Visual Evoked Potential (VEP) Test

What is a visual evoked potential test?

A visual evoked potential (VEP) test measures the electrical signal your visual cortex (a region of your brain) generates in response to visual stimulation. The test is also called a visual evoked response (VER).

More specifically, a VEP test assesses the function of your visual pathway, which includes your:

• Eyes.
• Optic nerves.
• Optic chiasm (the place in your brain where some of the optic nerve fibers coming from one eye cross optic nerve fibers from the other eye).
• Optic tract (the pathway between the optic chiasm and your brain).
• Optic radiation (the part of your visual pathway that transmits visual input coming from your retina, optic nerve and optic tract).
• Cerebral cortex.

Any abnormality that affects the visual pathways or visual cortex in your brain can appear as an abnormality on the VEP test.

What does a visual evoked potential test diagnose?

Visual evoked potential tests can help diagnose or point to the following conditions:

• Multiple sclerosis (MS), which often causes optic neuritis. This is the most common reason why healthcare providers order a visual evoked potential test.
• A glioma on your optic nerve (usually due to neurofibromatosis type 1).
• Compression of your optic pathways, such as from hydrocephalus or a pituitary adenoma.
• Visual pathway damage related to head or brain trauma.
• Toxic optic neuropathy due to certain medications or other toxins like carbon monoxide.
• Other types of optic neuropathy, such as anterior ischemic optic neuropathy.
• Visual pathway issues due to infections, such as meningeal tuberculosis.

VEPs can also help providers assess the progression of certain already-diagnosed neurodegenerative conditions. Examples of these conditions include:

• Multiple sclerosis (MS).
• Leber optic neuropathy (LON).
• Kjer-type dominant optic neuritis (DON).
• Friedrich’s ataxia.

How does a visual evoked potential test work?

A visual evoked potential test involves electrodes placed on specific parts of your scalp and viewing a visual stimulus — most commonly, a flashing checkerboard pattern.

Electrodes on your scalp measure electrical signals (impulses) as they travel between brain cells. As the visual pattern flashes, your visual pathway responds to and interprets the stimulus. The electrodes record the electrical activity in your brain that results. Healthcare providers run VEP tests in the background of an electroencephalogram (EEG).

The evoked potential machine averages the EEG signals following multiple visual stimuli so it can assess the functioning of your visual pathway. The evoked potential machine records the electrical response to visual stimulation on several channels or traces. Your brain (visual cortex) has specific waveforms that occur at very specific times in response to a visual stimulus.

A specialist then interprets the wave patterns and looks for abnormal activity, which can point to various issues with your visual pathway.

How do I prepare for a visual evoked potential test?

Your healthcare provider will go over what to expect during the test and what to do prior to it. Don’t hesitate to ask questions or voice your concerns. Be sure to follow their preparation instructions.

Before the test, you’ll need to make sure your hair is clean and doesn’t have any hair sprays, oils or gels in it. These products can interfere with the function of the electrodes.

What can I expect during a visual evoked potential test?

There are several different types of VEP tests, so the process may vary.

You can generally expect the following:

• A healthcare provider will place electrodes (small metal disks) in specific places on your scalp with removable glue. The electrodes attach to a machine that gives your provider information about your brain’s activity.
• You’ll wear a patch over one eye and look at a flashing visual stimulus (usually a checkerboard pattern) projected on a monitor. Once the provider is finished testing one eye, you’ll repeat the process for your other eye.

How long does a visual evoked potential test take?

The test may take up to 60 minutes to complete.

What do the results of a visual evoked potential test mean?

Once a specialist has interpreted the results of your VEP, they’ll put together a report and send it to your healthcare provider. Your provider will then share the results with you.

VEPs can detect several different issues in your visual pathway. And each issue has a different presentation in terms of the shape of the wave and the length of time between the stimulus and your brain’s activity. For example, in demyelinating conditions of the optic nerve (like MS), the VEP test shows a delay in your brain’s responses.

Again, your healthcare provider will explain the results to you and let you know if you need any additional testing.