EEG Recording, 10-20 System & Montages

The technical setup of an EEG recording — electrode placement, montage selection, filter settings, and activation procedures — determines what the clinician sees. A focal source can be missed entirely if the wrong reference is used; a benign normal variant can be miscalled if the wrong filter setting is applied; an epilepsy diagnosis can hinge on whether sleep was captured. This page covers the practical mechanics of EEG recording: the 10-20 system, common montages, filter and amplifier settings, and activation procedures that increase diagnostic yield.

The International 10-20 System

Naming Convention

  • Letters indicate brain region: Fp (frontopolar), F (frontal), C (central), T (temporal), P (parietal), O (occipital), A (auricular).
  • Odd numbers = left, even numbers = right, “z” (zero) = midline (Fz, Cz, Pz).
  • Standard 21 electrodes: Fp1, Fp2, F7, F3, Fz, F4, F8, T3, C3, Cz, C4, T4, T5, P3, Pz, P4, T6, O1, O2, plus A1 (left ear) and A2 (right ear).

Modified Combinatorial Nomenclature (10-10)

  • Renames temporal electrodes: T3→T7, T4→T8, T5→P7, T6→P8.
  • Adds intermediate positions: F9/F10 (inferior temporal), FT9/FT10, AF7/AF8, etc.
  • Standard for modern digital EEG and source localization research.
  • Most clinical labs still report using the older T3-T6 convention.

Electrode Spacing Math

  • Distances are percentages of head dimensions, not absolute centimeters.
  • Nasion-to-inion measured along midline; total distance = 100%.
  • Fp is 10% from nasion, Cz at 50%, Oz at 90%, inion at 100%.
  • Same percentage logic applies preauricular-to-preauricular (T3-Cz-T4) and the chains in between.
  • This ensures comparable coverage across head sizes from children to large adults.

Standard Montages

Longitudinal Bipolar (“Double Banana”)

  • Chains: Fp1-F7, F7-T3, T3-T5, T5-O1 (left lateral); same on right; Fp1-F3, F3-C3, C3-P3, P3-O1 (left parasagittal); same on right; Fz-Cz, Cz-Pz (midline).
  • Excellent for localizing focal abnormalities via phase reversal.
  • Standard for routine EEG interpretation.
  • Limitation: bilateral widespread activity may be cancelled by adjacent-channel subtraction.

Transverse Bipolar

  • Chains run across the head: F7-Fp1-Fp2-F8, T3-C3-Cz-C4-T4, etc.
  • Complements longitudinal bipolar; better for left-right comparisons and detecting lateralization.
  • Often reviewed simultaneously in modern digital EEG software.

Referential (Ear or Cz Reference)

  • Common reference electrodes: averaged ears (A1+A2/2), single ipsilateral ear, vertex (Cz), or balanced sternovertebral.
  • Shows true amplitude and polarity at each electrode.
  • Critical for amplitude measurements (e.g., spindle voltage on PSG, EP amplitudes).
  • Sensitive to reference contamination — if the reference itself is active (e.g., a midline IED contaminating an averaged-ear reference), every channel shows that activity.

Average Reference

  • Each electrode compared to the mean of all electrodes.
  • Works when most channels are inactive (focal abnormality).
  • Fails when activity is widespread (the reference is contaminated).

Laplacian (Source Derivation)

  • Each electrode compared to its weighted neighbors.
  • Emphasizes focal activity, suppresses distant sources.
  • Useful for confirming IED focality and distinguishing true focal from diffuse sources.

Filter Settings

Standard Routine EEG

  • Low-frequency (high-pass) filter: 1 Hz (or 0.5 Hz for slow-wave analysis).
  • High-frequency (low-pass) filter: 70 Hz.
  • Notch filter: 60 Hz (US) or 50 Hz (Europe) — off unless needed.
  • Sensitivity: 7 μV/mm standard for adult EEG; adjust to 10 or 5 as needed.
  • Time base: 30 mm/sec standard for routine review.

Special Applications

  • Sleep recording: high-pass 0.3 Hz to capture slow waves.
  • EMG monitoring channels: 10–100 Hz.
  • ECG channel: 1–100 Hz.
  • HFO analysis: requires 1000 Hz+ sampling rate; high-pass 80 Hz, low-pass 500 Hz.

Activation Procedures

Activation increases the yield of routine EEG by triggering otherwise rare abnormalities. ACNS guidelines recommend including all of the following unless contraindicated.

Hyperventilation (HV)

  • 3 minutes of deep, regular breathing at 20–30 breaths/min.
  • Produces alkalosis → cerebral vasoconstriction → relative ischemia → enhanced synchronization.
  • Typically triggers absence seizures (3 Hz spike-wave) in childhood absence epilepsy.
  • Less reliable for triggering focal IEDs.
  • Contraindications: recent stroke, severe cardiopulmonary disease, sickle cell disease, severe asthma, moyamoya disease, pregnancy with hypertension.
  • Build-up: progressive high-amplitude rhythmic slowing during HV; resolves within 1–2 minutes after stopping; if persistent, suggests structural pathology or hypoglycemia.

Photic Stimulation (PS)

  • Flashing light at 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 Hz (or similar range); typically 5-second trains separated by 5-second rest.
  • Patient lies with eyes closed; flashes at 30 cm.
  • Normal responses: photic driving (occipital sinusoidal at the stimulus frequency).
  • Photoparoxysmal response: generalized spike-wave triggered by photic stimulation; suggests genetic generalized epilepsy.
  • Photoconvulsive response: sustained 3-Hz spike-wave outlasting the stimulus; high specificity for photosensitive epilepsy.
  • Photomyoclonic response: rhythmic muscle artifact from facial muscles; common, benign.

Sleep

  • Spontaneous sleep during EEG dramatically increases IED yield (40–80% increase).
  • Sleep deprivation (3–4 hours of sleep the night before) further increases yield.
  • Drowsiness, light sleep (N1, N2), and REM each can activate different epileptiform patterns.
  • Routine EEG should include drowsy and at least N1 if possible.
  • Recording for at least 20 minutes is the minimum; longer (60+ min) increases yield substantially.

Other Activation

  • Reading: activates some reading epilepsies.
  • Music: activates musicogenic epilepsy.
  • Eating: activates eating-induced reflex epilepsy.
  • Mental tasks: activate specific syndromes.
  • Specific personal triggers: useful in patients with photosensitive or reflex epilepsies.

Recording Duration

  • Standard routine EEG: 20–30 minutes minimum, including wakefulness, drowsiness, hyperventilation, photic stimulation, and sleep if obtainable.
  • Extended outpatient EEG: 60+ minutes with focus on sleep capture.
  • Ambulatory EEG: 24–72 hours, captures more sleep and more opportunity for spontaneous events.
  • Video EEG monitoring (epilepsy monitoring unit, EMU): 3–7 days, captures seizures with semiology.
  • Continuous EEG (cEEG): days to weeks in ICU patients for non-convulsive seizure detection.

Special Technical Considerations

Bone Defects (Burr Holes, Craniotomies, Skull Defects)

  • Beneath skull defects, EEG amplitude is increased and sharpness may be enhanced (“breach effect” or “breach rhythm”).
  • Can mimic focal IEDs.
  • Recognize by location matching prior surgical history.

Recording in the ICU

  • Patient often unable to cooperate; activation procedures may be limited or absent.
  • Multiple electrical devices (ventilator, pumps, monitors) create artifact.
  • Frequent staff entry interrupts recording.
  • Standard practice: continuous monitoring (cEEG) with daily review and ACNS-standardized terminology.

Recording in Children

  • Pediatric EEG patterns differ substantially from adult.
  • Slower frequencies are normal in young children.
  • Normal patterns (hypnagogic hypersynchrony, anterior slowing in drowsiness, photic driving variants) can be miscalled as pathology if applying adult criteria.
  • Recording technique adapted: shorter sessions, parent presence, video synchronization, often during natural sleep.

Recording in Neonates

  • Dramatically different patterns from older children.
  • Conceptional age (CA, weeks from last menstrual period) determines expected pattern.
  • Behavioral state coding: active sleep, quiet sleep, awake.
  • Specialized expertise required.

Calibration and Quality Control

  • Daily impedance check before recording.
  • Calibration signal (square wave at known amplitude) at start of each recording.
  • Documentation of filter and sensitivity settings on recorded tracing.
  • Electrode location verification (especially in long-term monitoring).
  • Video synchronization for time-stamping.

🔍 Did You Know?

The original 10-20 system was developed by Herbert Jasper in 1958, designed to provide standardized electrode locations using head proportions rather than absolute centimeters. The genius of the system is that it scales naturally across head sizes — the same electrode names cover the same brain regions whether the patient is a small child or a large adult. This is essential because EEG patterns vary by age, and clinical comparisons require consistent coverage. The 10 and 20 in the name refer to the spacing percentages: electrodes along the midline are at 10%, 20%, 20%, 20%, 20%, and 10% of the nasion-inion distance. Modern high-density EEG uses 64, 128, or 256 electrodes following the 10-10 or 10-5 extensions of this same system, dramatically improving source localization for epilepsy surgery planning. For the clinician, the practical lesson is that despite electronic and software advances, the underlying geometric framework is essentially unchanged for 65 years. The 10-20 system’s robustness across decades and across pediatric-to-adult populations is a testament to good standardization at the field’s foundation. Knowing how to find F7 on a patient by palpation and proportion — without specialized equipment — remains a core neurology skill, particularly for emergency bedside EEG when full electrode arrays aren’t immediately available.

Pitfalls and Pearls

  • 10-20 system: head-proportion based; scales across patient sizes; standardized for 65+ years.
  • Longitudinal bipolar (double banana): best for localizing focal IEDs via phase reversal.
  • Average reference: works for focal abnormalities; fails for widespread activity.
  • Laplacian: emphasizes focal sources; useful for IED confirmation.
  • Standard filter: 1–70 Hz, sensitivity 7 μV/mm, time base 30 mm/sec.
  • Hyperventilation: triggers absence seizures; avoid in stroke, sickle cell, moyamoya.
  • Photic stimulation: photoconvulsive (sustained 3 Hz SW) is clinically significant; photomyoclonic is benign.
  • Sleep deprivation: dramatically increases IED yield.
  • Recording duration: 20–30 min routine; 60+ if seeking diagnostic IEDs; 24 hr ambulatory if still negative.
  • Breach effect: enhanced amplitude/sharpness over skull defects; not pathology.
  • ICU EEG: continuous monitoring with daily review; ACNS-standardized terminology.
  • Neonatal/pediatric EEG: age-specific norms; don’t apply adult criteria.
  • Activation procedures: include all four — sleep, hyperventilation, photic, sleep deprivation — to maximize yield.
  • Document technique: every report should state filters, electrode placement, activation procedures, recording time.
  • Calibration signal: at recording start; verifies amplifier integrity.
  • Impedance check: before every recording; <5 kΩ all channels.

References

  1. Jasper HH. Report of the committee on methods of clinical examination in electroencephalography. Electroencephalogr Clin Neurophysiol. 1958;10(2):370-375.
  2. Acharya JN, Hani A, Cheek J, Thirumala P, Tsuchida TN. American Clinical Neurophysiology Society Guideline 2: Guidelines for standard electrode position nomenclature. J Clin Neurophysiol. 2016;33(4):308-311.
  3. Sinha SR, Sullivan L, Sabau D, et al. American Clinical Neurophysiology Society Guideline 1: Minimum technical requirements for performing clinical electroencephalography. J Clin Neurophysiol. 2016;33(4):303-307.
  4. Ebersole JS, Husain AM, Nordli DR Jr, eds. Current Practice of Clinical Electroencephalography. 4th ed. Wolters Kluwer; 2014.
  5. Mendiratta A, Emerson RG. Postoperative and intraoperative monitoring. Continuum (Minneap Minn). 2009;15(1):155-170.