Eye Blinks

Eye blinks are one of the most common artifacts you'll see, and are marked by very high amplitude negative waveforms in the bifrontal regions. They arise due to Bell's Phenomenon. The eyes' cornea is positively charged and retina is negatively charged; when you blink, the eyes roll up slightly, and the cornea moves closer to the frontal electrodes Fp1 and Fp2, which thus see a positive signal that is reflected on EEG.

Eye blinks should really only be seen in the frontal leads, without any field into the posterior regions. A similar waveform is seen upon eye closure, and a sort of opposite waveform, of a large bifrontal negative charge, is seen upon eye opening. Eye blinks are a key component of a normal awake EEG. Be careful not to mistake them for frontal spike and waves, or anterior predominant generalized spike and waves.

lateral eye movements

Lateral eye movements are marked by opposing polarities in the F7 and F8 leads. This is due, again, to the positive charge of the cornea and negative charge of the retina. When you look to the right, the right cornea gets closer to the F8 electrode, which sees a positive charge; the left retina gets closer to the F7 electrode, which thus sees a negative charge. So, with lateral eye movements you look to the positive side.
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‍Recall that in bipolar montages you are comparing the first and second electrode in every chain, with the calculation basically being the first electrode minus the second electrode in every lead, So, with the Fp2-F8 lead if F8 is more positive than Fp2, you subract a larger number from a smaller one and wind up with a negative value and thus an upward deflection on the tracing, while in the next lead of F8-T4 if F8 is more positive than T4 as well, you subtract a smaller number from a bigger one, so you get a positive value and a downward deflection. So, you get a sort of "opposite" phase reversal and the leads open up away from one another. The opposite happens on the contralateral side because F7 is negative, and the leads move toward one another.

chewing & hypoglossal movement

Chewing and tongue (hypoglossal) artifact are rather hard to miss on EEG. Chewing artifact is really just muscle artifact from the temporalis muscle, and is marked by sudden onset, intermittent bursts of generalized very fast activity (muscle artifact). It is quite easy to characterize with video studies, as you can just look at the video to correlate, but even without video studies chewing artifact does not usually share a close morphology with any other important physiologic activity. Just be careful not to mix it up with generalized periodic fast activity, which tends to be slightly slower (beta frequency) and lower in amplitude.

Hypoglossal artifact is often but not requisitely seen with chewing artifact. It arises from the movement of the tongue, and appears as slower, diffuse delta frequency activity. Notice how, on the example below, all the tracings have essentially the same movement--this is much too organized for something like slow wave sleep, and while ictal patterns can certainly lead to such rhythmicity, there is no evidence on this single page of the evolution across time or location that characterize seizures.

One notable point about tongue movement artifact is that it is reproducible--you can ask the patient to move it, or to say "la la la" (aka a lingual phoneme) to see if the same pattern is brought out on the EEG.

eCg & cardiac artifact

ECG artifact is marked by waveforms that are time locked to the QRS complex on the ECG tracing. They tend to be present more so or entirely on the left side, because the heart is in the left half of the chest, and tend to be relatively low amplitude. However, as the example below shows, they can at times be quite prominent, but don't mistake them for posterior discharges or POSTS (which are not time locked to the QRS).

A much less commonly seen cardiac artifact is cardioballistic artifact, in which the EEG electrode is placed just above an artery, and each pulsation of the artery is picked up as motion artifact on the EEG.

electrical & electrode artifact

Electrical artifact most commonly arises from interference from the 60 Hz electrical activity (in the USA; 50 Hz in Europe and elsewhere) that that runs through wires; so, it can be caused by anything from an electrical appliance to a cell phone charging, though most modern EEG equipment is quite good at minimizing it. Electrical artifact is a very fast, very monotonous activity, and you can use the notch filter to selectively remove all the EEG activity at 60Hz (this won't affect your interpretation of the signal, as no cerebral activity is that fast on scalp EEG). In the example below, the notch filter is not on, and the Fp1 electrode is likely not on quite right, leading to a lot of electrical interference.

Electrodes themselves can give artifact as well, usually in the setting of them becoming loose over time or getting bumped by things in the environment. A common such finding is electrode pop, caused most often by a loose electrode. On EEG pop is marked by a single electrode showing a very sudden, steep upslope with a slower downslope and absolutely no field. On the example below, F7 is involved; if this continues, you should take a look at the electrode to make sure it's not falling off.

myogenic / muscle

Myogenic artifact comes from muscle movements, and is most commonly found in the frontal or lateral temporal regions, due to the frontalis and temporalis muscles. It is marked by high frequency, often low amplitude activity overlying the normal cerebral rhythms, and is usually most prominent in the awake state. Of note, there's typically only minimal myogenic artifact near the vertex so if you see fast activity there, be slightly more suspicious (although, realistically, myogenic activity is much faster than the cerebral activity that can be picked up on scalp EEG).

sweat & Other

Sweat artifact is marked by very slow (typically less than 0.5 Hz), relatively low amplitude activity that arises because the sodium chloride in sweat carries a charge, which is picked up by the EEG electrodes. Sweat artifact doesn't have to follow a particular pattern in terms of localization, and can be bilateral, unilateral, or even focal to just a few electrodes. The example below also has electrode artifact at P3.

In the hospital, particularly in patients intubated and sedated in the ICU, you'll commonly come across chest physiotherapy artifact. This is important to know about because it can appear rhyhthmic similar to a seizure, although you can easily distinguish the two (even without your keen electrophysiologic skills) by looking at the video, if available. On the EEG, it also typically differs from a seizure by its lack of a field or evolution. Chest PT tends to be more prominent posteriorly, but depending on the patient's position and where the PT is being applied, that can vary widely.

Movement artifact has a plethora of appearances, usually of chaotic looking, high amplitude activity that doesn't mimic any actual cerebral patterns. However, head shaking artifact can be slightly trickier and is marked by slow, low amplitude activity that is usually more prominent posteriorly if the patient is resting their head on a pillow. It can look similar to the roving eye movements of drowsiness, but eye movements are in the anterior leads.

While the above artifacts are perhaps the most commonly seen, you'll come across many other kinds of artifact as you continue to read EEG. On this example below, for example, in an intubated patient a small bubble on the tip of the nose fluctuated with breathing, leading to a sort of periodic burst of activity that is, in fact, just artifact.