3.2 Electrode Application & the 10-20 System
Key Takeaways
- The International 10-20 System places electrodes at 10% and 20% interelectrode distances measured from the nasion, inion, and the two preauricular points.
- Odd numbers mark left-hemisphere electrodes, even numbers mark the right, and 'z' marks the midline (Fz, Cz, Pz); Cz sits at 50% of both the nasion-inion and biauricular lines.
- The technologist applies electrodes with skin prep that lowers impedance while keeping skin intact, then verifies measured impedances.
- Recommended scalp electrode impedances are typically between 100 and 5,000 ohms, balanced across electrodes, with impedances kept consistent rather than near zero.
- Special electrodes (sphenoidal, true anterior temporal T1/T2, plus extended-array additions) improve detection of temporal and focal abnormalities.
Why the 10-20 System Is Foundational
Accurate electrode placement is one of the most testable skills in the Pre-Study domain because every downstream measurement depends on it. The International 10-20 System is the standardized method for positioning scalp electrodes so that recordings are reproducible across technologists, sessions, and laboratories. Misplaced electrodes mislocalize abnormalities and can invalidate the study.
The Four Cranial Landmarks
Measurements start from four bony landmarks identified by palpation:
| Landmark | Location |
|---|---|
| Nasion | The depression at the bridge of the nose, between the forehead and nose |
| Inion | The bump (external occipital protuberance) at the back of the skull on the midline |
| Left preauricular point | The depression just anterior to the tragus of the left ear |
| Right preauricular point | The depression just anterior to the tragus of the right ear |
The anteroposterior (sagittal) line runs from nasion to inion over the vertex. The coronal line runs between the two preauricular points. The head circumference is also measured for the transverse rows.
The 10-20 Measurement Logic
Electrodes are placed at distances that are 10% or 20% of the total measured line. Along the nasion-to-inion line, the first electrode (Fpz region/Fp row) is 10% up from the nasion, then successive points are spaced 20% apart, ending 10% above the inion. The same 10%/20% spacing is applied to the coronal and transverse circumference lines.
- Cz (vertex) sits at exactly 50% of the nasion-inion line and 50% of the line between the preauricular points — it is the central reference point of the array.
- The standard 21-electrode array includes Fp1/Fp2, F7/F3/Fz/F4/F8, T3(T7)/C3/Cz/C4/T4(T8), T5(P7)/P3/Pz/P4/T6(P8), O1/O2, plus the ear references A1/A2.
Naming Conventions
| Element | Meaning |
|---|---|
| Letters (Fp, F, C, P, O, T) | Brain region: frontopolar, frontal, central, parietal, occipital, temporal |
| Odd numbers (1, 3, 5, 7) | Left hemisphere |
| Even numbers (2, 4, 6, 8) | Right hemisphere |
| z (Fz, Cz, Pz) | Midline (zero) electrodes |
| Smaller number | Closer to the midline; larger number is more lateral |
Note the modified combinatorial nomenclature substitutions: T3 = T7, T4 = T8, T5 = P7, T6 = P8 in the updated naming system. Knowing both conventions prevents confusion across labs and exam items.
Electrode Types and Application
- Disc (cup) electrodes: Reusable gold, silver/silver-chloride, or tin discs applied with conductive paste or collodion; standard for routine EEG.
- Subdermal needle electrodes: Sterile, single-use; used in emergent, ICU, or operating-room settings where rapid application is needed.
- Electrode caps: Pre-positioned arrays for faster setup, often in long-term monitoring.
Application sequence: locate and mark the site, gently abrade and clean the skin to reduce impedance without breaking the skin, apply the electrode with conductive medium, secure it, and then measure impedance. The goal is a stable, low-impedance contact that does not injure the scalp.
Impedance Measurement and Targets
Impedance is the opposition to current flow at the electrode-skin interface; high or unequal impedances increase artifact and 60 Hz interference. The technologist measures impedance at every electrode before recording.
| Parameter | Practical Target |
|---|---|
| Recommended scalp electrode impedance | Generally 100-5,000 ohms |
| Balance across electrodes | Impedances should be similar to one another |
| Too low (near 0 ohm) | Suggests a salt bridge between adjacent electrodes |
| Too high (well above 5,000 ohm) | Causes artifact and instability; re-prep the site |
Keeping impedances low and matched between electrodes minimizes common-mode and mains interference. Note that impedances that are essentially zero are not ideal — they often indicate bridging from excess paste.
Conditions Affecting Impedance Values
The outline explicitly lists conditions affecting impedance values, so know why impedance drifts. High impedance results from inadequate skin prep, dried electrolyte, oily or calloused skin, hair under the electrode, a loose electrode, or a corroded/damaged electrode or lead. Low/near-zero readings between adjacent electrodes indicate a salt bridge from excess paste or sweat. Impedance also drifts upward over time as gel dries - a major issue in long-term monitoring, where electrodes must be checked and refreshed.
The correct response to a high-impedance electrode is to re-prep and re-apply that specific electrode, not to compensate with filters.
A Measurement Worked Example
Suppose the nasion-to-inion distance measures 36 cm. The 10% points (Fpz row and just above inion) sit 3.6 cm from each landmark, and the 20% steps between successive midline electrodes are 7.2 cm each. Cz lands at 50% - 18 cm from the nasion along the midline. The same logic applies to the coronal (ear-to-ear) line and the transverse circumference rows. Working a measurement like this confirms why precise landmark identification matters: a misidentified inion shifts every posterior electrode, mislocalizing any occipital finding.
Special Electrodes
Special electrodes extend coverage where the standard array is insufficient:
- True anterior temporal (T1/T2): Improve detection of anterior temporal discharges.
- Sphenoidal electrodes: Placed by a physician to record from the mesial temporal region in epilepsy evaluation.
- Additional 10-10 (extended array) electrodes: Provide closer spatial sampling for focal localization.
- Electrocardiogram (ECG/EKG) channel: Standard to distinguish cardiac artifact and identify rhythm.
The technologist must place special electrodes accurately and label channels clearly so the reader can correlate findings.
Which set of landmarks is used to begin International 10-20 System measurements?
In 10-20 nomenclature, an electrode labeled with an odd number indicates placement over which area?
Before recording, the technologist measures an impedance of essentially 0 ohms between two adjacent electrodes. What does this most likely indicate?
Which electrode is positioned at 50% of both the nasion-inion line and the line between the preauricular points?