What is a visual evoked potential?

A visual evoked potential is a diagnostic test that works based on picking up the electrical signals produced in the primary visual cortex time locked to a stimulus seen by the patient. It is essentially an EEG of the visual system. Electrodes are placed on the skull in such a way so as to allow the recording of the electrical potential changes, hence the name, in response to the stimulation. Many different types of stimulation can be used. One typical use has been by neurology in the diagnosis of demyelinating disease such as multiple sclerosis. By using a bright flash of light and seeing when the signal arrives at the primary visual cortex (V1) they can see if there is a slowing down of signal. If there is a slow down it is generally secondary to loss of myelin.

For our purposes, we care more about how detail from edges of various spatial frequency targets is being moved through the system and how the flows from the two channels are interacting with each other. The typical stimuli we use are varying size checkerboards that alternate, with white boxes changing to black and black boxes changing to white across the entire board several times per second.