1.4 Electrooculography 41
In 1948 Du Bois Reymond reported that in the normal eye there is a flow of electrical current that is oriented so the cornea is relatively more positive than the posterior pole of the eye. This potential difference is referred to as the stand- ing potential or resting potential of the eye. The electrooculogram (EOG) is an indirect measure of the amplitude of the standing potential, which changes during dark and light adapta- tion. To obtain an EOG, electrodes are placed at the inner and outer canthi of the eyes, and the patient is asked to look back and forth between a pair of fixation lights. When the cornea moves closer to one of the electrodes, it becomes more positive and the other electrode becomes more negative. The opposite happens when the eyes move to the other side.
The changes in the amplitude of the EOG in the dark-adapted and light-adapted state of a normal subject are shown in Fig. 1.38. The smaller amplitudes are recorded when the eyes make the saccadic eye movements in the dark;
this is called the “dark trough.” The peak ampli- tude is recorded against a steady light back- ground, which is called the “light peak.” The light peak/dark trough (L/D) ratio is an index (Arden index) used to assess retinal function [1]. A ratio of 1.80 is the lower limit of normal in our clinic.
The origin of the retinal standing potential is thought to be in the retinal pigment epithe- lium (RPE). However, the light rise is generated by light stimulation of the photoreceptor–RPE complex; and it is detected only if certain struc- tures in the middle retinal layer are normal.
Reference
1. Arden GB, Baradda A, Kelsey JH (1962) New clini- cal test of retinal function based upon the standing potential of the eye. Br J Ophthalmol 46:449–465
1.4 Electrooculography
Fig. 1.38. Electrooculography of a normal subject RDI1(104%) 9/9/05 5:33 PM Page 41
