An electrooculogram is a signal that can be used for measuring the resting potential of the retina in the eye. By taking this measurement one can determine which way the eye is looking.
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.
Figure 1. Top view of a cross section of an EOG measurement; demonstrating sensor placement as well as the dipole of the eye and equations used for determining electric potential between the sensors.
Using these leads we can translate each change in voltage into a change in the eye’s position. If the eye looks to the right for instance, the positive pole of the eye will be nearest the right side sensor causing it to read a positive voltage while the back of the eye will be nearest the left side sensor causing it to read a negative voltage. From the voltage potential between these two sensors the left/right position of the eye can be determined. The same technique is used to determine the vertical position.