Using the 10-20 placement system, if the head circumference is 58 cm, what is the distance between C3 and O1?

Using the 10-20 placement system, if the head circumference is 58 cm, what is the distance between C3 and O1?

• A. 14.5cm
• B. 29cm
• C. 11.6cm
• D. 2.9cm

Answer: (B)

In the 10-20 system, electrode locations are defined as percentages of the head circumference, so distances along the scalp reflect fractions of that circumference. C3 and O1 sit roughly on opposite sides along the back/left portion of the head, spanning about half of the head’s circumference when measured along the skull from central to occipital regions. Therefore, the distance between them is about half of the head circumference. With a head circumference of 58 cm, half of that is 29 cm, which is why the distance between C3 and O1 is about 29 cm. The other options correspond to much shorter arc lengths (roughly a quarter or less of the circumference or far less), which wouldn’t match the approximate arc between C3 and O1.

In the 10-20 system, electrode locations are defined as percentages of the head circumference, so distances along the scalp reflect fractions of that circumference. C3 and O1 sit roughly on opposite sides along the back/left portion of the head, spanning about half of the head’s circumference when measured along the skull from central to occipital regions. Therefore, the distance between them is about half of the head circumference.

With a head circumference of 58 cm, half of that is 29 cm, which is why the distance between C3 and O1 is about 29 cm.

The other options correspond to much shorter arc lengths (roughly a quarter or less of the circumference or far less), which wouldn’t match the approximate arc between C3 and O1.