What is an Evoked Potential?

An Evoked Potential is an electrical manifestation of the brain's reception and response to an external stimulus. This can be an auditory stimulus, such as a click in a Brainstem Auditory Evoked Potential (BAEP); a visual stimulus, such as a checkerboard pattern in a Visual Evoked Potential (VEP), or an electrical stimulus provided by a stimulator in a Somatosensory Evoked potential (SSEP).

Evoked potentials provide a measure of function of the motor and sensory system and tract. The clinical use of Evoked Potentials is based on their ability to:

• demonstrate abnormal motor and sensory function.
• reveal the presence of clinically unsuspected malformation in the nervous system.

A number of other Evoked potentials exist such as Dermasomal Evoked Potentials, Motor Evoked Potentials, Event Related Potentials, the Electroretinogram and the Electroocculogram.

Monitoring of spinal cord function during spinal surgery is performed in a number of Hospitals throughout Australia. Click on the Spinal Monitoring link to find out more about Spinal Monitoring.

The BAEP

The Brainstem Auditory Evoked Potential provides information about the auditory brainstem pathways.

In a BAEP, the potential changes are measured on the scalp. The potential fluctuations are caused by electrical events occurring in the cochlea, eighth nerve, and auditory pathways in the brainstem whenever impulses in response to an auditory stimulus travel to the cerebral cortex via the acoustic system.

Although the recording and stimulus parameters change between clinics, surface recording electrodes are usually placed on the mastoid or earlobe, at the vertex, and the forehead as a Ground electrode.

Testing of each ear using around 70dBHL clicks is repeated until a response is replicated at least once. Ears are tested monaurally with a white-band masking noise applied to the contralateral ear.

The BAEP consists of seven peaks and is measured in the 10msec following an acoustic stimulus (usually a click).

Below is an example of a brainstem auditory evoked potential.

The VEP

A VEP is a computer averaged brainwave response to a visual stimulus such as a checkerboard pattern on a computer screen. It records the latency of responses down the optic nerve after visual stimulation. The VEP is used as an objective measure of visual function.

Patients are assessed when presenting with eye movement disorders, optic nerve dysfunction, cortical blindness and assorted neuro-ophthalmological conditions.

Below is an example of a Visual Evoked Potential.