The electrode located 30% above the left pre-auricular crease is:

The electrode located 30% above the left pre-auricular crease is:

• A. C3
• B. P3
• C. O1
• D. F3

Answer: (A)

Understanding how EEG electrodes are placed helps decode this description. In the 10-20 system, letters denote the scalp region: C means central (midline between frontal and parietal areas). The numbers indicate left or right: odd numbers are left of midline, even numbers are right, with Z for midline. A point described as 30% above the left pre-auricular crease sits on the left side and along the central region toward the vertex. That places it at the left central site, which is designated as C3. It’s between the frontal and parietal lobes over the left sensorimotor cortex. The other options correspond to different lobes: frontal (F3), parietal (P3), and occipital (O1), which would be in more anterior, more posterior, or more occipital locations, not the central left spot described.

Understanding how EEG electrodes are placed helps decode this description. In the 10-20 system, letters denote the scalp region: C means central (midline between frontal and parietal areas). The numbers indicate left or right: odd numbers are left of midline, even numbers are right, with Z for midline.

A point described as 30% above the left pre-auricular crease sits on the left side and along the central region toward the vertex. That places it at the left central site, which is designated as C3. It’s between the frontal and parietal lobes over the left sensorimotor cortex.

The other options correspond to different lobes: frontal (F3), parietal (P3), and occipital (O1), which would be in more anterior, more posterior, or more occipital locations, not the central left spot described.