A visual evoked potential (VEP) refers to a systematic change in the electrical activity of someone's brain in response to a change in what the person is viewing. It is revealed by averaging over many trials the raw electrical activity, the electroencephalogram (EEG), recorded from 2 to 256 electrodes at standard positions on the person's head from each individual trial. For example, if a person is looking at a set of lines with one eye and at an oppositely oriented set of lines with the other eye, and then one set of lines changes to be identical to the other, the EEG from electrodes on the back of the head changes in a typical way over about 400 milliseconds (ms): a VEP (color insert, Figure ...