If C4 must be moved 1 cm posteriorly due to an incision, which other electrode must be moved accordingly?

The correct choice involves moving the C3 electrode 1 cm posteriorly when C4 is repositioned for anatomical reasons. This decision is based on maintaining spatial relationships between the electrodes, as they are part of a standardized system for electrode placement on the scalp, known as the 10-20 system.

In the 10-20 system, electrodes are placed at specific locations to ensure consistent and reliable readings across different patients and settings. When one electrode, such as C4, is moved, it is often necessary to adjust nearby electrodes to preserve the overall layout and spatial orientation. C3 is directly adjacent to C4 on the contralateral side of the head, and moving it in correspondence with C4 helps to maintain the relative distances between the electrodes, which is crucial for accurate EEG interpretation.

Maintaining these relationships is important as the signals detected by EEG electrodes are influenced by their position. If changes are made to the placement of one electrode without compensating for adjacent ones, it could lead to misinterpretation of the brain's electrical activity due to altered spatial representation of the electrical signals.