Which electrode is located 30% above the left preauricular crease?

The electrode located 30% above the left preauricular crease is C3. This placement is based on the International 10-20 system of electrode positioning used in electroencephalography (EEG) and polysomnography. In this system, "C" denotes a central electrode, and the number indicates its location relative to the left ear (preauricular crease) and the midline of the head.

The specific placement of C3 is important as it is intended to monitor the central region of the brain that is involved in motor control and sensory processing. The 30% positioning highlights the precise anatomical landmark, which is foundational for acquiring accurate and consistent neurophysiological data in sleep studies and other monitoring settings.

In contrast, the other electrodes—C2, P3, and O1—are located in different positions according to the International 10-20 system, directing them to other regions of the brain. C2 is positioned on the right side, P3 is located more toward the posterior part of the head compared to the central area, and O1 is placed over the occipital region. Hence, they do not correspond to the specific reference point indicated in the question.