Fig. 4. ( A ) In the 10/20 convention, odd numbers refer to placements over the left hemisphere, even numbers refer to placements over to the right hemisphere, and Z refers to midline locations. Uppercase letters indicate underlying brain regions: C = central; F = frontal; Fp = frontopolar; O = occipital; P = parietal; T = temporal. ( B ) Sample of electroencephalographic recording. Each tracing comes from 1 of the 19 electrodes in the 10/20 array, positioned according to the labels at the left side of each trace. An artifact-free segment lies in the region above the two arrowheads on the bottom line.