Visual Electrophysiology, ERG, VEP, and Psychophysics
New England Eye Center offers several electrophysiology tests, which are more sophisticated tests that are not usually performed in a routine eye examination. They are usually performed under the direction of a neuro-ophthalmologist or retina specialist when a patient is having vision problems.
Electroretinography (ERG) Test
An ERG test measures the electrical response of the light-sensitive cells in the eyes. There are millions of these cells known as rods and cones in the back of the eye (the retina). The cones are responsible for color sensitivity. Rods are more sensitive to light than the cones.
Ophthalmologists usually perform ERGs to determine whether patients have retinal degeneration or loss of vision of unknown causes.
The ERG usually takes 60-90 minutes. An ophthalmic technician will numb the patient’s eyes with anesthetic drops, and then place a small electrode on each eye. The electrode is about the size of a contact lens. Another electrode is attached to the skin. Patients then watch a flashing light in both normal light and in a darkened room. The retina’s electrical response to light transfers to a monitor via the electrodes, and appears as waves that the doctor can measure and evaluate. Patients may feel slight discomfort, but the test is not painful.
Visual Evoked Potential Test (VEP)
This test is performed when patients are experiencing certain changes in vision that may be related to problems with the optic nerve. This nerve helps transfer signals that allow patients to see. The VEP measure the time it takes for a visual stimulus to travel from the eye to occipital cortex in the brain. The ophthalmologist can determine whether there are abnormalities along the nerve pathway. The test takes about 45 minutes, and is painless and non-invasive. A technician will attach some wires to the top of the head to detect brain waves. Patients then look at a variety of visual patterns on a screen. Readings are taken through the wires on the head, and evaluated by the doctor.
Preferential looking can be performed with a VEP to assess vision in infants and nonverbal children.