The visual evoked potential (VEP) measures visual acuity, vision for detail and patterns. In adults, visual acuity is measured with the letter chart. For those who are unable to read the letter chart, the VEP is used. It measures acuity by assessing the response of the brain to alternating black and white stripes. This acuity test is most helpful in those youngsters for whom other vision tests, like the preferential looking test, give ambiguous or incomplete information about visual acuity.