What is a VEP test? What is an ERG? What do we learn from these special tests?
Case 1- A young infant, Chris, was referred to us because he had nystagmus, did not look at his mother’s face, and seemed to completely ignore all visual stimuli. He did not respond to any of the Teller Acuity Cards. Could he see at all?
From Norcia (1994) Two of the developers of the SVEP Acuity test are shown testing a young infant.
Chris was referred for the Visual Evoked Potential Test or VEP . The VEP is the electrical response of the brain to a simple patterned stimuli– similar to the gratings used in the Teller Cards.
The VEP stimulus alternates– the black bars become white, and the white bars black, and the stripes become narrower and narrower.
The narrowest stripe that the brain “sees” is the infant’s Acuity. Chris had an acuity of about 6 cycles per degree—this is equivalent to about 20/100. This is in the normal acuity range for 2 month old infants.
Because his family and ophthalmologist thought Chris could not see at all, we were asked to record his electroretinogram or ERG. The ERG is the massed electrical response of the retina to brief flashes of light. It is used in the diagnosis of retinal diseases.
The ERG is recorded using a small contact lens electrode that rests on the front surface of the eye.
The ERG response reflects the activity of retinal cells. We measure the amplitude and timing of the response components and compare them to age appropriate norms. b-wave a-wave
These are some of the responses we recorded from Chris. They are normal for a 2 month old infant.
Case 2. Pat was an infant who had roving disorganized eye movements, poor pupil responses to light, and did not appear to look at anything. He did not respond to any of the Teller cards, and we found no pattern response with the VEP . We were asked to do an ERG .
Pat Healthy Control Pat’s diagnosis: Leber Congenital Amaurosis
Case 3. Bob was a young child with poor vision and photophobia. His VEP acuity was only about 20/200– very low for his age. His ERG responses helped make the diagnosis.
Bob’s cone mediated ERG responses were markedly abnormal. Bob’s diagnosis was achromatopsia.
The VEP can also be used to assess the time it takes for a signal to go from the eye to the cortex. Normally this takes about a tenth of a second. Diseases that affect the optic nerve can cause delays From Norcia (1994)