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Three types of evoked potential testing are commonly used: visual evoked potentials, auditory evoked potentials and somatosensory evoked potentials.

Visual evoked potentials
Visual evoked potentials (VEPs), also sometimes referred to as visual evoked responses (VERs), are the electrical responses recorded from the scalp at the back of the head over the areas of the brain involved with vision.

All evoked potential studies use electrodes placed on the scalp. These are just the same kind of electrodes as are used to record the EEG, but only a few electrodes, carefully positioned, are needed for evoked potential recording. The test is completely painless.

The stimulation most commonly used now for visual evoked potential testing is a black and white checkerboard pattern, with the black and white squares “reversing” repeatedly throughout the test, so that the black squares change to white and the white squares to black, and then back again. As you can imagine, this gives a sensation of movement of the squares.

The person usually sits in a comfortable chair and is asked to stay alert during the test. The lights are dimmed. The alternating checkerboard pattern is presented on a TV monitor placed at a standard distance in front of the chair. The screen will normally have a coloured dot in the middle, which the person is asked to look at during the test, so that the direction of their gaze remains fairly steady and doesn’t wander too much. The two eyes are usually tested separately.

How well someone can see may affect the visual evoked potentials quite appreciably and it is routine to test this with an optician’s letter chart. The person will be asked to wear his or her usual glasses during the test, which will normally take about 30 minutes.

Although a reversing checkerboard pattern is most commonly used to record visual evoked potentials, in some circumstances a simple flash of diffuse light may be used instead (flash VEPs). This is commonly done in assessing visual function in babies and young children, who are not able to focus on a normal checkerboard pattern. Flash VEPs may also be used in the further assessment of people who have not shown any response to the standard checkerboard pattern.

What is the test used for?
The use of visual evoked potentials in the diagnosis of multiple sclerosis has already been described. The test is also useful in the investigation of a whole range of other disorders affecting any part of the visual system from the eye itself to the visual areas at the back of the brain. Visual evoked potentials are used by ophthalmologists (specialists in eye disease) as well as by neurologists.

Auditory evoked responses
Auditory evoked responses (AERs) are also referred to as brainstem auditory evoked responses (BAERS) and brainstem auditory evoked potentials (BAEPs). The term brainstem is used because this area of the nervous system, which connects the base of the brain with the top of the spinal cord, is where the electrical responses recorded in this test are believed to originate.

During the test, the person sits in an easy chair or lies on a couch or bed in a quiet room and is made as relaxed and comfortable as possible. Unlike visual evoked potentials, AERs are not altered by sleep, and if anything, sleep may improve the quality of the recording. In some departments, a mild sedative drug will be given before the test and this may be particularly helpful when recording from babies or small children.

The person hears the sound stimuli through a pair of headphones. Various types of sound may be used, but clicks are the most common for investigating diseases of the nervous system.

At the beginning of the test, the sound is turned up and down until the point at which the person can just hear the stimulus has been found. The sound is then increased slightly for the actual test.

When a sound is played into one ear through a headphone, some sound will be conducted through the bones of the skull and can reach and stimulate the opposite ear. To avoid this, white noise is usually played into the headphone on the side opposite the one being tested by the clicks. White noise is a soft hissing sound, rather like the background noise when a stereo is switched on. Standard recording electrodes are placed on the top of the scalp and on or around the ears to pick up the AERs. The whole procedure will usually take about 30 minutes.

What is the test used for?
A delay in the conduction of signals through nerve pathways in the brainstem may occur in multiple sclerosis and this can often be revealed by AER testing. It is also used to investigate a variety of other disorders affecting the brainstem and adjacent areas.

As well as diseases of the nervous system, AERs can also be used to test hearing. In particular, AER testing is used in many hospitals in the assessment of hearing in premature or other sick newborn babies who may be at risk of hearing loss.

Somatosensory evoked potentials
Somatosensory evoked potentials - SEPs or SSEPs for short - sound complicated but are really quite straightforward. SSEPs measure the electrical response in the brain and spinal cord when a nerve in an arm or leg is stimulated with a small electrical pulse.

During the test, the person either lies on a couch or bed or sits in a comfortable armchair in a quiet room. Relaxation is encouraged as muscle tension or movement may interfere with the results of the test. As with AERs, a mild sedative drug is sometimes given if a person has difficulty relaxing.

If the arms are being tested, a small repeating electric pulse is given to the skin over one of the large nerves, usually around the wrist. This usually produces a slight twitch in the thumb. The electrical pulse itself feels like a tapping sensation on the skin, which most people do not find too unpleasant.

Following stimulation of the nerve at the wrist, the electrical signal produced by the pulse will pass up the arm along the course of the nerve and through a large network of nerves under the collarbone called the brachial plexus. An electrode is usually placed on the skin over the collarbone to detect the signal as it passes this point. The signal then travels on through the nerve roots in the neck and into the spinal cord. As it moves upwards in the spinal cord it can be picked up by electrodes placed on the skin at the back of the neck. Finally, the signal passes up through the brainstem into the areas of the brain that receive information about sensation from the body. These brain responses can be recorded from a small number of electrodes placed on the scalp.

If the legs are being tested, the electrical pulses will be applied to the skin over one of the large nerves either at the ankle or the knee and may produce a slight twitch of the ankle. The signal will pass up the leg along the course of the nerve and through the nerve roots into the lower end of the spinal cord. Electrodes placed on the back can pick up this signal as it travels up the spinal cord to the brain. The brain responses can again be recorded from electrodes on the scalp.

By following the course of these electrical signals as they travel towards the brain from the nerve in an arm or leg and by measuring the time they take to make this journey, it is possible to find out whether the spinal cord and the brain are functioning properly. It may also be possible to pinpoint the location of any problem areas. The test takes about 30 minutes and it is not at all unpleasant.

What is the test used for?
In multiple sclerosis, there may be a delay in the passage of the signals up the spinal cord or through the brainstem. This will show up in the test, which may also give information about where in the nervous system this delay has occurred. The test is also used to help with the diagnosis of a range of other disorders known to produce abnormal SSEPs.

SSEPs are now increasingly being used during surgery to monitor the spinal cord during delicate operations on the vertebrae or the spinal cord itself. Many people believe that recording SSEPs during spinal surgery can reduce the risk of serious injury to the spinal cord during this type of operation.

Motor evoked potentials
Motor evoked potentials are the electrical responses of muscles in the limbs to stimulation applied over the brain or spinal cord. The speed of these responses is also tested in some specialised departments.

Magnetic stimulation is used to produce the responses. This is done by creating a magnetic field by sending pulses through a hand-held coil of wire covered in plastic. The magnetic field then activates the nerve cells. Magnetic stimulation is not uncomfortable and no special preparation is required. The coil is held over the scalp to stimulate the brain or over the neck to stimulate the spinal cord. The responses are recorded from electrodes placed over muscles in the arms or legs.

The responses may be delayed in a variety of conditions, including multiple sclerosis, disorders of the spinal cord, stroke and coma as well as a range of other less common neurological disorders. This technique is also used to monitor the spinal cord during surgery.