An electroencephalogram (EEG) records voltage differences detected by electrodes on the scalp over time. It is not a brain image or a readout of thoughts: clinicians interpret its changing waveforms in light of recording quality, whether a person is awake or asleep, symptoms, and other clinical information.
What an EEG records
Scalp electrodes detect small electrical potential differences between selected locations. The system amplifies and digitizes those signals, then displays them as channels over time. A montage specifies which electrodes are compared in each channel; changing the montage changes how the same recording is viewed, not what was recorded. Digital recordings can be reformatted for review. [ACNS technical guidance]
Because the electrodes sit on the scalp, an EEG describes electrical activity as detected there. It does not show brain anatomy like an imaging scan. The number and placement of electrodes affect how well the recording samples activity across the scalp.
How a clinical EEG is recorded
1. The team records context
The clinical question guides the session. The record includes identifying and clinical information, such as why the test was requested, relevant event history, the person’s state, and medications. These details help the reader distinguish a finding that matters from one that may be expected in that setting. [ACNS technical guidance]
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2. Electrodes are placed in a standardized pattern
A technologist prepares the scalp and applies electrodes, commonly using paste or gel, or a suitable cap. The conventional placement approach is the international 10–20 system. The joint International Federation of Clinical Neurophysiology (IFCN) and International League Against Epilepsy (ILAE) standards, published in 2023, suggest using their 25-electrode array when feasible; otherwise, the 10–20 array is acceptable. These are professional recording recommendations, not a guarantee that every clinic uses the same setup. [IFCN-ILAE standards]
3. Contact and signal quality are checked
The technologist checks electrode connections and signal quality. The 2023 IFCN-ILAE standards suggest electrode impedance below 5 kΩ and consider below 10 kΩ acceptable, while emphasizing balanced impedance. They also note that evidence relating impedance levels to expert-perceived signal quality is limited. These are technical recommendations for clinical acquisition, not steps for a patient to perform. [IFCN-ILAE standards]
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4. The system acquires the signal
Amplification, digitization, calibration, filters, and other acquisition settings affect the recorded scale and which frequencies are visible. The 2023 IFCN-ILAE standards propose a minimum sampling rate of 256 Hz for routine EEG. That figure is a professional technical recommendation, not a measure of diagnostic accuracy or a patient-facing target. [IFCN-ILAE standards] [ACNS technical guidance]
5. The session may capture different states or events
Depending on the clinical question and local protocol, a session may include awake recording, sleep, eye opening and closure, photic stimulation, or hyperventilation. Video, ECG, muscle activity (EMG), or eye-movement channels may also be recorded to help relate the EEG trace to what was happening. The clinical team chooses these procedures; do not try to reproduce activation procedures at home. [IFCN-ILAE standards]
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How clinicians interpret the tracing
A trained reader first considers whether the recording is technically reliable, then examines the signal’s background, waveform shape and distribution, changes over time, and relationship to wakefulness, sleep, or observed events. If video or auxiliary channels are available, these can help determine whether a waveform coincides with a movement or other event. Findings are reviewed in appropriate montages because the channel comparisons affect how patterns appear. The interpretation is then related to the reason for the test and the person’s clinical information. [ACNS technical guidance] [IFCN-ILAE standards]
Artifacts must be separated from cerebral activity
Not every visible waveform comes from the brain. Blinking and eye movement, muscle activity, movement, sweat, poor electrode contact, and electrical equipment can create signals that obscure or resemble cerebral activity. A suspicious pattern is assessed across channels and montages, with signal quality and video or session context considered before it is classified. [IFCN-ILAE standards] [EEG atlas: artifacts]
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What a normal EEG does—and does not—mean
A normal routine EEG means that the recording did not show an abnormality identified by the reader in that session. It does not, by itself, prove that a suspected condition is absent or settle whether a person’s episodes are seizures. EEG samples electrical activity during a particular recording; it must be interpreted alongside symptoms and other clinical information.
Sleep can reveal findings that are not seen during an awake session. The 2023 IFCN-ILAE standards report that epileptiform discharges are more frequent during NREM sleep than wakefulness and that sensitivity for detecting them increases with repeated EEG recordings. For a second study, the group recommends a sleep EEG. The standards also characterize the overall evidence quality as low and their recommendations as conditional and consensus-based. [IFCN-ILAE standards]
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Recording standards and clinical judgment
Technical guidance describes how to acquire a useful, interpretable recording; it does not replace clinical judgment. The American Clinical Neurophysiology Society (ACNS) lists its “Minimum Technical Requirements for Performing Clinical EEG” as revised in August 2016. Its guideline index lists a November 2025 update to guideline materials, so professional guidance may evolve and local protocols can differ. [ACNS guideline index]
This overview explains the recording and interpretation process; it cannot diagnose an individual. A clinician should explain what a specific report means in the context of the symptoms and reason for testing.
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