In EEG (electroencephalography) analysis, filters are used to improve signal quality by removing unwanted noise and artifacts. The three main types of filters commonly used in EEG are:
- Low Frequency Filters (LFF) – High Pass Filters
- High Frequency Filters (HFF) – Low Pass Filters
- Notch Filter
1. Low Frequency Filters (LFF) - High Pass Filters
Low frequency filters are used to remove frequencies below a certain threshold, allowing higher frequencies to pass through. These filters are important for eliminating slow-moving artifacts like sweat or drifts that are not part of the brain's electrical activity.
Purpose: Removes slow artifacts and drifts.
Typical Range: 0.1 Hz to 1 Hz.
Common Standard: 1 Hz.
Examples:
- LFF Off: No low frequency filtering is applied. Slow artifacts like sweat or movement may remain visible.
- LFF 1 Hz: This is a common setting to filter out very slow movements and artifacts while preserving important slower brain activity, such as delta waves.
- LFF 5 Hz: A higher cutoff that will further reduce low-frequency artifacts but could also filter out some important slow-wave activity (e.g., deep sleep).
2. High Frequency Filters (HFF) - Low Pass Filters
High frequency filters are used to remove frequencies above a certain threshold, allowing lower frequencies to pass through. This filter is essential for eliminating high-frequency noise from muscle movements, electrical equipment interference, or skin potential artifacts that can obscure brain wave patterns.
Purpose: Removes high-frequency noise such as myogenic artifacts (muscle activity), electrical noise, or interference from devices.
Typical Range: 40 Hz to 100 Hz.
Common Standard: 70 Hz.
Examples:
- HFF Off: No high frequency filtering is applied. High-frequency noise, like muscle movements or equipment interference, may be visible.
- HFF 5 Hz: A very restrictive setting that may cut off most brain activity (including beta and gamma waves) and is rarely used for standard EEG recordings.
- HFF 70 Hz: The standard filter that removes high-frequency noise while allowing most brain activity, including beta waves, to pass through. It helps in preserving important brain signals while eliminating muscle and electrical interference.
3. Notch Filter
The notch filter is a specific type of filter that removes a narrow frequency band, typically designed to eliminate electrical interference from power lines. This is usually set to 50 Hz in Europe or 60 Hz in countries with different electrical grid frequencies.
Purpose: Removes electrical interference, often from power lines or electrical equipment.
Typical Range: 50 Hz (Europe) or 60 Hz (North America).
Common Standard: 60 Hz (US), 50 Hz (Europe).
Examples:
- Notch Filter Off: Electrical interference from power lines (e.g., 60 Hz or 50 Hz hum) will be visible on the EEG, often distorting the signal.
- Notch Filter On: Removes the 60 Hz or 50 Hz interference, cleaning up the signal without impacting brain wave activity.
Summary of EEG Filter Types
| Filter Type | Purpose | Common Settings |
|---|---|---|
| Low Frequency Filter (LFF) | Removes low-frequency artifacts like sweat and slow drifts | 1 Hz, 5 Hz |
| High Frequency Filter (HFF) | Removes high-frequency noise like muscle artifacts and electrical interference | 70 Hz, 5 Hz |
| Notch Filter | Removes power-line interference (60 Hz or 50 Hz) | 60 Hz (US), 50 Hz (Europe) |
Key Takeaways:
- LFF (Low Frequency Filter) helps clean up slow artifacts like movement or sweat, with 1 Hz being a standard.
- HFF (High Frequency Filter) removes high-frequency noise from muscle activity and electrical interference, with 70 Hz as a common setting.
- Notch Filter selectively removes power-line noise (60 Hz or 50 Hz), improving signal clarity.