What is the VEP test?

VEP stands for "Visual Evoked Potential". It is one of the special visual tests which are not done as part of a routine exam at Children's Hospital. Both the VEP and the ERG are tests designed to give very specific information about what is happening in the eye that cannot be determined any other way.

Two of the developers of the SVEP Acuity test are shown testing a young infant. A young infant taking the SVEP Acuity test

In the VEP, electrodes are mounted over the visual part of the child's brain, towards the back of the head. The child sits in front of a television screen where a pattern is presented. A toy is dangled in front of the television to attract the child's attention to the center of the screen. The VEP is the electrical response of the brain to a simple patterned stimuli similar to the gratings used in the Teller Acuity cards. The VEP stimulus alternates, the black bars become white, the white bars become black, and the stripes become narrower and narrower. In a ten-second trial the gratings go from very wide to very narrow. The big ones make big signals and the small ones make smaller signals until you can no longer distinguish them from the ongoing electrical activity of the brain. A calculation is then done to fit a line to the data in order to come up with an estimate of acuity.

An example of a VEP result:
An example of a VEP result

In cases where a disease of the optic nerve is suspected, such as optic nerve hypoplasia, the VEP is done a little bit differently. A waveform is extracted in order 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. An example of such a result is shown in the figure below. Diseases which affect the optic nerve can cause delays.

Example of a VEP waveform done in the case of suspected optic nerve disease Example of a VEP waveform done for a case of suspected optic nerve disease
Additional Resources about the VEP: