1. Eye Blink Artifact

Eye blinks are a common source of artifact in EEG recordings, particularly in the frontal region. The artifact occurs because of a phenomenon known as Bell's Phenomenon, where the eye moves upward and outward during a blink. This movement creates an electrical potential due to the movement of the eye's muscles and tissues. The result is a large frontal positive deflection on the EEG. The deflection is typically sharp and transient, and it can obscure underlying brain activity, especially in the frontal leads. This artifact is most noticeable in awake individuals, particularly when they blink frequently.

2. Lateral Eye Movements Artifact

Lateral eye movements, or side-to-side eye movements, produce a distinctive pattern in the EEG. When the eye moves laterally, the frontal positive deflection occurs on the side to which the eye is looking, while the contralateral side (opposite) shows a negative deflection. This happens because the electrical potential generated by the muscles that move the eye is detected by the electrodes placed over the frontal area of the scalp. This artifact is often seen in both frontal leads, but the size and polarity depend on the direction of the gaze.

3. Myogenic (Muscle) Artifact

Myogenic artifacts arise from muscle activity, and they are typically characterized by fast frequency, often low-amplitude activity, which can be seen over the frontal and lateral temporal regions. These artifacts are generated by the contraction of muscles, such as the frontalis (forehead) and temporalis (side of the head) muscles. Myogenic artifacts are particularly noticeable in awake patients, as the muscles tend to contract in response to movements like frowning, jaw clenching, or head shaking. The key feature of myogenic artifacts is their high-frequency nature, which often mimics cerebral activity, but it is distinct because it is usually more localized to specific regions rather than widespread like brain activity.

4. Chewing Artifact

Chewing generates a diffuse burst of high-frequency myogenic activity due to the movements of the muscles involved in chewing, such as the masseter and temporalis. The artifact is usually diffuse and can appear across multiple electrodes, especially in the temporal and lower frontal regions. In contrast to normal brain activity, which tends to have a more rhythmic or less chaotic appearance, chewing artifact manifests as sharp, sudden bursts of myogenic signals. The amplitude can be higher than typical cerebral rhythms, making it an easily identifiable artifact in EEG data.

5. Hypoglossal (Tongue) Artifact

Hypoglossal artifact arises from the movement of the tongue, and it is usually marked by diffuse, slow, synchronized activity. The hypoglossal nerve controls the muscles of the tongue, and when these muscles contract—either due to voluntary or involuntary movement—an electrical signal is generated that can be detected by EEG electrodes. This artifact typically has a slow frequency, making it easy to differentiate from faster, higher-frequency brain activity. However, it can be confused with slow-wave brain activity, so it’s important to consider the patient's behavior and context when interpreting this artifact.

6. ECG (Electrocardiogram) Artifact

The ECG artifact is generated by the heart’s electrical activity, specifically from the QRS complex of the electrocardiogram, which represents the depolarization of the ventricles. On EEG, the artifact appears as posterior negative spikes that are time-locked to the QRS complex of the heartbeat. The artifact is more noticeable on the left side of the scalp, as the heart's electrical field is primarily directed towards the left side of the body. The QRS complex’s sharp, transient signal can appear similar to a spike or sharp wave in the EEG, but it is distinguishable because it occurs at a fixed interval (i.e., the heartbeat) and has a regular, rhythmic pattern.

7. Sweat Artifact

Sweat artifact arises from the ionized sodium chloride in sweat, which can interfere with the EEG signal. Sweat is a conductor of electricity, and when sweat accumulates on the scalp, it can create a slow, undulating wave on the EEG. This artifact usually has a frequency of less than 0.5 Hz and is typically of low amplitude. Sweat artifact can occur anywhere on the scalp, but it is often seen in areas of heavy sweating, which can be induced by stress, anxiety, or temperature changes. It can sometimes be difficult to distinguish from low-frequency brain activity, so it’s important to consider environmental factors when interpreting EEG data.

8. Movement Artifact

Movement artifact is the most variable and often the most disruptive to EEG recordings. It is typically chaotic, high-amplitude, and irregular, making it difficult to distinguish from actual brain activity. Movement artifact can arise from a variety of sources, such as the patient shifting positions, clenching fists, or moving their head or limbs. In particular, head-shaking artifact is a form of movement artifact characterized by slow, low-amplitude waves, often seen when a patient rests their head on a pillow. This artifact can look similar to roving eye movements seen in the drowsy state, but it can be differentiated because eye movements typically show up in the anterior leads, while movement artifact is often more posterior if the patient’s head is supported.

9. Other Artifacts

Beyond the common types discussed, there are various other sources of artifact that can affect EEG readings. For example, air bubbles near the nose in an intubated patient can cause periodic bursts of activity. Additionally, chest physiotherapy (chest PT) in ICU patients can generate rhythmic activity that resembles a seizure on the EEG, though it can be distinguished by the absence of evolution or field spread, and by the absence of seizure-like features on video recordings.

Conclusion

EEG artifacts can complicate the interpretation of brain activity, but understanding their characteristics helps in identifying and distinguishing them from true cerebral signals. Artifacts can arise from a variety of sources, including muscle contractions, eye movements, heart activity, sweat, and patient movements. Knowing the typical patterns, locations, and frequencies associated with each artifact type ensures more accurate EEG interpretation and reduces the risk of misdiagnosis.