ERG evaluates the functioning of the retina—the light-sensitive layer of tissues that lines the back of the eye. Normally, retinal photoreceptor cells (called cones and rods) receive light and transform it into electrical impulses that are sent to the brain. In ERG, small sensors (electrodes) embedded in contact lenses are applied to the eyes to record the electrical responses of the retina to light stimuli.

Purpose of the ERG

  • To assess functioning of photoreceptors and relay cells in the retina
  • To help diagnose or evaluate certain retinal disorders, including hereditary conditions such as retinitis pigmentosa; toxic retinopathies resulting from medications, chemicals, or metallic foreign bodies; circulatory problems in retinal blood vessels; degenerative diseases such as diabetic retinopathy; and remote ocular effects of some cancers
  • To determine if retinal surgery should be performed

Who Performs ERG

Before the ERG

  • The examiner administers eye drops to dilate your pupils.
  • You will wait in a dark room or a room lit only with a soft red light.
  • When your pupils are dilated, anesthetic eye drops are given to numb the eyes.

What You Experience during ERG

  • Contact lenses (containing electrodes) are placed on each eye. A background patch electrode is applied to the forehead or earlobe.
  • Electrical activity of the retina, induced by a flash of light, is first recorded in the dark and then after your eyes have adapted to the light. Different light stimuli, such as a reversing checkerboard light pattern, may be used. The intensity, color, and frequency of the light may be varied.
  • The procedure usually takes about 30 to 45 minutes.

Risks and Complications of ERG

  • In some cases, the electrode causes a temporary scratch on the surface of the cornea.

After the ERG

  • Do not rub your eyes for at least 30 minutes (until the anesthesia wears off) to avoid injuring the cornea.
  • Your pupils may stay dilated for 3 to 4 hours. You may find it more comfortable to wear dark glasses until your vision has returned to normal.
  • Arrange for someone else to drive you home.

Results of ERG

  • The electrical responses of the retina are displayed as waves on a viewing monitor and recorded on paper or photographic film. The patterns of waveforms are examined for abnormalities.
  • A doctor will review the test results. These findings will be considered along with your symptoms, your eye exam, and the results of other tests—such as fluorescein angiography and visual field testing—in order to diagnose any retinal disease or condition.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 12 Jan 2012

Last Modified: 16 Dec 2014