A, Sevy (2010); B, Dewey (2015); C, Pollonini (2014); D, Our new honeycomb-shaped headpiece. The optode arrangement in all headsets is based on the International 10/20 system: A is centered at the T3/T4 position; the optode located in the middle of the bottom horizontal line in B, C and D is aligned with the T3/T4 position.