1. What are Myogenic Artifacts?

Myogenic artifacts in EEG are electrical signals that arise from muscle movements, not brain activity. These artifacts are caused by the electrical activity generated by the contraction of muscles near the scalp. The most common muscles responsible for myogenic artifacts are the frontalis and temporalis muscles, which control facial expressions, eye movements, and jaw activity.

When these muscles contract, they generate electrical potentials that are detected by the EEG electrodes on the scalp, leading to signals that resemble brain activity but are unrelated to actual cerebral function. These signals can interfere with the clarity and accuracy of the EEG, making it harder to interpret the brainwave patterns.

2. Causes of Myogenic Artifacts

Myogenic artifacts can be caused by various muscle activities, including:

  • Facial movements: Actions like blinking, frowning, or smiling can activate the frontalis muscle, causing interference in the frontal EEG channels.
  • Jaw movements: Chewing, clenching, or other jaw-related activities involve the temporalis muscle, which can lead to artifacts in the temporal regions of the scalp.
  • Eye movements: While eye movements are primarily detected by electrooculography (EOG), they can also influence muscle groups near the eyes, causing artifacts in the frontal regions.

3. Characteristics of Myogenic Artifacts

Myogenic artifacts are characterized by the following features:

  • High-frequency activity: Myogenic artifacts typically occur at much higher frequencies than normal brain activity, often between 20–100 Hz. This is much faster than typical brainwaves such as alpha (8–12 Hz), beta (13–30 Hz), or theta (4–8 Hz) waves.
  • Low amplitude: The amplitude of myogenic artifacts is usually lower than that of normal EEG signals. However, depending on the strength of the muscle contraction, the amplitude can occasionally appear more significant.
  • Appearance over normal cerebral rhythms: Myogenic artifacts can overlap with normal brainwave activity, making it challenging to distinguish between the two. They may be seen as sharp, brief bursts of activity that mask or distort the underlying brain rhythms, especially in the frontal and temporal regions.

4. Myogenic Artifacts in Specific Regions

Myogenic artifacts are most commonly found in the following regions of the scalp:

  • Frontal regions: The frontalis muscle, responsible for forehead movements, can lead to muscle activity that interferes with the EEG in the frontal lobe (e.g., Fp1, Fp2, F3, F4 electrodes).
  • Lateral temporal regions: The temporalis muscle, which controls jaw movement, generates signals that can be detected in the temporal areas (e.g., T3, T4 electrodes).

Myogenic activity is generally minimal near the vertex (the top of the head), as muscle activity tends to be less prominent in this area. Therefore, if fast activity is detected near the vertex, it is more likely to be a myogenic artifact than a cerebral signal.

5. Identifying Myogenic Artifacts

Myogenic artifacts can be challenging to identify because they may resemble actual brainwave patterns, particularly if they occur at high frequencies. However, several factors can help in recognizing myogenic interference:

  • Location: Artifacts from muscle movements are typically more prominent in the frontal or temporal regions due to the muscles' proximity to these areas. If high-frequency activity is detected in these regions, it may be a myogenic artifact.
  • Frequency: The frequency of the artifact is often higher than typical brain activity. If the frequency is in the range of 20–100 Hz, it is likely caused by muscle activity.
  • Duration: Myogenic artifacts tend to appear in brief, sharp bursts rather than sustained brainwave patterns. If the activity is quick and discontinuous, it may be a muscle artifact.

6. Impact of Myogenic Artifacts on EEG

Myogenic artifacts can significantly distort the EEG data, especially when muscle movements overlap with cerebral signals. This can make it difficult to identify brainwave patterns and accurately interpret the EEG. The presence of muscle artifacts can lead to:

  • False positives: High-frequency activity may be mistakenly interpreted as brain activity.
  • Masking of brainwaves: Muscle activity can mask underlying brainwaves, especially in the frontal and temporal regions, leading to incomplete or incorrect interpretations.

7. Minimizing Myogenic Artifacts

There are several ways to minimize the impact of myogenic artifacts in EEG recordings:

  • Patient preparation: Instructing the patient to remain still and avoid unnecessary facial or jaw movements can help reduce muscle artifacts.
  • Electrode placement: Ensuring that electrodes are securely attached to the scalp can reduce the likelihood of muscle-related interference.
  • Artifact rejection algorithms: Many modern EEG systems come with built-in algorithms designed to detect and reject myogenic artifacts based on their frequency and location.
  • Signal processing: Applying filters, such as high-pass filters, can help attenuate high-frequency muscle artifacts while preserving the desired brainwave activity.

8. Conclusion

Myogenic artifacts are a common and challenging source of interference in EEG recordings, primarily caused by muscle contractions in the frontal and temporal regions. Recognizing and differentiating myogenic activity from actual brain signals is crucial for accurate EEG interpretation. By understanding the characteristics of these artifacts and employing strategies to minimize their impact, clinicians and researchers can improve the quality of EEG recordings and ensure more accurate analysis.