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 in response to the stimulation, hence the name. 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.

Why does it have two names (Visual evoked potential/visual evoked response)?

Over time this procedure, has been called by different names. The word evoked is part of the name since active and controlled stimuli are used to drive the visual system. Because we care about how detail is being moved through the system the stimuli are various size checkerboards of different spatial frequencies. It is the changing patterns that drive or evoke the response.

Those who use the word potential are generally referring to the changes in the electrical firing levels in the primary visual cortex. Those who use the word response are simply looking at the entire pattern of the graph as the response the primary visual cortex is making secondary to the triggering of the stimuli. The two terms are essentially interchangeable.

What information does this add to my diagnostic battery?

The main extra piece of information that the VEP/VER adds to one’s diagnostic battery is a test of the neurology of the primary flow from the retina back to the primary visual cortex (V1). It is good to know as soon as possible whether or not there may be any frank neurological involvement that would preclude a positive outcome. One would very much like to know if there is an active problem that requires care from other disciplines or as an aid in deriving an accurate prognosis. The VEP/VER does not answer all of these questions unequivocally, but it does add significantly to the clinical picture of these types of cases. It only takes one good recording to tell us the neurology is intact and the potential for improved vision exists.

What kinds of cases could benefit from this information?

VEP/VER can be used with any patient. It yields the greatest insights when working with amblyopia, strabismus, non-malingering syndrome, and with some non-verbal patients. The loss of visual acuity associated with these conditions can mimic other conditions that may be secondary to neurological problems. Some have used the devices to monitor changes in binocularity before and after treatment, and to do a form of refraction in some cases. The VEP/VER can be helpful in determining if the loss of visual acuity is functional or developmental, and therefore more treatable, or if the loss is secondary to neurological damage.

If I don’t have the device, what specifications for testing parameters should I ask for when ordering the test?

Since the typical non-optometric use of these devices normally is only looking for gross responses, most testing centers only have bright flash stimuli capability. It is essential in talking with a center to find out if they have the ability to do pattern reversal stimuli.

The typical testing protocol involves testing at least five different spatial frequencies (different size boxes or checkerboards) in a number of different test conditions such as whether a patch is to be used or not. In non-strabismic patients it is recommended that testing go from binocular, to one eye, the other eye and back to binocular. If the electrode attachments are variable or if a youngster pulls off a lead one can get decreasing amplitudes over the course of the testing. Thus, if the amblyopic eye is left for the end, it may be difficult to know if the lowered amplitudes are from the amblyopia or from the decreasing sensitivity of the electrode. Thus, it is suggested that after the first binocular testing that the amblyopic eye be the first monocular eye tested followed by the non-amblyopic eye. Thus, if the amplitudes jump back up, then you can be certain that the electrodes are still in place. By repeating the binocular testing at the end, this also acts as a kind of redundancy check. If the amplitudes return to the early levels from the first set of binocular recordings then one can assume that variations in the monocular testing data is not from electrode problems but represents true signal variations through the channels.

Sometimes in strabismic testing an additional testing step may be added and that’s a binocular recording set with compensatory prism in place. This is to see if binocular summation emerges as a result of alignment changes. For relatively small deviations of the eyes the repetitive nature of the checkerboard stimuli takes care of the need for prism as there most likely is overlap of the signal due to it being repeated over and over. They may be overlapping the wrong set of checks but they are still overlapping checks and this is generally enough to get the binocular summation if it is present. With large angle turns the two images may be separated too far to get simultaneous stimulation anywhere: thus the need for the compensating prisms.