Using the 10-20 system, the C4 lead is located what percentage of the circumference away from C3?

Using the 10-20 system, the C4 lead is located what percentage of the circumference away from C3?

• A. 20%
• B. 30%
• C. 40%
• D. 50%

Answer: (C)

In the 10-20 system, electrode positions are defined by percentages of the head’s circumference, with Cz at the midline and lateral sites like C3 and C4 located off that midline by equal steps. C3 and C4 are each about 20% of the head circumference away from Cz on opposite sides, so the path around the head from one to the other goes through Cz and spans about 40% of the circumference. That’s why the correct distance is forty percent. The other options would correspond to only reaching Cz (twenty percent) or attempting a longer wrap around the head (fifty percent or more), which isn’t the spacing used for these two sites.

In the 10-20 system, electrode positions are defined by percentages of the head’s circumference, with Cz at the midline and lateral sites like C3 and C4 located off that midline by equal steps. C3 and C4 are each about 20% of the head circumference away from Cz on opposite sides, so the path around the head from one to the other goes through Cz and spans about 40% of the circumference. That’s why the correct distance is forty percent. The other options would correspond to only reaching Cz (twenty percent) or attempting a longer wrap around the head (fifty percent or more), which isn’t the spacing used for these two sites.