EEG (electroencephalography) measures the electrical activity of the brain through various frequency bands, each associated with different mental and physiological states. The main EEG frequencies, in increasing order of frequency, are:
1. Delta Waves (0-4 Hz)
- Seen during deep sleep (slow-wave sleep) and in certain pathological conditions like brain injuries, tumors, or encephalopathy.
- High amplitude.
2. Theta Waves (4-8 Hz)
- Common in childhood and during drowsiness or light sleep in adults.
- Associated with states of relaxation, meditation, or deep thinking.
3. Alpha Waves (8-13 Hz)
- Characteristic of a relaxed but alert state, especially in the awake, calm adult brain.
- Known as the posterior dominant rhythm (PDR) in normal brain activity.
4. Beta Waves (13-30 Hz)
- Linked to active thinking, problem-solving, or heightened mental focus.
- Often appear as muscle artifacts or during states like drug use (e.g., benzodiazepines).
- Lower amplitude than delta and theta waves.
5. Gamma Waves (Above 30 Hz)
- Not commonly seen on standard scalp EEG, but related to higher cognitive functions like memory and attention.
- Higher frequency but not typically measured in routine EEG tests.
Frequency-Amplitude Relationship:
In general, lower frequencies (delta, theta) tend to have higher amplitude, while higher frequencies (alpha, beta) have lower amplitude. However, EEG tracings often show a mixture of frequencies overlapping each other.
Understanding these frequencies is key to interpreting EEG, as each frequency band corresponds to different mental states, levels of consciousness, or neurological conditions.