ERG is a test that measures the changes in the standing potential of the eye induced by brief light stimulation. Many different types of ERG exist (to learn more please follow the Further reading link below). The two tests offered by the USF Eye Institute are named based on the type of light stimulation used.
What to expect from the test?
- Diffuse light: brief (typically 4 msec or less) light stimulus illuminating the whole surface of the retina. The resulting signal and, generally, the test are referred to as flash or full-field ERG. Sometimes this test is called also “standard ERG”, of just “ERG”, as it is the most frequently administered type of ERG test
- Patterned light: stimulus illuminating the central part of the retina, which is temporally modulated (changes its appearance during the test, typically at least once per second), but is of constant mean luminance. The resulting signal and the test are referred to as pattern ERG (PERG)
Full-field ERG provides information about the functioning of all cell types in the retina. Pattern ERG provides information mostly about the functioning of neurons in the inner retina (e.g. ganglion cells, etc.).
How to prepare for the ERG test?
The full-field ERG test lasts about one and a half hours; the pattern ERG test lasts about one hour.
Please arrange and plan in advance the mode of transportation to and form the clinic. Full-field ERG typically requires the dilation of both pupils and the effects on your vision can last for several hours. Pattern ERG does not require pupil dilation, but requires distance correction, therefore, it is recommended that you bring your most recent eyeglasses for distance or the equivalent prescription.If you are not sure which test will be performed, please ask our office.
Electrodes will need to be placed on your forehead and earlobe. Please do not apply make-up in the morning and remove all earrings before the test. If you are a contact lens user, please make arrangements to remove and store your contact lenses for the duration of the test (this requirement is valid for both full-field and pattern ERG).
Changes in the standing potential of the eye resulting from long-duration (in the order of several minutes) changes in retinal illumination can be recorded indirectly by measuring voltage changes in skin electrodes induced by horizontal movements of the eye following an alternating fixation target. The recording of these changes over time is called electrooculography (EOG). The type of test most frequently used in the clinic typically analyzes the ratio between the decrease in the standing potential in the dark (“dark trough”) vs. the increase of the potential in the light (“light peak”).
What to expect from the EOG test?
The EOG provides information about the functioning of the retinal pigment epithelium and the overlying photoreceptors.
How to prepare for the test?
This test takes about 45 minutes. Pupil dilation is not required.
Visual Evoked Potential (VEP)
Electrophysiological signals extracted from the electroencephalographic activity of the visual cortex (recorded from the overlying scalp) as a result of light stimulation are called visual evoked potentials (VEPs). There are several different types of VEP tests. As with ERG, the two types of tests performed at the USF Eye Institute are named based on the type of stimulation used:
What to expect from the test?
- Flash stimulation, and therefore the test is called flash VEP (fVEP)
- When pattern stimulation is used the test is called pattern VEP (pVEP)
Both the flash and the pattern VEP test evaluate the functioning of the optic nerve and the part of the brain responsible for primary analysis of the visual information (“primary visual cortex”). However, the normal functioning of both the optic nerve and the primary visual cortex are depended on a normal functioning of the eye (the retina), therefore, the VEP reflects the overall health of the visual system.
How to prepare for the VEP test?
The test takes about one hour.
The flash VEP test requires dilation of the pupils, and the effects on your vision can last for several hours, therefore, arrange and plan in advance the mode of transportation to and form the clinic. The pattern VEP does not require pupil dilation, but requires distance correction, therefore, it is recommended that you bring your most recent eyeglasses for distance or the equivalent prescription. If you are not sure which test will be performed, please ask our office.
Electrodes will need to be placed on your head. If you are a contact lens user, you would need to remove temporarily your contact lenses for flash VEP, but not for pattern VEP.