An electroencephalogram (EEG) is a non-invasive medical test that measures the brain’s electrical activity. Small electrodes placed on the scalp detect the tiny electrical charges produced by communicating brain cells. Analyzing these brainwave patterns helps diagnose conditions affecting brain function and identify irregularities.
Fundamental Brain Rhythms
The brain produces distinct types of electrical waves, each associated with different states of consciousness. Delta waves, the slowest (0.5-4 Hz), are observed during deep, dreamless sleep. In contrast, theta waves (4-8 Hz) are prominent during periods of drowsiness, light sleep, or deep relaxation.
Alpha waves (8-13 Hz) characterize relaxed wakefulness, especially with eyes closed. These waves tend to diminish when a person opens their eyes or engages in active mental tasks. Beta waves (13-30 Hz) are the fastest, prevalent during active, alert states such as focused thinking and concentration.
Key Features of a Healthy EEG
A healthy EEG tracing exhibits several consistent characteristics. Normal brain activity generally appears symmetrical, meaning the amplitude and frequency of brainwaves are similar across both hemispheres. The typical amplitude of EEG signals on the scalp usually ranges from 10 to 100 microvolts.
Reactivity is another feature, where normal brainwaves respond predictably to stimuli. For instance, the alpha rhythm, prominent with eyes closed, attenuates or “blocks” when a person opens their eyes or performs mental arithmetic. A normal EEG also displays a consistent background rhythm, free from excessive slowing, sharp waves, or spikes. The posterior dominant alpha rhythm is a defining feature of a normal adult EEG.
Variations in Normal EEG Patterns
Normal EEG patterns are not static and can show variations based on age and state of consciousness. Brainwave patterns shift significantly from infancy through adulthood. Young children typically exhibit a greater presence of slower waves, and the alpha rhythm reaches 8 Hz. Gradual slowing of brain activity can be a normal finding in older adults.
The state of consciousness profoundly influences EEG patterns. During wakefulness with eyes open, the EEG is dominated by low-voltage, high-frequency beta waves, indicating an alert state. As a person transitions to drowsiness, the alpha rhythm may become less prominent, and theta activity can emerge. In light sleep (Stage 1 and 2 NREM), theta waves become more prevalent, and distinct patterns like sleep spindles (11-16 Hz bursts) and K-complexes, which are large, sharp waves, appear, particularly in Stage 2. Deep sleep (Stage 3 NREM) is characterized by high-amplitude delta waves, while REM sleep features low-amplitude, high-frequency beta waves.