An evoked potential measures how long it takes for nerves to respond to stimulation. The response speed in people with MS may be slower, indicating a lesion somewhere in the CNS. The tests measure the electrical activity of the brain in response to stimulation of specific sensory nerve pathways and can help determine a diagnosis of MS.

There are three types of evoked potential responses:

1. Visual evoked potential (VEP) is measured when the eyes are stimulated with test patterns. A checkerboard pattern on a computer screen or a strobe light might be used.

2. Somatosensory evoked potential (SSEP) is measured when the arms or legs are stimulated by a very mild electrical impulse.

3. Brain stem auditory evoked potential (BAEP) is measured when the hearing is stimulated by listening to a test tone.

The doctor is looking for both the size of the response and the speed with which the brain receives the signal. Weaker or slow signals may indicate that demyelination has occurred and that MS is a possibility. Each type of response is recorded from brain waves by using electrodes that are taped on the scalp.

The placement of the electrodes depends on which potential is being tested. To test visual potential, for example, the electrodes are taped near the back of the head over areas in the brain that are responsible for visual stimuli.

Currently, neurologists use the results of the VEP to aid in diagnosing MS as it identifies difficulty in transmission of the vision pathway — a common finding in MS. The results are not specific for MS, so evoked potentials by themselves cannot confirm a diagnosis. Other tests must be performed to complete the clinical picture.