- Electroretinography (ERG) in patients with suspected retinal degeneration. This is a test of the electrical function of the eyes. Before the test, patients sit in a dark room for 30 minutes with their eyes patched. A small electrode (silver disk) is taped to their forehead. The eye patches are then removed, the eyes are numbed with drops, and contact lenses are placed in the eyes. The contact lenses sense small electrical signals generated by the retina when lights flash. During the ERG recording, the patient looks inside a large empty bowl. A light flashes, first in the dark and then with a light turned on inside the bowl. The test takes 1 hour or less.