Evoked Potentials (EP) are electrical responses produced in the nervous system by stimulation of some kind. Clinical neurophysiology is most commonly concerned with stimulation of one of the senses - usually vision, hearing or the sensory nerves in the limbs. A single EP resulting from one stimulus is normally quite small and difficult or impossible to distinguish from the background EEG activity.

If the evoked responses, to many stimuli, rather than just one, are collected and averaged the signal emerges clearly. All the electrical activity not directly related to the stimulus is averaged out and disappears from the recording, allowing the response caused by the stimulus itself to be seen quite easily.

Visual Evoked Potentials (VEPs)
Visual evoked potentials (VEPs), also sometimes referred to as visual evoked responses (VERs), are the electrical responses recorded from the occipital area of the brain.

Although a traversing checkerboard pattern is most commonly used to record VEPs, 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.

Auditory Evoked Potentials (AEPs)
Auditory Evoked Potentials (AEPs) are also referred to as Brainstem Auditory Evoked Responses (BAERS) and Brainstem Auditory Evoked Potentials (BAEPs). Unlike visual evoked potentials, AERs are not altered by sleep, and if anything sleep may improve the quality of the recording.

Various types of sound may be used, but clicks are the most common for investigating diseases of the nervous system. 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 (SEPs)
Somatosensory Evoked Potentials (SEPs or 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 pulseIn 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 SEPs, including being used during spinal surgery to monitor the spinal cord function. This can reduce the risk of serious injury to the spinal cord during this type of operation.