The EOG is a test done to evaluate the health of the Retinal Pigment Epithelium (RPE) layer. Skin electrodes are placed on the patient on both sides of each eye, with a fifth electrode placed on the forehead.
Imagine the eye as a solar-powered battery, where the + terminal is the front of the eye, and the – terminal at the back. As the patient looks back and forth, the voltage recorded at each input swings up and down as the terminals approach and depart the inputs on each side of the eye.
The EOG starts with the patient in the dark. As the patient follows the fixation light back and forth, the “charge” in the RPE diminishes and the signal amplitude declines until it reaches the lowest point (the “Dark Trough”). Halfway through the test, the lights in the ganzfeld turn back on, the RPE “battery” begins to re-charge, and the signal amplitude rises until it reaches the “Light Peak”). The ratio of the Light Peak to the Dark Trough is called the Arden ratio. A healthy retina has an Arden ratio of at least 1.8 or 180%.
[Note: a full, more accurate explanation of the EOG can be found here: