An electroretinogram (ERG) is an electrical test of the retina. Similar to the recording of the electrical wave and rhythm of the heart or electrocardiogram (ECG), the response of the retinal photoreceptors to light also has an electrical wave that can be recorded. The ERG is a test of the ability of the retinal photoreceptors, the rods and cones, to respond to light. ERGs are performed when an ophthalmologist cannot visualize the retina on examination due obstruction, like cataracts, or when the retina can be visualized but looks normal or abnormal. Patients that are having cataract surgery will have an ERG to test the function of the retina so that after surgery we a confident that the patient will be visual. Patients that have sudden blindness and no retinal examination abnormalities may have a condition call Sudden Acquired Retinal Degeneration Syndrome (SARDs) or may have central blindness from a dysfunction of the brain or optic nerve. The diagnosis of SARDs is based no response of the photoreceptors on ERG or a flat ERG. If a patient is blind and the ERG and ophthalmic exam are normal then central blindness is the likely diagnosis. These are the most common ways that ophthalmologists used ERG in practice. Conditions like early progressive retinal atrophy (PRA) and specialized ERGs can also be performed.