An electric potential is produced in the retina of the eye, which makes the front of the retina and the cornea electrically positive and the back of the retina and the bulb negative. When the retina is stimulated by light this potential increases, and an electric action potential, the electroretinogram, or ERG, develops. The appearance of the curve formed by these potentials differs in various animal species and also varies with the intensity of light stimulus, as well as with the dark and light adaptation of the retina.
For clinical purposes, the potential is led off from an active electrode, consisting of a chlorinated silver rod inserted in a contact glass of plastic or glass, as first used by Riggs (1941). Contact is established between the corneal surface of the eye and the electrode by means of saline, which fills the space between contact glass and
KARPE G. Indications for Clinical Electroretinography. AMA Arch Ophthalmol. 1958;60(5):889-896. doi:10.1001/archopht.1958.00940080909009