An electroencephalogram (EEG) is a medical test that measures the electrical activity of the brain. Tiny metal discs, called electrodes, are placed on the scalp to detect the electrical signals produced by brain cells communicating with each other. This activity is recorded as wavy lines, providing insights into brain function. The primary purpose of an EEG is to aid in diagnosing various brain conditions, such as epilepsy or sleep disorders. This article explores the characteristics of a normal EEG tracing.
Basics of Brainwave Activity
The brain’s electrical activity is categorized into different types of brainwaves, distinguished by their frequency, measured in Hertz (Hz). These frequencies correspond to different states of consciousness or activity.
Delta waves, the slowest, range from 0.5 to 4 Hz and are prominent during deep sleep, playing a role in physical restoration. Theta waves (4-8 Hz) are associated with light sleep, deep relaxation, and creativity. While normal in children and during sleep, their presence in awake adults can indicate an abnormal state. Alpha waves (8-12 Hz) characterize a relaxed, yet awake state, particularly when the eyes are closed, and are linked to mental coordination and calmness. Beta waves (12-30 Hz) signify an alert, active, and focused state, associated with conscious thought, problem-solving, and decision-making.
Hallmarks of a Normal EEG
A normal EEG tracing exhibits distinct characteristics across different states of consciousness, including wakefulness, drowsiness, and sleep. These patterns are organized, symmetrical, and appropriate for the individual’s age. The amplitude of normal EEG waveforms ranges from 10 to 100 microvolts, with most falling between 10 and 50 microvolts.
During relaxed wakefulness with eyes closed, a normal adult EEG is dominated by a posterior alpha rhythm (8-12 Hz), symmetrical over both hemispheres. This alpha activity attenuates or disappears when the eyes open or during intense mental activity, demonstrating normal reactivity. Beta frequencies, low in amplitude and over 13 Hz, are also present, particularly in the fronto-centro-temporal regions during wakefulness. As an individual transitions into drowsiness, the alpha rhythm becomes less prominent, and slower theta and delta frequencies emerge, particularly in children and young adults.
In the initial stages of sleep (N1), the EEG shows a gradual loss of the alpha rhythm, the appearance of slow eye movements, and the emergence of vertex waves and positive occipital sharp transients of sleep (POSTS). Stage N2 sleep is characterized by sleep spindles (bursts of 12-14 Hz activity) and K-complexes, which are distinct, high-amplitude negative sharp waves followed by a positive component. Deeper sleep stages, particularly N3 (slow-wave sleep), are marked by a significant increase in high-amplitude delta activity (0.5-2 Hz), indicating profound rest and recovery.
Factors Affecting EEG Readings
Several non-pathological factors can influence EEG readings, causing variations without indicating a medical problem. Age is a significant factor, as normal EEG patterns evolve from infancy through old age. For example, diffuse slow delta-theta activity is normal in young children, whereas its presence in an awake adult can be abnormal.
The individual’s state of consciousness (fully awake, drowsy, or in different sleep stages) profoundly alters brainwave patterns, naturally introducing slower frequencies and specific waveforms. Certain medications, such as benzodiazepines and barbiturates, can also affect brainwave activity, increasing beta activity. Additionally, technical artifacts, such as muscle movements or external electrical interference, can appear on the tracing and must be distinguished from actual brain activity.
The Role of Professional Interpretation
Interpreting an EEG tracing is a complex task requiring specialized expertise. A trained medical professional, a neurologist or clinical neurophysiologist, interprets these results. They consider waveform nuances, integrate findings with the patient’s clinical history, and account for the recording’s context. This comprehensive approach ensures an accurate diagnosis. This information is for general knowledge and should not substitute professional medical advice or diagnosis.