The numbers in the 10-20 System are used to describe electrode positions relative to the brain's midline. They are essential for maintaining standardized, symmetrical, and reproducible electrode placement in EEG recordings.

1. Odd and Even Numbers

The numbers identify the hemisphere in which the electrode is placed:

  • Odd Numbers: Indicate electrodes on the left hemisphere (e.g., F3, P7).
  • Even Numbers: Indicate electrodes on the right hemisphere (e.g., F4, P8).
  • Z (Zero): Represents electrodes on the midline of the head (e.g., Fz, Cz, Pz).

2. Relationship to the Midline

The numbers also indicate the electrode's distance from the brain's midline:

  • Lower Numbers (e.g., C1, F3): Closer to the midline.
  • Higher Numbers (e.g., C5, F7): Farther from the midline.

This numbering ensures that electrodes are symmetrically placed across the left and right hemispheres.

3. Proportional Distances

The "10-20" name reflects the proportional distances between electrode positions:

  • Electrodes are spaced at 10% or 20% of the total head circumference.
  • For example, Fp1 and Fp2 are positioned 10% of the total distance from the nasion (bridge of the nose).
  • The distances between Fp1, F3, C3, and P3 are all 20% of the head circumference.

4. Clinical Relevance

  • Localization of Brain Activity: Numbers help pinpoint areas of activity or dysfunction (e.g., seizures or lesions).
  • Symmetry and Abnormalities: Comparing electrical activity between symmetrical electrodes (e.g., F3 vs. F4) can reveal conditions like strokes or brain damage.
  • Advanced Systems: Systems like the 10-10 or 10-5 System, which include additional electrodes, use the same numbering principles for finer spatial resolution.

5. Example of Electrode Placement

Here is how the numbers relate to electrode placement:

  • F3: Frontal lobe, left hemisphere.
  • F4: Frontal lobe, right hemisphere.
  • Cz: Central region, midline.
  • P7: Parietal lobe, left hemisphere, far from the midline.

Note: The standardized numbering system in the 10-20 System ensures accurate, consistent, and reproducible data for clinical and research applications.