Department of Neurology
Visual Evoked Potentials
Department of Neurology October 1996
Visual evoked potentials (VEPs) are computer averaged brain-wave responses to visual stimuli which may be performed in a variety of clinical situations as an objective measure of visual function.
We assess infants and children with retinal disorders, eye movement disorders, optic nerve dysfunction, delayed visual development, cortical blindness and assorted neuro-ophthalmological conditions. We also monitor optic nerve function pre- and post- craniofacial surgery which involves manipulation of the facial bones.
The visual stimulation and evoked potential recording protocol is adapted to the requirements of the individual patient. VEPs may be performed in their own right or in conjunction with electro-retinograms.
The two main categories of VEPs are those generated by a patterned stimulus such as a black and white checkerboard pattern (PVEP) and those generated by a diffuse flash of light (FVEP) such as from stroboscopes and clusters of LEDs.
PVEPs reflect perception of the patterned stimulus and generally require a waking cooperative patient. They are sensitive to visual acuity so correction of refractive error is required, where possible. PVEPs provide a precise measure of speed of optic nerve conduction, useful in detecting sub-clinical lesions of the optic nerve. Varying the check size to find response latencies and thresholds in young infants can be helpful in measuring visual maturity. When PVEPs are not feasible or successful, FVEPs may provide clinically useful information particularly if evoked potential distributions are mapped. Flash stimuli penetrate the closed eye-lid enabling recordings during sleep. Mapping of potentials elucidates asymmetries of cortical responses and assists in identifying artefacts such as blink artefact.
Skin at each electrode site is prepared with "OMNI PREP" so as to reduce impedances to similar low (less than 5kohms) values.
Occipital Pattern VEPs.
Active: Mid-Occipital (Oz), Right Occipital (Oz+3cms), Left Occipital (Oz+3cms), Far Right Occipital (Oz+6cms), Far Left Occipital (Oz+6cms). Reference: Mid Frontal (Fz).
When testing a young infant or the patient's cooperation is limited, a single mid-occipital electrode active electrode is often used.
Occipital Flash VEPs.
Active: The above electrodes or with the addition of parietal electrodes or a complete EEG 10-20, 21 channel montage. Reference: Linked Ears.
Occipital Pattern VEPs. Three options...
Digitimer D112 slide-projector based unit placed 1m from eyes. Check size 11mm (Angle 38' at eyes)
TV stimulator placed 1m from eyes. Check size controlled by CADWELL SPECTRUM32. (Standard checks 10mm, Angle 35' at eyes)
LED-Check stimulator for CADWELL SPECTRUM32. Check size 6.5mm (Distance 15cm, Angle 150' at eyes), Check size 13mm (Distance 15cm, Angle 300' at eyes)
Rate of patterned stimulation 2/sec.
Recordings are made under dim light conditions. Testing is performed monocularly although difficult and apprehensive patients are usually tested binocularly first. Older patients are routinely tested with 35' checks. Visual acuities are recorded and comparisons of recordings with and without glasses may be done. Infants are usually tested with a range of check sizes including 9',18',35',70' and 140' at the eyes.
Occipital Flash VEPs
Recordings are made in darkness.
Strobe light stimulation is at 15cms distance. (May be increased if blink response is strong)
Rate of stimulation is randomized about 1/sec.
LED cluster stimulation is used to ensure monocular stimulation.
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last modified: 08 Apr 2016