The human eye is polarized, with the front of the eye being positive and the back of the eye being negative. This is caused by a concentration of negatively charged nerves in the retina on the back of the eye. As the eye moves the negative pole moves relative to the face and this change in the dipole potential can be measured on the skin in micro volts. To translate this voltage into a position, two sets of electrodes are used to measure the differential voltage in the vertical and horizontal direction. Figure 1 indicates how the electrodes are placed on the face. The red and black leads measure movement in the horizontal direction and the white and brown leads measure movement in the vertical direction. The green sensor is placed behind the ear or on the ear lobe to provide a ground reference.