The Electroencephalogram (EEG) is a non-invasive medical test that records the brain’s electrical activity using electrodes placed on the scalp. This activity is generated by the synchronized firing of millions of neurons, producing subtle electrical fields that the equipment can detect. The EEG tracing provides a functional snapshot of the brain, making it a valuable tool for monitoring brain health, especially in the diagnosis and management of conditions like epilepsy, sleep disorders, and altered states of consciousness. Interpreting the resulting waveforms requires understanding both the biological signals and the technical parameters that shape the display.
Technical Parameters and Display Configuration
The visual representation of brain activity on an EEG screen or paper is governed by specific technical settings that determine how the raw electrical data is displayed. The most important of these settings are the montage, the time scale, and the sensitivity.
A montage refers to the specific arrangement of electrode pairs that feed into the recording channels, essentially creating a map of the electrical potential differences across the scalp. The two main types are bipolar montages, which compare adjacent electrodes in a chain, and referential montages, which compare all electrodes to a common reference point. Selecting the appropriate montage is necessary for accurately localizing the source of any abnormal activity.
The time scale controls the horizontal axis of the tracing, dictating how quickly the paper or screen moves. This setting is directly related to how easy it is to count the frequency of the waves, which is measured in cycles per second, or Hertz (Hz). A slower time scale can compress the waveforms, making fast activity difficult to distinguish, while a faster speed spreads them out for more detailed scrutiny.
The sensitivity parameter adjusts the vertical height of the waveforms, measured in microvolts per millimeter. Since the brain’s electrical signals are extremely small, this setting is used to amplify the signal for visual analysis. A high sensitivity setting will make the waves appear taller, but it risks cutting off the peaks of high-amplitude waves.
Interpreting Normal Brain Rhythms
The fundamental task in reading an EEG is recognizing the normal background rhythms, which are categorized into four frequency bands, each associated with a different state of brain function. Frequency describes the speed of the waves, or how many cycles occur in one second.
The slowest normal frequency is the Delta rhythm (0.5 to 4 Hz). These waves are the hallmark of deep, non-rapid eye movement sleep in adults and are also the dominant rhythm in infancy. Finding Delta waves in an awake adult can suggest global brain dysfunction or a structural lesion, depending on whether the activity is generalized or focal.
Slightly faster is the Theta rhythm (4 to 8 Hz). Theta waves typically appear during states of drowsiness, light sleep, or deep meditative relaxation. While Theta activity is a normal finding in children, excessive amounts in an awake adult are considered a sign of mild brain slowing.
The Alpha rhythm (8 to 13 Hz) is the primary rhythm of the normal, relaxed adult brain. This rhythm is most prominent over the posterior regions of the head. It is characteristically seen when a person is awake but resting with their eyes closed, and it should diminish or disappear when the eyes open or when the person concentrates.
The Beta rhythm (greater than 13 Hz, up to 30 Hz) is typically low in amplitude. Beta waves are associated with active concentration, alertness, and anxiety. Beta activity is often seen in the frontal regions and can become more diffuse in individuals taking certain medications, such as benzodiazepines.
Identifying Pathological Waveforms
Pathological waveforms represent deviations from the expected normal rhythms and are the main focus when searching for signs of neurological dysfunction. These abnormal patterns can be broadly classified by their shape, speed, and distribution across the scalp.
One of the most clinically significant types of abnormal activity is epileptiform activity, which strongly suggests a predisposition to seizures. This includes spikes, which are transient, sharp-contoured waves lasting less than 70 milliseconds, and sharp waves, which have a similar pointed morphology but a longer duration (70 to 200 milliseconds). These are often followed by a slow wave, forming a spike-and-wave complex, which is the electrographic signature of certain seizure types.
Another major category of abnormality is slowing, where the background rhythm is replaced by waves that are slower than expected for the patient’s age. Generalized slowing, where Delta or Theta activity appears diffusely across both hemispheres, is a non-specific sign of global cerebral dysfunction, often seen in metabolic disorders or toxic encephalopathy. Conversely, focal slowing is confined to a specific area, suggesting a localized issue such as a stroke, tumor, or structural lesion underneath that area.
The concept of asymmetry is also a powerful indicator of pathology, referring to a significant difference in the amplitude or frequency of activity between the two hemispheres. If the normal Alpha rhythm is present on one side but markedly reduced or absent on the other, it can point to a structural problem or injury affecting the less active hemisphere. The presence of triphasic waves, which are distinctive, high-amplitude, three-phase complexes, suggests a severe metabolic derangement, such as hepatic encephalopathy.
Recognizing Artifacts and Noise
Artifacts are electrical signals that appear on the EEG tracing but do not originate from the brain itself. Identifying and distinguishing these non-cerebral signals is necessary to prevent them from being mistaken for genuine brain activity, which could lead to misinterpretation. Artifacts are generally divided into two types: physiological and external.
Physiological artifacts are generated by the patient’s body. The most common is muscle artifact (EMG), which appears as fast, jagged, low-voltage, high-frequency activity, often seen over the temporal and frontal regions due to jaw clenching or scalp muscle tension. Another frequent physiological artifact is the eye movement artifact (EOG), which creates large, slow-wave deflections, particularly in the frontal electrodes, as the eyeball’s charge dipole moves during blinking or lateral gazing.
External artifacts originate from sources outside the patient’s body, typically the environment or the equipment itself. A common example is 60 Hz AC noise (50 Hz in some regions), which appears as a rhythmic, high-frequency interference pattern caused by nearby electrical power lines or equipment. Another frequent issue is electrode popping, which presents as an abrupt, transient spike in a single channel, usually caused by a sudden, temporary change in the electrical contact between the electrode and the scalp. Recognizing these characteristic appearances allows the reader to isolate the true activity of the brain.