The visually evoked potential (VEP) measures the electrical response of the brain's primary visual cortex to a visual stimulus. To measure the electrical response, you first place three electrodes on the scalp. One electrode, which measures the response itself, goes over the primary visual cortex, slightly above the inion at the back of the head.  Another electrode is placed at a reference location, typically on forehead or top of the head. The third electrode, for grounding, goes on the ear. These electrodes connect to an LKC visual electrodiagnostic test system.

A visual stimulus is presented creating an evoked potential from the visual cortex. The amplitude of VEPs are quite variable, but are typically in the range of 3 - 20 microvolts.  Because of the small amplitude of the signals, multiple potentials are averaged to create a clean, accurate reading.

Most of the variability between patients occurs because of different skull thicknesses, visual cortex morphology, and electrode placement. Any disorder or pathology of the optic nerve or primary visual cortex can also affect the shape and timing of the VEP response.

Pattern VEP

Most clinical VEP recordings involve placing a patient in front of a black and white checkerboard pattern displayed on a video monitor.  The checks alternate black/white to white/black at a rate of 2 Hz.  Every time the pattern alternates, the patient's visual system generates an electrical response that can be detected by the electrode on the back of the head.  A typical VEP response comprises of a well-defined Positive peak at around 100 milliseconds.  Hence this peak is commonly called P100.  The latency of P100 increases with smaller checks.  

Flash VEP

Similar to the pattern VEP, the flash VEP elicits electrical responses from the brain’s visual cortex. The variation falls in the stimulus used. In place of a pattern monitor, a Ganzfeld stimulator is used. This can be used with uncooperative or unconscious patients. The resulting peaks will have different latencies with the flash VEP as opposed to the pattern VEP.

Sweep VEP

The sweep VEP (SVEP) uses the same pattern monitor as the pattern VEP. Typically, however, the display will alternate vertical bars instead of checks. The main difference between the two is that the sweep VEP will use increasingly small bar sizes. The decreasing size will allow for an estimation of visual acuity in the patient. The key benefit is that the test is completely objective. This allows testing to be performed on infants, suspected malingerers, disabled subjects, and animals. Depending on the protocol selected, a typical sweep takes from 10 to 45 seconds.
Learn more about Sweep VEP