measure the half-way point in the sagittal and coronal planes. Make a small mark with a grease pencil to mark each location on the patient's scalp, and you're done. As you will see in later lessons, the other electrode positions in the audiology clinic don't really require measurement (e.g., the mastoid, back of the hand, ear canal). And to be fair, even researchers don't measure all the 10-20 system sites by hand - they use prepared electrode nets or caps that hold electrodes in place with flexible wires of the appropriate lengths (Figure 4). Because the nets and caps still have to scale with the head, they come in many different sizes.

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FIG. 4. © A geodesic electrode net.

Decoding the 10-20 System

Let's briefly run through the organization of the 10-20 system. For the most part, the letters correspond to the brain region immediately below their position. Table 1 shows what each letter stands for.

Letter Meaning
A Auricular (Earlobe)
C Central
F Frontal
Fp Fronto Polar (Low Forehead)
Fpz (Low Forehead - Center)
Fz (High Forehead - Center)
M Mastoid
P Parietal
O Occipital
T Temporal
z (Lowercase) Zero (Midline)

The last part to tackle concerns the numbers next to the letters. The system is simple: even numbers refer to the right side, and odd numbers to the left side. The lowercase "z" is used instead of the number 0 to avoid confusion with the letter "O" and indicates the midline. So an electrode position of F3 would be over the frontal lobe, to the left of the midline. Cz would be over the center of the head, at the midline. From here on, whenever you record a waveform you'll refer to the electrode montage associated with that particular potential.

The most important electrode sites to know for the audiology clinic are listed below:

  • A (earlobe)
  • Cz or Vertex (center of the top of the head)
  • Fpz (low forehead - center)
  • Fz (high forehead)
  • M (mastoid)


Test Your Understanding

Answer One
Answer Two
Answer Three


REFERENCES


1Klem, G. H., L├╝ders, H. O., Jasper, H., & Elger, C. (1999). The ten-twenty electrode system of the International Federation. Electroencephalography and Clinical Neurophysiology, 52(3), 3-6.
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