Electrophysiology in adults

When you arrive, please follow the signs to Electrophysiology on the first floor and report to the reception desk in the department.


What is visual electrophysiology? 

When you look at something, an image of the object is projected onto the retina at the back of your eye. The retina converts this optical image into very small electrical signals, which pass along the optic nerve to the brain, where the sensation of ‘seeing’ occurs. Visual electrophysiology measures these very small signals produced by the eye and the brain.


Why am I being referred?

Your consultant has asked us to examine the function of your visual system. This is usually to help diagnose the cause of your visual problem. The tests are also used to monitor disease or the effects of any treatment you may be receiving.


What is involved

The tests are non-invasive and do NOT involve injections.

To perform the tests

  • Small contacts are fastened to the skin around the eyes or scalp (depending on test),using tape or paste.
  • Some tests also use a thin silver thread that rests on the lower eyelid. These are not uncomfortable but may initially make your eyes water a little.
  • All contacts will be removed at the end of testing and the paste will be removed as much as possible.
  • Dilating drops are needed for most of the tests, and a 10 - 20 minute period is required for them to take effect.
  • Some tests (ERG) require you to spend 20 minutes in the dark before testing, to fully assess retinal function.
  • The testing can take up to 3 hours depending on the number of tests.


What tests can I expect?

When you attend you will have a selection of the tests over the page, depending on the reason for referral:

Pattern electroretinogram (PERG)

  • Measures the function of the central retina (the macula), used for central vision and for viewing fine details e.g., reading.
  • You will be asked to look at a moving checkerboard pattern on a TV whilst we record the small electrical signals generated in the macula.

Visual evoked potential (VEP)

  • Investigates the optic nerves and how well signals are transmitted from the eye to the brain.
  • You will be asked to observe a moving checkerboard pattern on a TV or flashes of light, while electrical signals are recorded using contacts placed at the back of the head.

Electroretinogram (ERG)

  • Records the electrical signals from the whole of the retina to flashes of light in both darkness and in light.
  • The brightness, flash rate and colour of light are varied to allow the rod system (night vision) and cone system (daytime vision) to be examined separately.

Multifocal electroretinogram (MfERG)

  • Measures the function of the central macula and surrounding small retinal areas.
  • You will be asked to watch a rapidly changing pattern made up of 61 hexagons, while responses are recorded.

Electro-oculogram (EOG)

  • Measures the function of the whole of the retinal pigment epithelium (the layer under the retina).
  • You will be asked to make eye movements between two small alternating red lights, for about 20 seconds each minute, for 15 minutes in the dark and 15 minutes in the light.


Are there any side effects?

Dilating drops will cause blurred vision and increased sensitivity to bright light/sunlight, but this will wear off after a few hours. Dark glasses may help on a sunny day.

You should NOT drive for several hours after the testing has finished, until the effects of the eye drops have worn off.


What will happen next?

The results of the tests will be analysed, and a report will be sent to the consultant who referred you for testing.

This leaflet is intended to provide general information for patients. If you require further information or advice, please contact the electrophysiology department:




Author: Andrew Carter, Technical Manager

Review date: October 2026