An electroencephalogram, or EEG, is a safe and non-invasive test that measures the electrical activity produced by the brain’s cells, which constantly communicate through electrical impulses. This activity is recorded using small metal discs, known as electrodes, placed on the scalp. The primary purpose of an EEG is to detect changes in brain activity, which can assist in diagnosing various neurological conditions, such as epilepsy or sleep disorders. A normal EEG trace visually represents the healthy, rhythmic electrical patterns expected from a functioning brain in different states of consciousness.
Understanding the EEG Display and Measurement
The output of an EEG is a continuous, wavy line tracing that plots electrical voltage against time. Each line corresponds to a specific recording channel, reflecting the difference in electrical potential between two or more electrodes. This arrangement, called a montage, allows specialists to localize brain activity to specific regions of the scalp. The visual characteristics of these waves are defined by two main measurements: frequency and amplitude.
Frequency is measured in cycles per second (Hertz or Hz) and describes how fast the waves oscillate, determining the brainwave band. Amplitude refers to the vertical height of the wave, indicating the voltage or intensity of the electrical activity. The display’s sensitivity (gain) is often adjusted to change the height of the waveforms, making the activity easier to visualize.
Defining Normal Brainwave Frequencies and Amplitude
The normal adult brain exhibits four primary frequency bands. Beta waves (12 to 30 Hz) are the fastest. Beta activity is characteristic of an alert, focused, or actively thinking mind, and these waves are typically low in amplitude. They are most prominent over the frontal and parietal regions.
When the eyes close and the mind relaxes, the dominant rhythm shifts to Alpha waves (8 to 12 Hz). Alpha waves are generally higher in amplitude than Beta waves and are most prominent over the posterior regions of the head. This rhythm is considered the posterior dominant rhythm in a healthy, relaxed adult. Theta waves (4 to 8 Hz) are slower and are linked to drowsiness, daydreaming, or light sleep.
Delta waves (0.5 to 4 Hz) are the slowest and highest in amplitude, characteristic of deep, dreamless sleep in adults. While Delta and Theta activity are normal during sleep, their presence in an awake adult can indicate cerebral dysfunction. The overall pattern must be appropriate for the patient’s age and state of consciousness to be considered normal.
How Normal EEG Patterns Change During Sleep and Wakefulness
The normal EEG is highly dynamic, changing profoundly as a person transitions from alert wakefulness to deep sleep. In the fully awake state, the EEG is dominated by low-voltage, fast Beta waves anteriorly and rhythmic Alpha activity posteriorly. Opening the eyes or focusing attention suppresses the Alpha rhythm, replacing it with faster, lower-amplitude Beta activity.
As a person moves into drowsiness and light sleep (NREM Stage 1), the Alpha rhythm decreases and is replaced by slower Theta waves. This transition is marked by slow, rolling eye movements visible on the EEG. Deeper sleep stages (NREM Stage 2 and 3) introduce specific normal transient waveforms, such as sleep spindles and K-complexes. Sleep spindles are brief bursts of high-frequency activity, while K-complexes are large, sharp waves seen over the central regions.
During the deepest phase of sleep (NREM Stage 3), the tracing becomes dominated by high-amplitude, slow Delta waves. Paradoxically, Rapid Eye Movement (REM) sleep features an EEG pattern that closely resembles the alert, awake state. This pattern is characterized by low-amplitude, mixed-frequency activity, including Beta waves, which is why REM sleep is sometimes called paradoxical sleep.
Common Normal Findings That May Look Unusual
A normal EEG often contains patterns that might appear alarming to an untrained observer but are considered benign variations or artifacts. Artifacts are electrical signals originating outside the brain but picked up by the electrodes. These include signals from eye movements, muscle activity, or the heart.
Eye blinks or rapid eye movements produce large, sharp waveforms, especially in the frontal leads, that can mimic abnormal brain activity. Muscle movement (electromyographic or EMG activity) creates a high-frequency, “fuzzy” appearance on the tracing that can obscure the true brainwaves. The electrocardiogram (EKG) signal from the heart is also frequently recorded and appears as regular, sharp spikes.
Beyond external noise, some brainwave patterns that look sharp or unusual are recognized as normal physiological phenomena. Examples include Positive Occipital Sharp Transients of Sleep (POSTS) during light sleep. Minor differences in amplitude between the two hemispheres are also common.