ElectroretinogramAn ERG might be necessary to evaluate the function of the retina when the ophthalmologist suspects retinal disease or when the retina can not be visualized due to some opacification of the eye (usually a cataract). An ERG records the total electrical response of the retina to a light stimulus by measuring the difference in potential between the retina and the cornea. An ERG is recorded by placing a special contact lens on the cornea and then flashing a series of bright lights into the eye.