 - Electroretinogram ERG to measure electrical responses generated from within the retina. For this test, the patient sits in a dark room for 30 minutes with his or her eyes patched. Then, a small silver disk electrode is taped to the forehead, the eye patches are removed, the surface of the eye is numbed with eye drops, and contact lenses are placed on the eyes. The patient looks inside an open white globe that emits a series of light flashes for about 20 minutes. The contact lenses sense small electrical signals generated by the retina when the light flashes. 