Frequently Asked Questions - Electrodiagnostics

Electrodiagnostics is a type of testing that allows us to get a better view of how well the connections from the eye to the brain are working. In cases of amblyopia and strabismus, head injury, brain injury and in conditions where a person is non-verbal or has trouble responding to health care providers, as in the case of infants, electrodiagnostic testing can give insights into the degree a signal is getting from the eyes to the brain.

Electrodes are placed on the head. NOTE: These are paste on electrodes that do not require the hair to be cut, nor are any needles used. In some instances ear lobe clips are used as well, which also do not penetrate the skin and as such they do not hurt. The patient sits in front of a stimulator, generally a television screen with changing checkerboard patterns on them of different sizes. The signal from the electrodes goes into the computer and we measure the response of the primary visual cortex to the changing checkerboard patterns.

We look at several things. The first is how much signal we get from the brain that is time-locked to the checkerboard patterns. The more signal we get the better AND we are looking to get about the same amount of signal from each eye. We also look to see if we get a bigger signal from both eyes together than with either eye alone. Most people know that they see better with both eyes open. This "binocular summation" gives us insight into how well this two-eyed type of seeing is present in our patient.

We also look at the latency of the response; which is how long does the signal take to get to the brain. If it takes too long or is long on one side, this may be an indication that some other disease process is taking place that may need to be addressed by other health care providers.

Below are questions that are of a technical nature. They are here for those interested in this aspect of these testing protocols.


The following are some theoretical sample form four made up subjects.  This show only the amplitudes of the electrical signal measured at V1 (visual cortex).  All measurements are in micro-volts.  


Eye Subject 1 Subject 2 Subject 3 Subject 4
Right 18 18 18 18
Left 18 9 9 9
Both 23 18 12 22


Subject 1 shows the normal binocular summation with the signal from both eyes being significantly bigger than the signal from either eye alone, and both eyes signals are the same.

<p> Subject 2 shows decreased amplitude in the left eye as compared to the right (9 μV compared to 18 μV) with the binocular signal the same as the non-amblyopic eye.  Here no binocular summation is occurring but neither is the amblyopic channel causing a decrease in the signal from the non-amblyopic channel.  

<p> Subject 3 shows decreased amplitude in one eye that is actually interfering with the signal from the non-amblyopic eye and indicates a problem NOW that will lead to more of a problem over time.  This is seen by the binocular amplitude actually being less than the right eye amplitude alone.  This implies that the reduced left eye flow is actually causing “noise-on-the-line” when both eyes are open causing a reduction in the binocular amplitude to less than that of the right eye. Most people will not stay in this condition for a long time as it can be quite bothersome.

<p> Subject 4 shows decreased amplitude in one eye but that eye's flow is being used and there is a bigger binocular signal than with the non-amblyopic eye alone.  This is rare but it does happen.

<p> The following slides are real data from a real patient shared by Paul Lederer, OD from Arlington HeightsIL Time increases from left to right and the amount of electrical potential are measured in the deflection up and down of the lines. 

The right eye here shows a classic normal pattern.  The right eye recordings show a small dip down before rising to their peaks.  The peaks, marked with the small vertical line at the highest point of each line, occur at the normal time of around 100 milliseconds.  The left eye has the dip down at about the right time but the lines keep rising to a smaller degree at a later time (more to the right).  Here the peaks are easily 40-50 milliseconds later than they should be and the total amount of signal from the lowest point to the highest point is smaller than for the right eye.



This second diagram shows two binocular recordings group together at the top with the right eye recording below that and the amblyopic left eye being the lowest one.  You can see that the shape of the right and binocular recordings are very similar in pattern and timing of the highest peak.  In the left eye recording here there is very little of the dip down before the slow rise to a later time.  Here the late shift is smaller than in the example just prior to this.




Here there are two right eye recordings grouped at the top, with two left eye recordings in the middle with the two binocular recordings at the bottom.  Now all of the recordings have the same pattern but with differing amplitudes.  The right eye by itself has a larger amplitude than the left eye.  However, the timing of the peaks is nearly the same (115-116 milliseconds for the right eye to 122-123 milliseconds for the left eye).  It can easily be seen that the binocular recording has the largest amplitude yet and the timing of the large peak is between the timing of the peak times of the right and left eyes respectively at 119-120 milliseconds.   The following table has the averages of the six recordings from above.


Eye Amplitude Latency
Right 15.4 115.5
Left 12.0 122.5
Both 17.8 119.5


Here even though the amplitude of the left eye is below that of the right it is obviously contributing to the binocular signal because the binocular pattern is larger than the right eye amplitude alone.  This is a powerful demonstration of the ability of optometric treatment to change signal transmission from the eye to the primary visual cortex and to normalize a system that appeared to be beyond care.


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.

The main extra piece of information that the VEP/VER adds to ones diagnostic battery is a test of the neurology of the primary flow from the eye 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. 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.

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.

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.