Which electrode site is half the distance from T3 to T4?

The correct answer is Cz, as it is positioned at the midpoint between the temporal electrodes T3 and T4 on the International 10-20 electrode placement system. This system is used in EEG (electroencephalography) for accurate and standardized electrode placement on the scalp.

To understand why Cz is the correct answer, it's important to recognize the layout of the 10-20 system. The T3 and T4 electrodes are located on the left and right sides of the head, respectively, above the ears. The Cz electrode is placed centrally on the scalp, specifically at the midline equidistant from T3 and T4, thereby fulfilling the requirement of the question for being at half the distance between these two temporal sites.

The other options do not fit the criterion. Tz, for example, is an electrode placed slightly anterior to Cz but not at the midpoint between T3 and T4. T5 is located further down the side of the head and is not positioned between the temporal electrodes. P4 is located above the right ear and is also not on the midline. Therefore, Cz is the only option that accurately meets the description of being half the distance from T3 to T4.