Give me detailed notes of EEG basics. Like a foundation for undertsadning and starting to learn EEG.

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International Federation of Clinical Neurophysiology EEG recording standards electrode placement 10-20 system guideline

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EEG 10-20 electrode placement diagram scalp montage

This anatomical diagram displays a three-dimensional head model illustrating a functional Near-Infrared Spectroscopy (fNIRS) optode montage. The setup is mapped according to the international 10-20 EEG system (represented by large orange dots) and the extended 10-10 system (small orange dots). Three red dots represent light source optodes placed at locations FC3, C3, and CP3. Six blue dots represent detector optodes positioned at FC1, C1, CP1, FC5, C5, and CP5. Red lines connecting the sources and detectors visualize 14 active fNIRS channels, where each line represents a specific source-detector pair across the scalp. The montage is localized over the left-hemispheric fronto-parietal region, specifically covering the sensorimotor cortex, including the premotor and parietal areas. This visual provides an educational representation of neuroimaging electrode/optode placement and the geometric relationship between sources and detectors in cognitive neuroscience research and Brain-Computer Interface (BCI) applications.

EEG basics can be learned in a logical sequence:

What EEG measures - summed postsynaptic cortical electrical activity recorded from the scalp
Waveform language - frequency, amplitude, morphology, polarity, field, synchrony, and reactivity
Electrode placement - International 10-20 system and the meaning of labels such as Fp1, F3, C3, O1
Technical fundamentals - referential versus bipolar montages, sensitivity, filters, impedance, sampling, and calibration
Normal awake EEG - posterior dominant rhythm, beta activity, eye opening response, drowsiness
Normal sleep EEG - vertex waves, sleep spindles, K-complexes, slow-wave sleep, REM
Artifacts - eye movements, blink, muscle, ECG, sweat, electrode pop, movement, and 50/60-Hz interference
Abnormal EEG terms - focal/generalized slowing, epileptiform discharges, periodic patterns, electrographic seizures
Clinical interpretation - how to systematically read an EEG and correlate it with the patient
Limits and applications - epilepsy, encephalopathy, coma/ICU monitoring, and why a normal EEG does not exclude epilepsy

Foundation overview

EEG records very small voltage differences at scalp electrodes. It does not directly record individual neuronal action potentials. The signal largely reflects synchronized postsynaptic activity of large populations of cortical pyramidal neurons. Scalp EEG is therefore strongest for superficial cortical activity and less sensitive for deep or very small seizure foci.

Electrodes are placed using reproducible skull landmarks in the 10-20 system. Letters indicate region: Fp = frontopolar, F = frontal, C = central, P = parietal, O = occipital, T = temporal. Odd numbers are left sided, even numbers right sided, and z means midline. Standardized placement matters because localization depends on comparing the distribution, field, phase reversal, and evolution of activity across channels. The ACNS electrode nomenclature guideline describes the standard system.

A beginner should first distinguish normal rhythms and state changes from pathology. In a relaxed awake adult with eyes closed, the key normal rhythm is the posterior dominant rhythm, typically around 8-13 Hz and maximal over the occipital regions. It should attenuate with eye opening. Drowsiness and sleep introduce organized normal patterns, including vertex waves, spindles, and K-complexes. These can look unusual until you understand the state of the patient.

Before diagnosing a waveform, exclude artifact and inspect its field. Ask: Is the activity biologically plausible? Does it have a spatial distribution across neighboring electrodes? Is it linked to blinking, eye movement, muscle tension, ECG, or a faulty electrode? Then describe it before naming it: onset, duration, frequency, amplitude, morphology, location, symmetry, reactivity, and evolution.

For epilepsy, an EEG may show interictal epileptiform discharges such as spikes, sharp waves, spike-wave complexes, or polyspikes. But a sharp-looking transient alone is not enough. True epileptiform activity usually has an appropriate field and background disruption. A seizure is usually identified by a sustained evolving rhythmic pattern, with change in frequency, amplitude, morphology, and/or spatial spread, ideally with clinical correlation.

Textbook sources emphasize normal activity, recording technique, artifact recognition, and clinical correlation as the core of introductory EEG interpretation: Adams and Victor’s Principles of Neurology, “Electroencephalography (EEG)”; Bradley and Daroff’s Neurology in Clinical Practice, “Physiological Principles of Electroencephalography.”

Which section should I turn into detailed study notes first?

EEG signals and waveforms
10-20 electrodes and montages
Normal awake and sleep EEG
Artifacts and troubleshooting
Epileptiform discharges and seizures
A complete beginner-to-reader guide

Give me a complete beginner-to-reader EEG study guide

A complete EEG guide spans basic neurophysiology, technical recording, normal variants, and clinical interpretation. To make it useful rather than a very long surface-level note, here is the learning map.

Asking for Preferences

Which EEG module should I cover in detailed beginner-to-reader notes next?

EEG basics can be learned in a logical sequence:

What EEG measures - summed postsynaptic cortical electrical activity recorded from the scalp
Waveform language - frequency, amplitude, morphology, polarity, field, synchrony, and reactivity
Electrode placement - International 10-20 system and the meaning of labels such as Fp1, F3, C3, O1
Technical fundamentals - referential versus bipolar montages, sensitivity, filters, impedance, sampling, and calibration
Normal awake EEG - posterior dominant rhythm, beta activity, eye opening response, drowsiness
Normal sleep EEG - vertex waves, sleep spindles, K-complexes, slow-wave sleep, REM
Artifacts - eye movements, blink, muscle, ECG, sweat, electrode pop, movement, and 50/60-Hz interference
Abnormal EEG terms - focal/generalized slowing, epileptiform discharges, periodic patterns, electrographic seizures
Clinical interpretation - how to systematically read an EEG and correlate it with the patient
Limits and applications - epilepsy, encephalopathy, coma/ICU monitoring, and why a normal EEG does not exclude epilepsy

Foundation overview

Which section should I turn into detailed study notes first?

EEG signals and waveforms
10-20 electrodes and montages
Normal awake and sleep EEG
Artifacts and troubleshooting
Epileptiform discharges and seizures
A complete beginner-to-reader guideA complete EEG guide spans basic neurophysiology, technical recording, normal variants, and clinical interpretation. To make it useful rather than a very long surface-level note, here is the learning map.A complete beginner-to-reader EEG guide includes:
EEG signal foundations
What scalp EEG records, cortical generators, voltage, frequency, polarity, synchrony, and why deep sources may be missed.
Electrodes and the 10-20 system
Scalp landmarks, electrode names, left-right and midline labels, referential electrodes, and localization basics.
Montages and display settings
Bipolar and referential montages, phase reversal, sensitivity, time base, low-frequency and high-frequency filters, notch filter, impedance, and sampling.
Normal background EEG
Posterior dominant rhythm, beta, theta, delta, symmetry, continuity, reactivity, and expected changes with eye opening and activation.
Drowsiness and sleep
Normal transitions through N1, N2, N3, and REM: vertex waves, spindles, K-complexes, slow waves, and REM features.
Artifacts and benign variants
Eye movements, muscle, ECG, sweat, movement, electrode pop, electrical interference, plus common normal variants that can mimic epileptiform activity.
Abnormal background patterns
Focal slowing, generalized slowing, attenuation, asymmetry, discontinuity, periodic patterns, and their broad clinical implications.
Epileptiform discharges and seizures
Spikes, sharp waves, spike-wave complexes, focal versus generalized patterns, seizure evolution, and the difference between interictal and ictal findings.
Systematic EEG reading
A reproducible reading order, how to form an impression, and how to avoid overcalling normal activity or artifact as epilepsy.
Clinical use and limitations
Role in epilepsy, altered consciousness, encephalopathy, ICU monitoring, and why a normal routine EEG does not rule out epilepsy.

High-level overview: EEG reading is primarily pattern recognition anchored in physiology and technical quality. Beginners progress fastest by first learning normal wakefulness, drowsiness, and sleep, then artifacts, and only then epileptiform abnormalities. The key discipline is to describe before interpreting: identify the waveform’s frequency, amplitude, morphology, location, field, reactivity, and evolution before assigning a diagnostic label.

Reply with the number you want first. I recommend 1. EEG signal foundations, then proceeding in order.

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EEG: Complete Beginner-to-Reader Study Guide

Purpose: Build the foundation to understand routine scalp EEG and begin reading it systematically. This is educational material, not a substitute for supervised clinical EEG training or formal reporting by a qualified electroencephalographer.

1. What is EEG?

Electroencephalography (EEG) records electrical potential differences from electrodes placed on the scalp.

It is a continuous recording of voltage over time:

Horizontal axis: time
Vertical axis: voltage, usually microvolts (µV)
Each line: a recording channel, representing the voltage difference between electrodes or between one electrode and a reference

EEG is not a direct recording of individual neuronal action potentials. Scalp EEG mainly reflects the summed, synchronized excitatory and inhibitory postsynaptic potentials of large populations of cortical pyramidal cells. The cortex must generate a large, synchronized signal for it to become visible at the scalp. Thalamocortical and brainstem networks help organize rhythms such as alpha activity and sleep spindles. Adams and Victor’s Principles of Neurology, 12th ed., p. 40.

What scalp EEG detects well

Superficial cortical electrical activity
Generalized cerebral dysfunction
State changes: wakefulness, drowsiness, sleep
Many focal or generalized epileptiform discharges
Electrographic seizures, especially with continuous video-EEG monitoring

What it detects poorly

Very deep sources, such as mesial temporal, deep frontal, cingulate, or insular sources
Very small cortical areas
Brief, infrequent events during a short recording
Structural lesions themselves

A normal EEG does not rule out epilepsy. It means no diagnostic epileptiform abnormality or seizure was captured under the recording conditions.

2. Core EEG language

Learn to describe a waveform before trying to name it.

Feature	What it means	Questions to ask
Frequency	Cycles per second, Hz	Is it delta, theta, alpha, beta, or faster?
Amplitude	Height of waveform, µV	Is it low, normal, or high voltage?
Morphology	Shape	Sinusoidal, sharp, spiky, rhythmic, polymorphic?
Location	Where it is maximal	Frontal, temporal, central, parietal, occipital?
Distribution	How broadly it spreads	Focal, regional, unilateral, bilateral, generalized?
Symmetry	Left versus right comparison	Is the pattern similar on both sides?
Synchrony	Timing between regions	Are bilateral discharges simultaneous?
Reactivity	Change with stimulation or state	Does it change with eye opening, voice, touch, sleep?
Evolution	Meaningful change over time	Does frequency, amplitude, morphology, or location change?
Field	Spatial distribution across electrodes	Is there a physiologically believable scalp pattern?

Frequency bands

Band	Frequency	Usual context
Delta	<4 Hz	Normal in deep sleep; abnormal when prominent in an awake adult, especially if focal
Theta	4-7 Hz	Normal in drowsiness and children; may reflect dysfunction if persistent while awake in adults
Alpha	8-13 Hz	Posterior dominant rhythm in relaxed awake adults with eyes closed
Beta	>13 Hz	Low-amplitude fast activity, often frontocentral; increased by benzodiazepines and some sedatives
Gamma	>30 Hz	Often difficult to assess in routine scalp EEG because muscle artifact contaminates this range

Frequency alone never makes a pattern normal or abnormal. For example, delta is normal in N3 sleep but can indicate focal cortical dysfunction in a fully awake adult.

3. EEG polarity: why “up” and “down” can mislead you

By standard display convention:

Negative at input 1 produces an upward deflection.
Positive at input 1 produces a downward deflection.

However, beginners should not overinterpret whether a wave points up or down. The direction changes when the montage changes. The more useful early skill is identifying:

Where the waveform has its greatest amplitude
How it distributes across neighboring electrodes
Whether it reverses phase in a bipolar montage
Whether it has a believable field
Whether it evolves over time

4. The international 10-20 electrode system

Routine scalp EEG commonly uses the International 10-20 system. “10” and “20” refer to electrode distances measured as 10% or 20% of specific skull dimensions. This creates reproducible electrode positions rather than placing electrodes by visual estimation.

The system uses standard cranial landmarks:

Nasion: bridge of the nose
Inion: palpable prominence at the back of the skull
Preauricular points: just anterior to each ear

The system is the internationally standardized foundation for routine scalp EEG. The ACNS electrode nomenclature guidance describes 21 standard placements and extensions such as the 10-10 system.

International 10-20 electrode placement and EEG channels

EEG electrode placement and channel display. Adapted from Adams and Victor’s Principles of Neurology, 12th ed., p. 40.

Electrode labels

Letter	Region
Fp	Frontopolar
F	Frontal
C	Central
P	Parietal
O	Occipital
T	Temporal
A or M	Auricular or mastoid reference area
z	Midline

Numbering

Odd numbers: left hemisphere
Even numbers: right hemisphere
z: midline, for example Fz, Cz, Pz

Examples:

Fp1: left frontopolar
F3: left frontal
C3: left central
P4: right parietal
O2: right occipital
F7: left anterior temporal/frontotemporal
T7: left mid-temporal
P7: left posterior temporal
F8, T8, P8: corresponding right-sided sites

Important localization caution

Electrode names are approximate surface labels, not exact anatomical maps. For example, a temporal discharge at the scalp does not prove that the seizure originates only in the temporal lobe. EEG localization requires the full spatial field, multiple montages, clinical semiological correlation, imaging, and sometimes invasive EEG.

5. What is a montage?

A montage is the way electrode signals are arranged and compared on the display.

The same underlying EEG can look different in different montages. Do not diagnose an abnormality from a single montage alone.

A. Bipolar montage

Each channel compares two neighboring electrodes, for example:

Fp1-F7
F7-T7
T7-P7
P7-O1

Uses

Excellent for seeing local fields
Helpful for recognizing phase reversal
Helps distinguish focal activity from widespread artifact

B. Referential montage

Each scalp electrode is compared with a common reference, for example:

F3-Avg
C3-Avg
P3-Avg
O1-Avg

Common references include average reference, linked ears/mastoids, Cz, or other laboratory-specific references.

Uses

Helps identify areas of maximal amplitude
Useful for generalized patterns
Can make a focal discharge’s distribution easier to see

Limitation: if the reference itself contains abnormal activity, it can distort the appearance of multiple channels.

C. Why you must review several montages

A real cerebral waveform should remain recognizable when you change montage, even though its shape and polarity may change. A misleading reference or localized electrode artifact may become obvious when viewed another way.

6. Phase reversal

A phase reversal occurs when a waveform changes polarity between two adjacent bipolar channels sharing a common electrode.

Example:

F7-T7      ↑
T7-P7      ↓

The reversal suggests that the voltage maximum is around the shared electrode, here T7.

Key warning

Phase reversal indicates a maximum of voltage in that montage. It is not automatically the “site of seizure onset,” nor does it prove the generator lies exactly beneath that electrode.

Think:

Phase reversal = local maximum on the scalp display, not a final anatomical diagnosis.

7. EEG technical basics

A technically poor EEG can produce false abnormalities or hide genuine findings.

Sensitivity or gain

Sensitivity tells you how much waveform height is displayed for a given voltage.

It is usually expressed in µV/mm.

Lower µV/mm number = more sensitive display = waveform looks taller
Higher µV/mm number = less sensitive display = waveform looks shorter

Example:

At 5 µV/mm, a 50 µV wave spans 10 mm.
At 10 µV/mm, the same 50 µV wave spans 5 mm.

Routine recording commonly begins around 5-10 µV/mm, though settings should be adjusted to display low- or high-amplitude activity appropriately. The ACNS minimum technical guideline recommends digital recording and standard 10-20 electrode placement for routine clinical EEG.

Time base

Time base determines how much time is displayed across the screen.

A faster display spreads waveforms out and helps inspect morphology.
A slower display compresses time and helps identify rhythmicity, periodicity, and seizure evolution.

Filters

Filters alter the displayed signal and must be used carefully.

Filter	Main purpose	Main danger
Low-frequency filter, also called high-pass	Reduces very slow drift, sweat artifact	Can attenuate or distort slow waves and long-duration components
High-frequency filter, also called low-pass	Reduces high-frequency noise and muscle artifact	Can blunt spikes and fast activity
Notch filter	Reduces 50/60-Hz line noise	Can mask an underlying technical issue and alter signal appearance

A beginner rule:

First inspect the raw or minimally filtered signal. Do not solve every problem by applying stronger filters.

Impedance

Impedance is resistance to current flow at the electrode-skin interface.

High or unstable impedance may cause:

Excessive artifact
Poor signal quality
Electrode pops
Increased 50/60-Hz interference

The practical goal is stable, appropriately low impedance according to the laboratory protocol and equipment manufacturer.

Sampling rate and digital EEG

Digital EEG enables review with different montages, gain, filters, and time bases after recording. This is an advantage, but post-processing does not replace good acquisition. If an electrode is detached or a patient is moving, lost signal quality cannot be completely restored later.

8. Routine EEG protocol and activation procedures

Routine EEG is a short sample of brain activity. A longer recording, sleep, and appropriate activation procedures increase the chance of capturing abnormalities. The ACNS guidance identifies at least 20 minutes of artifact-free recording including activation procedures as a minimum baseline, while longer recordings improve yield.

Usual recording conditions

Relaxed wakefulness, often eyes closed
Eye opening and eye closure
Drowsiness and, when possible, sleep
ECG channel
Video when available
Documentation of medications, sleep deprivation, clinical events, and activation procedures

Hyperventilation

Usually involves deep breathing for about 3 minutes when safe.

Expected effect: generalized high-amplitude theta and delta slowing, especially in children and younger people.

Clinical value: may provoke generalized spike-wave activity, particularly in absence epilepsy.

Safety: should be avoided or modified in selected patients, such as those with significant cardiopulmonary disease, cerebrovascular risk, or other contraindications determined by the clinical team.

Intermittent photic stimulation

Flashing light is presented at a sequence of frequencies, generally with eyes open and closed.

Possible responses:

Photic driving: normal posterior rhythmic response time-locked to flashes
Photoparoxysmal response: epileptiform discharges provoked by flashes, sometimes generalized and sometimes associated with myoclonus or seizures

Do not confuse normal photic driving with a photoparoxysmal response.

Sleep

Sleep is one of the most useful “activators” because it can reveal interictal epileptiform discharges that are absent in wakefulness.

9. Normal awake EEG

Posterior dominant rhythm, PDR

The posterior dominant rhythm, formerly called posterior alpha rhythm, is the main normal awake rhythm to learn first.

Typical adult features:

Around 8-13 Hz
Best seen posteriorly at O1 and O2
Usually symmetric or only mildly asymmetric
Most apparent when the person is relaxed with eyes closed
Attenuates or blocks with eye opening and alerting

The PDR is assessed by:

Frequency
Amplitude
Symmetry
Reactivity to eye opening
Organization of the overall background

A slow PDR in an alert adult can suggest diffuse cerebral dysfunction, but interpretation depends on age, drowsiness, medication effects, and technical factors.

Beta activity

Low-amplitude fast activity, usually frontocentral
Often symmetric
Can increase with benzodiazepines, barbiturates, and some sedating medicines
May also reflect muscle artifact if it is irregular, very fast, and concentrated over temporal/frontal areas

Mu rhythm

A normal central rhythm, typically 8-13 Hz, often seen over C3/C4.

Features:

Arch-shaped or “comb-like”
May look sharp
Blocks with movement or intention to move the contralateral hand
Can be mistaken for epileptiform activity by beginners

Lambda waves

Sharp transients in occipital leads during visual scanning, particularly while awake with eyes open.

They disappear when eyes close and are non-epileptic.

10. Drowsiness and normal sleep EEG

State recognition is mandatory. Many normal sleep waveforms look sharp or high amplitude.

Drowsiness, N1 sleep

Typical changes:

Posterior dominant rhythm becomes less organized and fades
Generalized theta activity increases
Slow rolling eye movements may appear
Vertex sharp transients may occur

Vertex waves

Brief sharp transients maximal at the vertex, often Cz
Common in drowsiness and light sleep
Can be asymmetric but should not have a consistent pathological field

N2 sleep

Defined by sleep spindles and/or K-complexes.

Sleep spindles

Brief waxing and waning bursts
Usually around 11-16 Hz
Often maximal over central regions
Bilateral and generally symmetric

Spindles are generated through thalamocortical network activity. Neuroscience: Exploring the Brain, 5th ed., sleep EEG section.

K-complexes

Large, biphasic waveform
May occur spontaneously or after an auditory stimulus
Often followed by a sleep spindle
Normal in N2 sleep

N3 sleep

High-amplitude delta activity
Deep sleep
Slow waves are broadly distributed and normal in this state

REM sleep

Low-voltage mixed-frequency EEG, sometimes resembling wakefulness
Rapid eye movements
Reduced chin muscle tone on polysomnography
Sawtooth waves may occur

A beginner safeguard

Before labeling a sharp transient as “epileptiform,” ask:

Is this a vertex wave, K-complex, spindle-related transient, or other normal sleep feature?

11. Artifacts: the first differential diagnosis

An artifact is recorded activity not generated by cerebral cortex. It may be physiologic, arising from the patient, or extraphysiologic, arising from equipment or the environment.

Artifacts are common. A reader who cannot recognize them will overcall epilepsy.

A. Eye blink and eye movement artifact

Features:

Prominent in Fp1 and Fp2
Large, slow frontal deflections
Often symmetric with blinking
Horizontal eye movements produce opposite polarity across left and right frontal electrodes

Eye artifact is strongest at frontal poles and attenuates posteriorly. Kaplan & Sadock’s Comprehensive Textbook of Psychiatry, “EEG Artifacts.”

B. Muscle, EMG artifact

Features:

Fast, irregular, low-amplitude or high-amplitude activity
Common in frontal and temporal electrodes
Seen with jaw clenching, talking, chewing, swallowing, frowning, or tension
May obscure real cerebral activity

Ask the patient to relax the jaw and facial muscles. Do not simply apply strong low-pass filtering, because filtering can also blunt real fast EEG activity.

C. ECG artifact

Features:

Repetitive waveform synchronized with heart rate
Often greatest near ear, temporal, or lower electrodes
Confirm by comparing with the ECG channel

D. Sweat artifact

Features:

Very slow, drifting baseline movement
Often frontal
May improve as the patient cools or the electrode-skin interface is corrected

E. Electrode pop

Features:

Abrupt, often high-amplitude transient
Usually isolated to one electrode or a small number of channels
May have a vertical or sharply discontinuous appearance
Often no plausible cerebral field

F. Movement artifact

Features:

Often large amplitude and irregular
May involve many channels simultaneously
Check video and technologist annotations

G. 50/60-Hz line noise

Features:

Regular, monomorphic fast oscillation
Usually widespread
May occur in one region if a specific electrode is poorly connected

Do not use the notch filter as the only response. Check electrode quality and environmental electrical sources first.

Artifact checklist

When a suspicious waveform appears, ask:

Is it confined to one electrode?
Does it have a credible field in adjacent electrodes?
Does it correlate with video, ECG, EOG, EMG, breathing, or movement?
Does it persist in another montage?
Is it state-linked or stimulus-linked?
Does its morphology look too abrupt, too perfectly regular, or technically implausible?

12. Normal variants that can mimic epileptiform activity

These are commonly encountered patterns that should not be automatically labeled as epileptiform.

Pattern	Typical clue
Mu rhythm	Central, arch-shaped, blocks with contralateral movement
Lambda waves	Occipital, during visual scanning with eyes open
Wicket rhythm	Temporal arch-like rhythm, no after-going slow wave, often in older adults
Rhythmic mid-temporal theta of drowsiness	Drowsiness-related, brief temporal theta bursts without evolution
Benign small sharp spikes	Low-amplitude, brief temporal transients in drowsiness/light sleep, usually no disruptive slow wave
14-and-6-Hz positive bursts	Brief posterior temporal positive bursts, usually in youth
Breach rhythm	High-voltage, sharp-contoured fast activity near a skull defect; non-epileptic by itself

A general principle:

Epileptiform discharges typically have a physiologic field and are often followed by a slow wave or background disruption. Benign variants commonly lack these features.

This is a guide, not a standalone rule. Formal interpretation requires repeated review across states and montages.

13. Abnormal background activity

Background abnormalities usually indicate cerebral dysfunction but are often nonspecific.

Diffuse slowing

Generalized excess theta or delta activity in a properly awake adult may reflect diffuse encephalopathy.

Potential causes include:

Toxic-metabolic disturbance
Sedative or anesthetic medications
Systemic infection or organ failure
Diffuse hypoxic-ischemic injury
Neurodegenerative disease
Reduced alertness or sleep, which must be excluded first

Describe:

Frequency and amount of slowing
Continuity
Reactivity
Symmetry
State dependence
Presence of superimposed epileptiform activity

Focal slowing

Focal polymorphic delta or theta slowing, especially persistent over one region, may indicate focal cerebral dysfunction.

Potential causes include:

Structural lesion
Postictal state
Focal ischemia
Hemorrhage
Tumor
Infection/inflammation
Focal cortical dysfunction

Focal slowing is not synonymous with epilepsy. It is a sign of focal dysfunction and needs clinical/imaging correlation.

Asymmetry

Asymmetry can involve:

Amplitude
Frequency
Organization
Reactivity
Continuity

Persistent focal attenuation may reflect cortical dysfunction, a lesion, or an extracranial issue such as a skull defect or poor electrode contact. Always check montage and artifact before assigning clinical meaning.

Attenuation, suppression, and discontinuity

These describe reduced or interrupted background activity and are particularly important in critically ill, sedated, or post-anoxic patients. Interpretation depends strongly on clinical context, medications, temperature, and reactivity. This is advanced EEG territory and should be interpreted with standardized critical-care EEG terminology.

14. Epileptiform discharges

An interictal epileptiform discharge suggests increased seizure propensity but is not itself a seizure.

Common terms

Term	Approximate duration	Description
Spike	<70 ms	Pointed transient with rapid rise and fall
Sharp wave	70-200 ms	Pointed transient, broader than a spike
Spike-wave complex	Variable	Spike followed by a slow wave
Polyspike	Multiple spikes	Cluster of spikes, often followed by slow wave
Sharp-and-slow-wave complex	Variable	Sharp wave followed by a slow wave

Duration is only one criterion. A waveform should not be called epileptiform merely because it is sharp.

Features supporting an epileptiform discharge

Distinctive sharp or spiky morphology
Clear physiologic field across adjacent electrodes
Voltage maximum in a plausible location
Background disruption
Often an after-going slow wave
Consistency across montages
Occurrence in a characteristic state, often drowsiness or sleep
Clinical correlation with an epilepsy syndrome when applicable

Features arguing against an epileptiform discharge

Only one electrode is involved
No field in surrounding channels
Perfectly stereotyped technical appearance
Clear relation to eye movement, ECG, muscle, or movement
Disappears or changes implausibly with montage
Occurs only in a setting suggestive of artifact

Focal epileptiform discharges

These are localized or regional, for example:

Left temporal sharp waves
Right frontal spikes
Centrotemporal spikes

They suggest a focal region of epileptogenic potential but do not alone define the seizure onset zone.

Generalized epileptiform discharges

These are bilaterally synchronous and broadly distributed, often frontally predominant.

Examples include:

Generalized spike-wave
Polyspike-wave
Generalized paroxysmal fast activity

They can support a generalized epilepsy syndrome when interpreted alongside age, seizure type, history, and imaging.

15. What makes an electrographic seizure?

A seizure is not just “repetitive spikes.” The key concept is evolution.

An electrographic seizure usually shows a sustained abnormal pattern that evolves in one or more of:

Frequency
Amplitude
Morphology
Spatial distribution
Rhythmicity

A typical progression might look like:

Rhythmic theta in right temporal region
→ increases in amplitude
→ frequency changes
→ spreads to adjacent regions
→ slows and terminates

Think in terms of evolution

Feature	More consistent with seizure	Less consistent with seizure
Duration	Sustained, usually tens of seconds or longer	Single brief transient
Evolution	Clear change over time	Static, unchanging pattern
Field	Plausible regional spread	One-electrode phenomenon
Clinical correlation	May have behavioral or motor change	No correlation does not exclude seizure, especially in ICU
Offset	Clear termination or postictal slowing	Abrupt technical disappearance

A focal seizure can be scalp-negative, particularly if it is deep, small, or obscured by artifact. Conversely, not every rhythmic EEG pattern is a seizure.

16. A systematic method to read a routine EEG

Use the same order every time. This reduces missed findings and overcalling.

Step 1: Confirm clinical context

Review:

Age
Indication
Event description
Epilepsy diagnosis and seizure types
Medications, especially antiseizure drugs and sedatives
Wakefulness, sleep deprivation, and sleep during the study
Brain imaging and previous EEGs
Recent seizure or postictal state

Step 2: Check technical quality

Is the recording sufficiently long?
Are all standard electrodes present?
Is the ECG channel visible?
Are video and event markers available?
Is there excessive artifact?
Are settings and montages appropriate?
Are there loose or noisy electrodes?

Step 3: Identify state

Determine whether the patient is:

Awake
Drowsy
N1, N2, N3, or REM sleep
Sedated
Stuporous/comatose

Never call diffuse slowing abnormal until you know whether the patient is drowsy or asleep.

Step 4: Describe the posterior dominant rhythm

In an awake adult, document:

Frequency
Symmetry
Organization
Reactivity to eye opening
Whether it is appropriate for age and state

Step 5: Evaluate background organization

Assess:

Continuity
Symmetry
Amount and distribution of theta/delta
Beta activity
Reactivity
Focal attenuation or excess fast activity
Whether normal sleep elements are present when expected

Step 6: Review activation procedures

Hyperventilation response
Photic driving
Photoparoxysmal response
Eye opening/closure
Sleep

Step 7: Search for focal abnormalities

Look for:

Focal slowing
Asymmetry
Attenuation
Focal spikes or sharp waves
Rhythmic focal activity
A consistent maximum and field

Step 8: Search for generalized abnormalities

Look for:

Generalized slowing
Generalized spike-wave or polyspike-wave
Periodic or rhythmic patterns
Lack of organization or reactivity

Step 9: Determine whether any event is a seizure

For any suspicious rhythmic pattern, evaluate:

Onset
Evolution
Spread
Duration
Offset
Clinical/video correlate
Post-event background change

Step 10: Write the impression

A basic impression should answer:

Is the EEG normal or abnormal?
If abnormal, what is the main abnormality?
Is there epileptiform activity?
Were electrographic seizures recorded?
What is the most appropriate clinical interpretation, stated cautiously?

17. Template for a beginner’s structured EEG description

Background

The awake background is organized, symmetric, and continuous. 
A posterior dominant rhythm of approximately __ Hz is present over the occipital regions and attenuates with eye opening.
Low-amplitude beta activity is seen frontocentrally.

Sleep

Drowsiness and N2 sleep are captured, with vertex waves, symmetric sleep spindles, and K-complexes.

Activation

Hyperventilation produces expected generalized slowing without epileptiform activation.
Photic stimulation produces symmetric photic driving without photoparoxysmal response.

Abnormalities

Intermittent polymorphic delta slowing is present over the left temporal region.
No definite epileptiform discharges are identified.
No electrographic seizures occur.

Impression

Abnormal EEG due to intermittent left temporal slowing, indicating focal cerebral dysfunction in this region. No epileptiform discharges or electrographic seizures are recorded.

Do not copy this format without adapting it to what is actually present.

18. Common beginner mistakes

Calling every sharp waveform a spike
Review morphology, field, after-going slow wave, state, and artifact correlation.
Ignoring sleep stage
Vertex waves and K-complexes can be mistaken for epileptiform activity.
Reading only one montage
Switch between bipolar and referential displays.
Treating phase reversal as exact localization
It identifies a scalp voltage maximum, not necessarily the source.
Ignoring the ECG channel
Cardiac artifact may look sharply periodic.
Calling muscle artifact “fast activity”
Examine distribution and video. Jaw and temporalis EMG are common.
Using filters to hide a problem
First identify the source of noise and inspect minimally filtered data.
Equating a normal EEG with absence of epilepsy
Routine EEG has limited sampling time and may not capture interictal discharges.
Equating interictal spikes with a seizure during the recording
Spikes indicate seizure tendency, while seizures show sustained evolving ictal activity.
Ignoring the patient
EEG is interpreted with the event history, neurological examination, imaging, medications, age, and level of consciousness.

19. Clinical uses and limits of EEG

Major uses

Evaluation of possible seizures and epilepsy
Classification of focal versus generalized epileptiform abnormalities
Detection of nonconvulsive seizures or status epilepticus
Evaluation of altered mental status and encephalopathy
Continuous EEG monitoring in critical care
Sleep studies and polysomnography
Supportive testing in selected conditions, including brain death protocols and Creutzfeldt-Jakob disease

EEG remains central to seizure assessment, encephalopathy evaluation, sleep recording, anesthetic monitoring, and selected diagnostic settings. Adams and Victor’s Principles of Neurology, 12th ed., p. 40.

Major limits

EEG cannot independently:

Diagnose epilepsy solely from symptoms without electroclinical correlation
Exclude epilepsy when normal
Reliably define every seizure onset zone
Replace MRI or CT for structural lesion localization
Determine the cause of diffuse slowing without clinical correlation

20. Suggested learning sequence

Week 1: Signal and electrode basics

Learn frequency bands
Memorize 10-20 locations
Understand left/right and midline naming
Learn bipolar versus referential montage
Practice identifying eye and muscle artifact

Week 2: Normal wake and sleep EEG

Posterior dominant rhythm
Eye opening response
Beta activity
Drowsiness
Vertex waves
Sleep spindles and K-complexes
N3 delta and REM background

Week 3: Abnormal backgrounds and artifacts

Focal versus generalized slowing
Symmetry, continuity, reactivity
Electrode pop, ECG, sweat, EMG, eye movement, line noise
Common normal variants

Week 4: Epileptiform patterns and seizures

Spikes, sharp waves, spike-wave
Focal versus generalized discharges
Field and phase reversal
Ictal evolution
Electroclinical correlation

Practice rule

For each EEG, write five lines before looking at the official report:

State
Background/PDR
Symmetry and reactivity
Artifacts and activation response
Focal, generalized, epileptiform, or ictal findings

Then compare your description with the formal interpretation.

21. One-page revision checklist

Before calling an EEG abnormal, confirm:

I know the patient’s state.
I have checked electrode quality and artifact.
I reviewed more than one montage.
I assessed PDR, symmetry, continuity, and reactivity.
I looked at wakefulness, drowsiness, and sleep.
I considered activation procedures.
The waveform has a credible field.
I considered normal variants.
I distinguish interictal discharges from seizures.
A suspected seizure demonstrates sustained evolution.
My final interpretation is compatible with the clinical context.

Evidence update

A recent systematic review examined AI-assisted EEG interpretation rather than changing the fundamentals of human EEG reading. It does not replace expert review or clinical correlation: PMID 41956141.

2nd, 3rd and 4th please

Reading File

Finding Sources

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electroencephalography AND artifact

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ACNS routine clinical EEG guideline 10-20 electrodes montage sensitivity filters artifact

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EEG electrode artifacts eye blink muscle ECG artifact waveform examples

EEG Study Notes: 2. Electrodes and Montages, 3. Normal Awake EEG, 4. Sleep and Artifacts

2. Electrodes and montages

A. The International 10-20 electrode system

The International 10-20 system is the standard method for positioning scalp EEG electrodes. Its purpose is to place electrodes reproducibly despite differences in head size. Measurements are made between skull landmarks using 10% and 20% intervals, hence the name.

The 10-20 system and examples of EEG channels. Adams and Victor's Principles of Neurology, 12th Edition, p. 40.

Important anatomical landmarks

Nasion: depression at the bridge of the nose
Inion: bony prominence at the posterior skull
Left and right preauricular points: immediately anterior to the ears

Electrodes are measured along:

The nasion-inion line
The left-right preauricular line
Head circumferences and parasagittal arcs

The 10-20 system is based on proportions of these measured distances, not fixed distances in centimeters.

Meaning of electrode names

Prefix	Approximate scalp region
Fp	Frontopolar
F	Frontal
C	Central
P	Parietal
O	Occipital
T	Temporal
A/M	Auricular/mastoid reference area
z	Midline

Left, right, and midline

Odd numbers: left side, for example F3, C3, O1
Even numbers: right side, for example F4, C4, O2
z: midline, for example Fz, Cz, Pz

High-yield electrode positions

Electrode	Region	Practical relevance
Fp1, Fp2	Frontopolar	Very sensitive to blink and eye-movement artifact
F3, F4	Frontal	Frontal activity and frontal slowing
F7, F8	Anterior temporal/frontotemporal	Eye movement and temporal/frontal abnormalities
T7, T8	Mid-temporal	Important for temporal discharges
P7, P8	Posterior temporal	Important in posterior temporal fields
C3, C4	Central	Mu rhythm, sleep spindles, central spikes
Cz	Midline central	Vertex waves and sleep features
P3, P4	Parietal	Parietal activity
O1, O2	Occipital	Posterior dominant rhythm and photic responses

Older versus current temporal terminology

You may see two naming systems:

Older label	Current 10-20 label
T3	T7
T4	T8
T5	P7
T6	P8

Know both. Older reports and textbooks often use T3/T4/T5/T6.

B. Electrodes, channels, and references

An EEG electrode does not independently display “brain activity beneath it.” EEG machines record voltage differences.

A channel is the difference in voltage between two recording inputs.

For example:

F7 - T7

means the channel displays the voltage difference between F7 and T7 over time.

Scalp electrodes are usually silver or silver-silver chloride discs applied with conductive paste or gel. Adams and Victor's Principles of Neurology, 12th Edition, p. 40.

Reference, ground, and electrode confusion

These terms are different:

Active/scalp electrode: electrode over the scalp region being recorded.
Reference electrode: comparator for a referential channel.
Ground electrode: improves electrical safety and reduces common electrical noise. It is not a recording reference.

C. What is a montage?

A montage is the arrangement of channels on the EEG display.

Changing the montage changes the appearance of a waveform, but it does not change the underlying brain activity. Therefore, review suspicious activity in more than one montage.

1. Bipolar montage

In a bipolar montage, each channel compares two nearby electrodes.

Example, left temporal chain:

Fp1-F7
F7-T7
T7-P7
P7-O1

This is a “double banana” or longitudinal bipolar style of display when both hemispheres are arranged in anterior-to-posterior chains.

Advantages

Shows local spatial relationships clearly
Helps identify phase reversal
Helps determine whether activity has a credible field
Useful for focal abnormalities

Limitations

A widespread pattern can be difficult to appreciate
The apparent maximum may be less obvious than in a referential montage

2. Referential montage

In a referential montage, each scalp electrode is compared with the same reference.

Examples:

F3-Avg
C3-Avg
P3-Avg
O1-Avg

Possible references include:

Average reference
Linked ears or linked mastoids
A single mastoid
Cz
Other laboratory-specific references

Advantages

Makes amplitude maxima easier to see
Useful for generalized patterns
Useful for comparing homologous right and left electrodes

Limitations

A contaminated or electrically active reference can create misleading activity across many channels
Activity at the reference can make other electrodes appear falsely abnormal

3. Transverse bipolar montage

This compares electrodes across the head, for example:

Fp1-Fp2
F7-F8
T7-T8
P7-P8
O1-O2

It is useful for comparing left versus right activity and identifying asymmetry.

D. Phase reversal

A phase reversal occurs in a bipolar montage when a waveform changes direction in two adjoining channels that share an electrode.

Example:

F7-T7      upward
T7-P7      downward

The shared electrode is T7, so the voltage maximum is around T7.

The rule

A phase reversal indicates a local voltage maximum in that montage.

It does not by itself prove:

Exact anatomical source
Seizure onset zone
Epileptogenic zone
That the waveform is pathological

A phase reversal may occur with normal rhythms, eye movements, artifact, and cerebral abnormalities. Always inspect the field and check another montage.

E. Field: one of the most important EEG concepts

A field is the spatial distribution of a waveform across electrodes.

Real cerebral activity typically:

Appears in several anatomically neighboring electrodes
Decreases gradually away from its maximum
Is recognizable in several montages
Has a plausible distribution over the scalp

Artifact often:

Appears in only one electrode
Has an abrupt or implausible distribution
Does not spread logically to neighboring electrodes
Is linked to movement, ECG, eye movement, muscle, or loose electrode
Changes dramatically or disappears in another montage

Example: plausible focal temporal field

F7-T7      moderate sharp wave
T7-P7      large sharp wave
P7-O1      smaller sharp wave

This suggests a field centered around T7/P7 rather than a single faulty electrode.

Example: likely electrode artifact

F7-REF     huge abrupt transient
F7-T7      huge abrupt transient
Fp1-F7     huge abrupt transient
Other channels: normal

When every channel containing F7 is affected but neighboring channels that do not contain F7 are normal, suspect a problem at F7.

F. Basic technical controls

Sensitivity or gain

Sensitivity is usually expressed as µV/mm.

Lower number, such as 5 µV/mm: waveform appears taller
Higher number, such as 10 µV/mm: waveform appears shorter

This can initially feel backward. Remember:

Lower µV per mm = more display sensitivity.

A routine starting sensitivity often falls around 5-10 µV/mm, with adjustment based on the amplitude of the EEG. The ACNS technical guidance recommends a routine sensitivity range of 5-10 µV/mm.

Time base

Time base controls how much time is displayed across the screen.

Faster time base: waveform is spread out. Better for inspecting spikes or morphology.
Slower time base: waveform is compressed. Better for identifying rhythmicity, periodic patterns, and seizure evolution.

Filters

Filter	What it reduces	Important risk
Low-frequency filter/high-pass filter	Very slow drift and sweat artifact	May attenuate slow waves or distort long-duration components
High-frequency filter/low-pass filter	Muscle and high-frequency noise	May blunt spikes and fast activity
Notch filter	50/60-Hz electrical interference	May hide a technical issue and alter waveform appearance

Use filters cautiously. The best first response to artifact is often to correct the source, not to filter it away.

3. Normal awake EEG

A. Start with state

Before deciding whether an EEG is normal, establish whether the patient is:

Alert and awake
Relaxed with eyes closed
Drowsy
Asleep
Sedated
Encephalopathic

An awake EEG and a drowsy EEG should not look the same. Misidentifying the state is a major source of beginner errors.

B. Posterior dominant rhythm

The posterior dominant rhythm, or PDR, is the main normal awake rhythm in a relaxed person with eyes closed. It was historically called the alpha rhythm.

Typical features in a healthy awake adult

Frequency approximately 8-13 Hz
Best seen in posterior regions, especially O1 and O2
More apparent with eyes closed
Attenuates, blocks, or becomes less organized with eye opening
Usually symmetric, though minor asymmetry can occur
Often has a smooth or sinusoidal appearance

Blink artifact, posterior alpha rhythm, eye-opening attenuation, and photic driving

The figure demonstrates posterior alpha activity, attenuation with eye opening, and a large frontal blink artifact. Adams and Victor's Principles of Neurology, 12th Edition, p. 40.

How to describe a PDR

Use five components:

Frequency: for example, 10 Hz
Location: posterior/occipital maximum
Symmetry: symmetric or asymmetric
Organization: well or poorly organized
Reactivity: attenuates with eye opening or not

Example:

A symmetric, well-organized 10-Hz posterior dominant rhythm is present,
maximal in the occipital regions and attenuating appropriately with eye opening.

When a PDR may be abnormal

Possible abnormalities include:

PDR too slow for age and alertness
Poorly formed or absent PDR in an alert adult
Marked persistent asymmetry
Lack of reactivity when the patient is clearly awake and cooperative

These findings are nonspecific. They must be interpreted in relation to age, sedation, fatigue, encephalopathy, visual impairment, and recording quality.

C. Beta activity

Beta activity is low-amplitude, fast activity above 13 Hz.

Typical features:

Often maximal over frontal and central regions
Usually symmetric
May become prominent with benzodiazepines, barbiturates, and other sedative drugs
Can be enhanced over a skull defect, called breach effect or breach rhythm

Beta versus muscle artifact

Beta	Muscle artifact
More regular and lower amplitude	Irregular, often more chaotic
Often symmetric frontocentral	Often frontal/temporal and uneven
Cerebral distribution	Linked to jaw clenching, frowning, talking, chewing
Persists when muscles relax	Reduces when patient relaxes

D. Mu rhythm

Mu rhythm is a normal central rhythm, often around 8-13 Hz, usually over C3 or C4.

Characteristics:

Arched or comb-like morphology
Often unilateral or asymmetric
Can resemble sharp waves
Attenuates with movement or intention to move the opposite hand
Does not behave like a posterior alpha rhythm

Key distinction:

Mu rhythm blocks with contralateral hand movement, whereas posterior dominant rhythm blocks with eye opening.

E. Eye opening and eye closure

During eye closure:

PDR becomes more evident posteriorly.

During eye opening:

PDR attenuates or blocks.
Blink artifact may appear prominently at Fp1/Fp2.

Failure to observe PDR attenuation must be interpreted carefully because the patient may not open the eyes fully, may be drowsy, may have visual impairment, or may have a poorly formed baseline rhythm.

F. Photic stimulation

Intermittent photic stimulation uses flashes at different frequencies.

Normal photic driving

This is a normal response:

Posterior rhythmic activity
Time-locked to flashes
Often more visible at harmonics of the flash frequency
Usually symmetric

Photoparoxysmal response

This is an epileptiform response:

Generalized or focal spikes, polyspikes, or spike-wave discharges triggered by flashing
May persist beyond the flash train
May correlate with myoclonus or a clinical event

Do not label all photic responses as abnormal. Photic driving is not a seizure.

G. Hyperventilation

Hyperventilation often produces:

Generalized slowing
Higher-amplitude theta and delta activity
Greater effect in children and younger adults

This can be normal during the procedure.

Hyperventilation is clinically useful because it may activate generalized spike-wave discharges, especially in absence epilepsy. It should be performed only when safe and clinically appropriate.

4. Sleep and artifacts

A. Why sleep matters

Sleep is important because:

It changes normal EEG patterns in predictable ways.
It increases the yield of epileptiform discharges in many patients.
It helps distinguish state-related normal activity from pathology.
It can reveal sleep-activated epileptiform abnormalities.

A beginner should learn normal sleep patterns before attempting to identify sleep-related epileptiform discharges.

B. Drowsiness and N1 sleep

Drowsiness

The transition from relaxed wakefulness to drowsiness often includes:

Attenuation and fragmentation of the PDR
Increased generalized theta activity
Slow rolling eye movements
Reduced responsiveness
Occasional vertex sharp transients

Slow rolling eye movements

These are often:

Large slow deflections
Maximal in frontal leads
Associated with drowsiness
Seen in EOG channels, if recorded

They should not be mistaken for frontal delta slowing or epileptiform activity.

C. Vertex waves

Vertex waves are normal sleep transients.

Features:

Brief, sharp-looking waves
Maximal at Cz or near the vertex
Seen in drowsiness and light sleep
Can be quite prominent in children
May occur singly or in brief trains

They can be asymmetric, especially in young people, but persistent marked asymmetry should prompt careful review.

A common beginner error is calling a vertex wave a central spike.

D. N2 sleep: spindles and K-complexes

N2 sleep is identified by the presence of sleep spindles and/or K-complexes.

Sleep spindles

Features:

Brief bursts around 11-16 Hz
Waxing and waning shape
Usually maximal over central regions
Often bilateral and symmetric
Can be asynchronous across the two sides, especially in younger individuals

Spindles reflect thalamocortical network activity.

K-complexes

Features:

Large, biphasic waveform
May occur spontaneously
May be provoked by sound or other stimuli
Often followed by a sleep spindle
Normal feature of N2 sleep

A K-complex may have a sharp initial component but is not automatically epileptiform.

K-complex versus epileptiform discharge

K-complex	Epileptiform sharp wave
Seen in N2 sleep	May occur in wake or sleep
Large, biphasic, broadly distributed	Usually has a more localized or syndrome-specific field
May be stimulus-induced	Not usually consistently stimulus-induced
Often followed by sleep spindle	Often followed by after-going slow wave, not spindle
Part of normal sleep architecture	May disrupt background or recur in a focal field

E. N3 sleep

N3 is deep sleep.

Typical EEG:

High-amplitude, generalized delta activity
Slow waves occupying a substantial part of the epoch
Reduced frequency of spindles and K-complexes compared with N2

Important rule:

Generalized delta can be normal in deep sleep but may be abnormal in an alert adult.

F. REM sleep

REM sleep EEG often resembles wakefulness:

Low-voltage mixed-frequency activity
Relative reduction in synchronized slow activity
Rapid eye movements
Low chin muscle tone, seen on polysomnography
Possible sawtooth waves

REM is identified best when EEG is interpreted with eye and chin EMG channels.

Artifacts

G. Definition

An artifact is activity in the EEG recording that does not arise from cerebral cortex.

Artifacts are divided into:

Physiologic artifacts: generated by the patient, but not the brain
Extraphysiologic artifacts: generated by electrodes, equipment, environment, or external electrical sources

The common physiologic artifacts include ocular, muscle, perspiration, and cardiac artifact. Kaplan & Sadock's Comprehensive Textbook of Psychiatry, “EEG Artifacts.”

H. Eye blink and vertical eye movement artifact

Features:

Large slow potentials
Maximal at Fp1 and Fp2
Often symmetric with blinks
Strongest frontally, decreasing toward posterior electrodes
May be captured directly on vertical EOG channels

Vertical and horizontal eye-movement artifact

Vertical eye movement and horizontal eye movement artifacts can spread into frontal EEG channels. Kaplan & Sadock's Comprehensive Textbook of Psychiatry, “EEG Artifacts.”

Recognition clue

If a large waveform is maximal at Fp1/Fp2 and occurs at the same time as blinking on video, it is an eye blink until proven otherwise.

I. Horizontal eye movement artifact

With lateral gaze shifts:

F7 and F8 are often involved
Left and right frontal electrodes can show opposite polarity
The pattern may mimic a frontal phase reversal

Look at:

Video
EOG channels
Timing with gaze changes
Symmetry/opposite polarity at lateral frontal sites

J. Muscle artifact, EMG

EMG artifact comes from contraction of scalp, face, jaw, neck, or temporal muscles.

Common sources:

Jaw clenching
Chewing
Talking
Swallowing
Frowning
Forehead tension
Shivering

Features:

Fast activity
Irregular and often chaotic morphology
Often most prominent in frontal and temporal electrodes
Can obscure true spikes or fast cerebral activity
Usually improves when the patient relaxes

Practical response

Ask the patient to:

Relax the jaw
Stop talking or chewing
Loosen facial muscles
Avoid reading or squinting
Remain still

Do not rely only on a stronger low-pass filter, as it can also attenuate real cerebral fast activity.

K. ECG artifact

ECG artifact is cardiac electrical activity picked up by scalp electrodes.

Features:

Repetitive sharp waveform
Occurs at the heart rate
Often more visible in inferior temporal, ear, or mastoid-related electrodes
Time-locks to the ECG channel

Recognition rule

If a sharp transient recurs exactly with every QRS complex, it is ECG artifact unless there is strong evidence otherwise.

L. Sweat artifact

Features:

Very slow baseline drift
Often frontal
Can mimic slow-wave activity
Usually irregular and not a stable cerebral field

Management:

Cool the patient if appropriate
Check skin and electrode contact
Allow time for sweating to reduce
Use technical adjustments carefully

M. Electrode pop

An electrode pop is an abrupt change in electrode potential, often from unstable contact.

Features:

Abrupt, high-amplitude transient
Often isolated to one electrode
May have a vertical edge or “pop”
No physiologic field
Can recur irregularly

Recognition strategy

Identify the common electrode across affected channels.

Example:

Fp1-F7     abrupt transient
F7-T7      abrupt transient
F7-REF     abrupt transient
Other channels: no corresponding event

The shared electrode, F7, is the likely artifact source.

N. Movement artifact

Movement can cause:

Large irregular waveforms
Widespread disturbance
Electrode displacement
Cable movement artifact
Poor baseline stability

Use video and technologist annotations. If the entire record changes abruptly when the patient moves, do not call it a generalized cerebral event.

O. 50/60-Hz electrical interference

This is line-frequency artifact from electrical sources.

Features:

Highly regular fast oscillation
Often widespread
Can be limited to one electrode if its impedance is poor
May worsen with electrical equipment nearby

Management:

Check electrode impedances and contact.
Check cables and nearby electrical equipment.
Inspect the unfiltered signal.
Use a notch filter only when necessary and document its use.

P. Artifact-versus-brain checklist

When you see an unusual waveform, ask:

Does it have a believable field?
Is it present in neighboring electrodes in a logical distribution?
Does it persist in a different montage?
Is it time-locked to eye movement, ECG, movement, or video?
Does it occur in a single electrode only?
Does it occur at 50/60 Hz or heart rate?
Does the pattern change when the patient relaxes or moves?
Could it be a normal sleep feature or normal variant?

High-yield revision table

Topic	Remember
10-20 system	Odd = left, even = right, z = midline
Channel	Difference in voltage between two inputs
Montage	Display arrangement, not a different recording
Bipolar montage	Best for local field and phase reversal
Referential montage	Useful for amplitude maxima and generalized activity
Phase reversal	Indicates local scalp voltage maximum, not exact seizure onset
PDR	Posterior, 8-13 Hz, eyes closed, attenuates with eye opening
Mu rhythm	Central, arch-shaped, blocks with contralateral hand movement
N2 sleep	Spindles and K-complexes
N3 sleep	High-amplitude delta is normal
Blink artifact	Frontal, especially Fp1/Fp2
EMG artifact	Fast, irregular, frontal/temporal, improves with relaxation
ECG artifact	Time-locked to QRS
Electrode pop	Abrupt, one-electrode phenomenon, no proper field
Line noise	Regular 50/60-Hz activity

Recent artifact-detection research, especially in wearable EEG, does not replace visual inspection of the raw signal and clinical recording context: 2025 systematic review.

Find recent guidelines for routine EEG recording

Reading File

Searching the Web

site:acns.org clinical EEG recording guideline routine EEG 2024 2025 2026 ACNS

Searching the Web

IFCN routine EEG recording guideline standards 2023 2024 clinical EEG

Searching PubMed

electroencephalography AND practice guideline

Finding Sources

Reading Article

PMID: 36762397

Reading Article

PMID: 28436800

Reading Article

PMID: 28436788

Reading Article

PMID: 28436792

Reading Web Page

https://acns.org/practice/guidelines

Most relevant recent guideline for routine EEG recording

The key current international guideline is:

Peltola et al., 2023, IFCN and ILAE: Routine and sleep EEG: Minimum recording standards of the International Federation of Clinical Neurophysiology and the International League Against Epilepsy. It is a joint international practice guideline with 16 consensus-based recommendations covering indications, technical standards, recording duration, sleep induction, and activation procedures. Practice Guideline . 2023 . PMID: 36762397

Essential recommendations to learn

Topic	Practical recommendation
Electrode placement	Use standardized electrode placement. The 10-20 system remains foundational; the IFCN standardized array supports 25 electrodes including inferior temporal electrodes for standard clinical recordings when feasible.
Recording duration	Record enough artifact-free EEG to assess the background and capture state changes. A longer routine recording improves detection of epileptiform abnormalities.
Sleep	Record sleep whenever feasible because it raises the yield of interictal epileptiform discharges. Sleep deprivation is generally preferred over sedation for sleep induction when appropriate.
Video	Use synchronized video whenever possible, especially when events, seizures, or unusual movements are a concern.
ECG channel	Include at least one ECG channel to identify cardiac artifact and to help correlate events.
Hyperventilation	Perform unless contraindicated. It is particularly useful for provoking generalized spike-wave discharges in absence epilepsy. Document effort and any reason for omission.
Intermittent photic stimulation	Perform according to a standardized frequency sequence unless contraindicated. Document photic driving, photoparoxysmal response, symptoms, and any clinical event.
Montages	Review EEG in more than one montage, particularly bipolar and referential reconstructions. Use additional electrodes or modified montages when temporal, anterior temporal, or mesial frontal localization is clinically important.
Technical quality	Record digitally, maintain good electrode contact, identify artifacts during acquisition, and avoid excessive filtering that can distort real cerebral signals.
Documentation	Record patient state, medications, activation procedures, sleep achieved, technical limitations, clinical events, and video correlation.

Supporting standards still commonly used

The most recent broad international routine/sleep EEG recording guideline is the 2023 IFCN-ILAE document. In contrast, the central ACNS routine EEG technical standards are still the 2016 series, listed by ACNS as current for this purpose.

ACNS Guideline 1: Minimum Technical Requirements for Performing Clinical EEG
This remains a main technical reference for routine scalp EEG. It supports:
- Full standard electrode coverage
- Digital recording
- At least 20 minutes of artifact-free EEG, including activation procedures, as a minimum baseline
- Routine display sensitivity around 5-10 µV/mm
- Hyperventilation for at least 3 minutes, with recording before and after, when safe
- Inclusion of a single ECG channel
  ACNS Guideline 1 . 2016 . PMID: 28436800
ACNS Guideline 3: Standard Montages in Clinical EEG
Useful for choosing bipolar and referential montages and for additional anterior temporal or mesial frontal coverage. ACNS Guideline 3 . 2016 . PMID: 28436788
ACNS Guideline 7: EEG Reporting
Recommends a structured routine EEG report with five sections:
- History
- Technical description
- EEG description
- Impression
- Clinical correlation
  Document activation procedures and state changes, and explain why hyperventilation or photic stimulation was omitted. ACNS Guideline 7 . 2016 . PMID: 28436792

The ACNS guideline index confirms that its core routine EEG technical, electrode nomenclature, montage, digital-recording, pediatric EEG, and reporting guidelines are the 2016 documents. Its newer 2022 standard concerns ambulatory EEG, rather than routine laboratory EEG.

Practical routine EEG checklist

For a standard adult routine EEG, document:

Patient identity, age, indication, antiseizure/sedating medicines, and last clinical event
Awake, drowsy, and sleep states achieved
Standardized 10-20 electrode placement, with extra electrodes where indicated
ECG and, ideally, synchronized video
Bipolar and referential montage review
Background, posterior dominant rhythm, symmetry, continuity, and reactivity
Hyperventilation and photic stimulation, or a documented reason not to perform them
Artifact burden and technical limitations
Interictal epileptiform discharges, seizures, and clinical correlation
A concise standardized final impression

For your notes, use the 2023 IFCN-ILAE guideline as the primary current standard, then use ACNS Guidelines 1, 3, and 7 for practical details on technical setup, montages, and report writing.

Compare IFCN-ILAE and ACNS EEG standards

Reading File

Reading Web Page

https://pmc.ncbi.nlm.nih.gov/articles/PMC10006292/

Reading Article

PMID: 36762397

Searching PubMed

electroencephalography AND practice guideline

Searching the Web

IFCN ILAE Routine and sleep EEG minimum recording standards 2023 recommendations routine duration electrodes sleep hyperventilation photic

IFCN-ILAE vs ACNS standards for routine EEG

Bottom line

The IFCN-ILAE 2023 guideline is the newer, internationally oriented minimum standard for routine and sleep EEG. The ACNS 2016 guidelines remain highly practical U.S.-based standards, with more operational detail on technical setup, montage selection, and structured reporting.

They are broadly consistent. The main differences are in scope, recency, electrode-array preference, methodology, and level of technical detail.

Feature	IFCN-ILAE	ACNS
Main routine EEG standard	2023 routine and sleep EEG minimum recording standard	Core routine EEG series mainly 2016
Organizations	International Federation of Clinical Neurophysiology + International League Against Epilepsy	American Clinical Neurophysiology Society
Geographic orientation	International, intended for broad global applicability	Primarily U.S. clinical practice
Scope	Indications, technical recording, routine and sleep duration, sleep induction, activation procedures	Technical requirements, electrode nomenclature, montage design, digital recording, pediatric EEG, reporting
Methodology	Systematic evidence review, GRADE, QUADAS-2, and modified Delphi consensus	Society guideline and expert consensus updates
Routine EEG duration	Suggests 20 minutes, excluding preparation, conditional recommendation	Minimum 20 minutes of artifact-free recording, including activation procedures
Sleep EEG duration	Suggests 30 minutes, excluding preparation	Sleep should be recorded whenever possible; no equivalent universal 30-minute sleep EEG specification in Guideline 1
Electrode array	Suggests the 25-electrode IFCN array, including six inferior temporal electrodes, when feasible	Standard 21-electrode 10-20 system is required for routine clinical EEG; additional electrodes as clinically needed
Activation	Hyperventilation, photic stimulation, eye opening/closure suggested unless contraindicated	Hyperventilation and photic stimulation are routine unless contraindicated; gives more procedure-specific operational detail
Reporting	Focuses chiefly on recording standards	Has a dedicated structured reporting guideline
Video	Supports video where available and useful	Time-locked video is incorporated into modern routine EEG reporting practice but is not universally mandatory in the core technical guideline

1. Scope and purpose

IFCN-ILAE 2023

The joint IFCN-ILAE guideline is designed to answer: What is the minimum acceptable routine or sleep EEG recording standard?

It provides 16 recommendations on:

Indications
Technical recording conditions
Electrode application
Recording duration
Sleep induction
Hyperventilation
Intermittent photic stimulation
Eye opening and eye closure

It was developed using systematic review methods, GRADE assessment, QUADAS-2 for relevant technical studies, and expert consensus where evidence was limited. The authors state that much of the evidence was low to moderate quality, so several recommendations are conditional. Peltola et al., 2023

ACNS 2016 series

ACNS distributes the content across several linked documents:

Guideline 1: minimum technical requirements for clinical EEG
Guideline 2: electrode nomenclature
Guideline 3: standard montages
Guideline 4: digital EEG recording
Guideline 5: pediatric EEG
Guideline 7: EEG reporting

This makes ACNS particularly useful as a laboratory implementation and reporting reference. The ACNS guideline listing continues to identify these as its current core EEG introduction standards.

2. Recording duration

Standard	Routine EEG	Sleep EEG	Interpretation
IFCN-ILAE 2023	20 minutes, excluding preparation	30 minutes, excluding preparation	Conditional recommendation due to limited evidence
ACNS 2016	At least 20 minutes of artifact-free EEG, including activation procedures	Sleep whenever possible, especially in suspected/known seizure disorders	Longer studies increase yield

Important practical difference

The standards agree that 20 minutes is a minimum, not an ideal universal duration.

IFCN-ILAE explicitly separates routine EEG from sleep EEG and specifies 30 minutes for sleep EEG.
ACNS emphasizes that the recorded sample must be artifact-free, and activation procedures plus sleep often make a longer study necessary.

For a laboratory protocol, a reasonable synthesis is:

Obtain at least 20 minutes of artifact-free routine EEG, and extend the study to obtain drowsiness/sleep and complete activation procedures when clinically indicated.

The IFCN-ILAE recommendation is available in the full guideline text.

3. Electrode number and placement

IFCN-ILAE

The IFCN-ILAE guideline suggests using the 25-electrode IFCN montage whenever feasible.

This comprises the usual 10-20 electrodes plus six inferior temporal electrodes. The intent is better coverage of anterior and basal temporal regions, where epileptiform activity may be missed by a standard 10-20 array.

It uses 10-10 nomenclature and includes an inferior temporal chain.

ACNS

ACNS Guideline 1 states that all 21 standard IFCN 10-20 electrodes and placements should be used for routine clinical EEG. It considers fewer electrodes inadequate for a comprehensive standard study, except in special circumstances.

Additional electrodes may be added for a suspected localized abnormality, for example:

Anterior temporal/subtemporal electrodes
Sphenoidal or other special temporal electrodes, depending on local practice
Extra electrodes near a skull defect
Expanded 10-10 coverage

Practical comparison

Question	IFCN-ILAE approach	ACNS approach
Minimum routine array	25-electrode array suggested when feasible	21-electrode 10-20 array required for a standard routine study
Temporal coverage	Stronger emphasis on routine inferior temporal coverage	Add electrodes when localization is clinically important
Naming	10-10 nomenclature integrated into the recommended array	10-20 standard, with 10-10 extension accepted
Resource sensitivity	Explicitly recognizes feasibility and resource limitations	More prescriptive about comprehensive standard coverage

Takeaway: If resources permit, the IFCN-ILAE 25-electrode array is a useful enhancement for routine clinical EEG, particularly in epilepsy assessment. In a lab using conventional ACNS practice, the full 21-electrode 10-20 array remains the minimum baseline, with additional temporal electrodes selected when indicated.

4. Technical recording standards

Areas of agreement

Both standards support:

Digital EEG recording
Standardized electrode placement
Good electrode contact and artifact control
Review in more than one montage
Recording wakefulness and sleep when possible
ECG recording
Careful documentation of technical limitations
Cautious and selective filtering

ACNS is more operational

ACNS gives more specific practical parameters. Examples from Guideline 1 include:

Typical routine display sensitivity: 5-10 µV/mm
At least one ECG channel
A review of an unfiltered system-reference segment is recommended before relying on a notch filter
Explicit technical considerations for calibration, filters, and montage changes

The ACNS technical standard is therefore especially useful for an EEG technologist's laboratory protocol.

IFCN-ILAE is more technology-aware and internationally framed

The newer IFCN-ILAE guideline discusses:

Cup electrodes and head caps
Need to verify impedance with caps
Current limitations of dry-electrode EEG, including movement and sweat artifact
MRI-compatible and needle electrode use in selected settings
Preference for enhanced temporal coverage when feasible

5. Sleep recording and sleep induction

Shared principle

Both organizations agree that sleep improves the likelihood of detecting epileptiform abnormalities and should be obtained whenever feasible.

IFCN-ILAE 2023

The IFCN-ILAE document provides a more explicit framework for:

Routine versus sleep EEG duration
Methods of sleep induction
Sleep deprivation
Balancing diagnostic benefit against patient practicality and safety

It recommends sleep EEG when clinically appropriate and favors non-pharmacologic methods where feasible.

ACNS 2016

ACNS says sleep recording should be performed whenever possible and is usually important in suspected or established seizure disorders. It recognizes sleep deprivation as a way to increase yield but leaves more protocol detail to local practice.

Takeaway: IFCN-ILAE offers the more current framework for selecting and structuring sleep EEG; ACNS reinforces that sleep should not replace an adequate waking record.

6. Hyperventilation

Agreement

Both recommend hyperventilation unless there is a valid contraindication.

ACNS: more detailed operational protocol

ACNS recommends:

At least 3 minutes of hyperventilation
At least 1 minute of pre-hyperventilation EEG using the same montage
At least 1 minute of EEG after hyperventilation
Documentation of the quality of patient effort
Recording the reason if hyperventilation is omitted

Examples of contraindications in ACNS include recent intracranial hemorrhage, important cardiopulmonary disease, sickle cell disease or trait, and inability or unwillingness to cooperate.

IFCN-ILAE: broader contraindication list and conditional recommendation

IFCN-ILAE supports routine hyperventilation unless contraindicated and gives a broader list, including:

Sickle cell disease or trait
Moyamoya disease or syndrome
Recent cerebrovascular event
Vascular malformations/aneurysm
Raised intracranial pressure
Recent myocardial infarction
Significant arrhythmia or severe cardiac disease
Severe pulmonary disease
Pregnancy

The international standard emphasizes verifying and documenting contraindications. IFCN-ILAE guideline

7. Intermittent photic stimulation

Areas of agreement

Both recommend:

Photic stimulation as a routine activation procedure unless contraindicated
Documentation of the procedure and response
Recording whether photic driving occurs
Documentation of photoparoxysmal response, symptoms, myoclonus, or induced seizure
Explicit documentation if omitted

Difference in emphasis

ACNS gives practical environment and sequence details, such as dim lighting and performing photic stimulation before hyperventilation or after hyperventilation-related EEG changes have resolved.
IFCN-ILAE emphasizes standardized activation protocols, eye-open/eye-closed baseline recording, and tailoring stimulation to clinical questions and patient safety.

8. Montages

ACNS

ACNS provides more detailed advice on montage selection. It recognizes that montage choice should help detect and localize abnormalities and includes standard bipolar and referential approaches. The ACNS montage guideline also addresses references and additions to improve sampling of anterior temporal and mesial frontal activity.

IFCN-ILAE

IFCN-ILAE focuses more on the electrode array and minimum acquisition standard than on prescribing a detailed set of display montages. It supports standardized recording that can be reconstructed and reviewed appropriately.

Practical conclusion

For learning and laboratory setup:

Use IFCN-ILAE to decide the minimum electrode coverage and routine recording elements.
Use ACNS to build the local montage library and display-review workflow.

9. Reporting

This is the strongest ACNS advantage.

ACNS Guideline 7 recommends a routine EEG report with five sections:

History
Technical description
EEG description
Impression
Clinical correlation

It also recommends documenting:

Number and placement of electrodes
Video and ECG channels
Montages and post hoc review settings
Recording duration
Wakefulness, drowsiness, and sleep
Hyperventilation and photic stimulation
Reason for omitting activation procedures
Technical limitations and relevant artifacts

See the ACNS reporting guideline.

The IFCN-ILAE guideline is not primarily a reporting template, so an EEG lab using IFCN-ILAE recording standards should still adopt a structured reporting system such as ACNS Guideline 7.

Recommended combined protocol

For a modern routine EEG service, combine the documents as follows:

Primary recording standard: IFCN-ILAE 2023
Use it for minimum routine/sleep recording duration, modern electrode considerations, sleep strategy, and activation procedures.
Baseline electrode coverage:
Use a complete 21-electrode 10-20 array at minimum. Add the IFCN inferior temporal electrodes, creating the 25-electrode array, when feasible, particularly for epilepsy evaluation.
Technical acquisition: ACNS Guideline 1
Use it for digital acquisition, sensitivity, filters, ECG, calibration, artifact management, activation procedure details, and minimum artifact-free recording.
Montage review: ACNS Guideline 3
Use bipolar and referential reconstructions. Include additional temporal or targeted electrodes when the clinical question requires improved localization.
Structured report: ACNS Guideline 7
Use the five-part report format and document state, technical parameters, activation, artifacts, findings, and clinical correlation.

Exam-style summary

IFCN-ILAE 2023 is the newer international standard for minimum routine and sleep EEG recording. It recommends a 20-minute routine EEG, a 30-minute sleep EEG, activation procedures unless contraindicated, and a 25-electrode array with inferior temporal electrodes when feasible.

ACNS 2016 is more detailed for day-to-day U.S. laboratory practice: it specifies at least 20 minutes of artifact-free recording, full 21-electrode 10-20 coverage, ECG, technical settings, detailed hyperventilation procedures, montage guidance, and standardized reporting.

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