Visually evoked potentials.

The term visually evoked potential (VEP) refers to electrical potentials recorded from scalp overlying visual cortex that have been extracted from the electroencephalogram by signal averaging. Usually the recording electrode is placed on the midline of the occipital scalp at the back of the head. VEPs are used to quantify the functional integrity of the optic nerves, pathways to the visual cortex of the brain, and occipital cortex. Any abnormality that affects the visual pathways or visual cortex in the brain can affect the VEP. Examples include slowing neuronal transmission, such as produced by myelin plaques common in multiple sclerosis, or gliomas on optic nerves in neurofibromatosis slowing the speed of the VEP wave peaks. Compression of the optic pathways, such as from hydrocephalus or from a pituitary tumor, affects the VEP. There are several methods of recording VEPs. In patients over about 3 years of age VEPs are usually recorded using a video monitor presenting patterned stimuli. In sedated patients and infants, flashes of light from a strobe flash or an array of LEDs are used to stimulate the eye. Multifocal VEPs expand the visual field topographic mapping to beyond 40 degrees of the central visual field.