Which of these electrodes is located 20% of the horizontal measurement of the head (pre-auricular point to pre-auricular point) to the left of Cz?

The correct answer is related to the system of electrode placement used in electroencephalography (EEG), which often utilizes the International 10-20 system for locating electrode positions on the scalp. In this system, the locations of electrodes are based on specific percentages of the head's total horizontal and vertical measurements.

Cz represents the central point on the head, and the task is to identify which electrode is positioned 20% of the horizontal distance from the midline (Cz) to the left side of the head. C3 is positioned to the left of Cz, directly in line with it in a lateral direction. The placement of C3 corresponds to a location that is 20% of the distance from Cz to the pre-auricular point, which effectively makes it the electrode designated for this position relative to Cz.

This electrode location is important in EEG studies as it serves specific functions in brain wave monitoring and can help in localizing brain activities related to motor functions, sensory experiences, and cognitive tasks. Understanding the placements of these electrodes is essential for sleep technicians when interpreting EEG results.