Did your grandmother ever tell you to scrub behind your ears? If she did, she’s about to be very pleased.

​The skin behind the ears will be the location for two more of our reference electrodes. 

​Swab the skin over the area behind both ears (the skin over the skull, not over the ear itself).

​Abrade the area, focusing particularly on the ‘bone bump’ if you can identify such a bump.

Given this is a difficult area to obtain a good signal, it may be worthwhile to spend extra time on the abrasion step.