Variations in the normal electro-oculogram.

THE existence of an electrical potential in the vertebrate eye, with the cornea positive to the fundus, was demonstrated over a century ago (Du Bois-Reymond, 1849). This potential, the standing or corneo-fundal potential, is often described as a steady potential to distinguish it from the rapidly oscillating potential of the electroretinogram. It is not, however, a truly steady potential, but is constantly changing, and among the factors causing it to vary is a change in illumination. During studies to elucidate the nature of the light-induced change of the standing potential, it was found that in the dark the potential falls to reach a minimum level, the "dark trough level", and on subsequent re-illumination there is a rapid rise in potential to reach a maximum within 8 or 9 minutes, the "first light peak level". The height of the potential rise after darkness depends upon the time in the dark and the intensity of the subsequent re-illumination (Arden and Kelsey, 1962a). On the basis of these studies, a clinical test was devised as an objective measurement of retinal function, known as the electro-oculogram (EOG). The basis of the test is a comparison of the level of the dark trough with that of the first light peak and to express the result as a ratio (Arden, Barrada, and Kelsey, 1962):