A test in which the brain's electrical activity in response to visual stimuli (eg a flashing checkerboard) is recorded by an electroencephalograph and analyzed by computer. Demyelination results in a slowing of response time. Because this test is able to confirm the presence of a suspected brain lesion (area of demyelination) as well as identify the presence of an unsuspected lesion which has produced no symptoms, it is extremely useful in diagnosing MS. VEP's are abnormal in approximately 90% of people with MS.
