An electroencephalogram (EEG) is a medical test that measures the electrical activity of the brain. Electrodes placed on the scalp detect the tiny electrical signals generated by brain cells, displaying them as wavy lines. This procedure allows healthcare providers to assess brain function and diagnose various neurological conditions. Understanding the characteristics of a “normal” EEG recording is fundamental to identifying deviations that might indicate underlying health issues.
Understanding EEG Waveforms
The fundamental components of an EEG recording are brainwaves, categorized by their frequency, measured in Hertz (Hz), and their amplitude, representing the height of the wave. Different frequencies correspond to distinct brain states. Delta waves, the slowest, range from 0.5 to 4 Hz and are prominent during deep, dreamless sleep. Theta waves, ranging from 4 to 8 Hz, are associated with light sleep, drowsiness, deep relaxation, and meditation.
Alpha waves, with a frequency between 8 and 13 Hz, characterize a relaxed yet awake state, observed when an individual is calm, with eyes closed. They can be prominent over the back of the head. Beta waves, ranging from 13 to 30 Hz, are the fastest and are prevalent during active thinking, alertness, concentration, and problem-solving. Low-amplitude beta waves are typical of an engaged and focused conscious state. The amplitude of these waves reflects the number of neurons firing synchronously, with higher amplitude indicating more synchronized activity.
Typical EEG Activity
A normal EEG changes depending on an individual’s state of consciousness and age. During an awake state, a healthy adult EEG shows a mix of alpha and beta activity. The alpha rhythm is most noticeable when the eyes are closed and a person is relaxed, appearing over the posterior regions of the brain. When eyes open or mental activity increases, the alpha waves tend to decrease in amplitude or disappear, replaced by faster, lower amplitude beta waves, especially in the frontal areas. Infants and young children exhibit more slower wave activity (delta and theta) than adults, which is considered normal for their developmental stage.
During sleep, the EEG progresses through distinct stages. Non-rapid eye movement (NREM) sleep begins with N1, characterized by a decrease in alpha waves and the appearance of slow eye movements and theta activity. N2 sleep is marked by the presence of sleep spindles (brief bursts of 12-14 Hz activity) and K-complexes (high-amplitude, low-frequency biphasic waves). Deep sleep, N3, is defined by the dominance of high-amplitude delta waves, signifying profound rest and physical restoration. Rapid eye movement (REM) sleep, in contrast, presents an EEG pattern similar to wakefulness, with low-voltage, mixed-frequency activity, including beta and theta waves, despite muscle paralysis.
Indications of Abnormal Activity
When an EEG deviates from these typical patterns, it can indicate abnormal activity. Excessive slowing, such as the persistent presence of delta or theta waves in an awake adult, can suggest a generalized brain dysfunction. Similarly, excessive fast activity, where beta waves are unusually prominent or widespread, can also indicate abnormal activity. A significant difference in activity or amplitude between the two hemispheres of the brain, known as asymmetry, can point to a localized issue.
The presence of sharp waves and spikes, which are transient, high-amplitude events with a pointed appearance, indicates abnormal neuronal discharges. Spikes last less than 70 milliseconds, while sharp waves last between 70 and 200 milliseconds, and both are associated with conditions like epilepsy. Another concerning pattern is suppression or flatlining, which signifies a significant reduction or absence of electrical activity, indicating severe brain impairment. These general indicators prompt further medical investigation to determine their underlying cause.