What are Pattern Visual Evoked Potentials and Flash Visual Evoked Potentials?
Visual evoked potentials (VEPs) are electrophysiologic responses recorded from electrodes placed over the occipital scalp that measure the electrical activity generated by the visual cortex in response to visual stimuli, with pattern VEPs using structured checkerboard stimuli and flash VEPs using brief light flashes. 1, 2
Pattern Visual Evoked Potentials (Pattern VEP)
Pattern VEPs are elicited by presenting structured visual stimuli—typically checkerboard patterns—that either reverse (pattern-reversal) or appear and disappear (pattern-onset/offset) on a video monitor. The ISCEV standard specifies two check sizes 1, 2:
- Large checks: 1° (60 minutes of arc)
- Small checks: 0.25° (15 minutes of arc)
Pattern-reversal VEPs are the most commonly used clinical protocol, where black and white checks alternate positions without changing overall luminance. The major component measured is the P100 wave—a positive deflection occurring approximately 100 milliseconds after stimulus presentation 1, 2.
Pattern-onset/offset VEPs present checks that appear for a defined period (typically 230 ms) then disappear into a background of mean luminance 3. These can be particularly valuable for detecting hemianopic field defects, as onset and offset components lateralize differently to the occipital hemispheres 3.
Clinical Applications of Pattern VEP
Pattern VEPs are the preferred method for patients over 3 years of age who can cooperate with fixation 4. They provide quantitative assessment of:
- Optic nerve function: Detecting demyelination (as in multiple sclerosis), compression, or other optic neuropathies 5, 6, 4
- Visual pathway integrity: Assessing conduction from retina through optic nerves to visual cortex 1, 2
- Prechiasmal lesions: Standard single-channel midline recordings effectively evaluate function anterior to the optic chiasm 2
Critical limitation: Pattern VEPs require adequate visual acuity and patient cooperation to maintain fixation on the stimulus. They cannot be obtained through dense cataracts or in uncooperative patients 7.
Flash Visual Evoked Potentials (Flash VEP)
Flash VEPs are elicited by brief luminance increments (flashes) from either a strobe or LED array that subtends at least 20° of visual field 1, 2. The flash stimulus does not require pattern recognition or precise fixation.
Key Components Measured
Flash VEPs generate several waveform components 7:
- P1: First positive deflection
- N2: Negative deflection following P1
- P2: Second positive deflection
- N2-P2 amplitude: The voltage difference between N2 and P2 peaks
Clinical Applications of Flash VEP
Flash VEPs are specifically indicated when pattern VEPs cannot be obtained 8, 4:
- Dense cataracts or media opacities: When optical clarity prevents pattern visualization 8, 7
- Infants and sedated patients: Who cannot maintain fixation on pattern stimuli 4, 9
- Nonverbal or uncooperative patients: Where behavioral responses cannot be reliably obtained 8
- Preoperative cataract assessment: Predicting visual potential when pattern VEP shows no response 7
Interpreting Flash VEP Results
In mature cataract evaluation, comparison with the contralateral normal eye is essential 7. The most reliable predictor of postoperative visual prognosis is the N2-P2 amplitude potential—a cutoff value of 6.07 μV provides 80% sensitivity and 88% specificity for predicting poor visual outcomes after cataract surgery 7. Delayed P2 latency and reduced N2-P2 amplitude compared to the fellow eye indicate worse visual prognosis 7.
Technical Considerations
Recording Parameters
Both pattern and flash VEPs use a midline occipital active electrode as the standard single-channel recording configuration 1, 2. The recording electrode is typically placed on the midline of the occipital scalp at the back of the head 4.
Advantages and Limitations
Pattern VEP advantages:
- More precise localization of visual pathway dysfunction
- Better reproducibility and narrower normal ranges
- Superior for detecting subtle optic nerve disease 6, 4
Flash VEP advantages:
- Can be obtained through media opacities
- Does not require patient cooperation or fixation
- Useful in infants and sedated patients 4, 9
Flash VEP limitations:
- Greater intersubject variability in latency and amplitude 9
- Less specific for localizing lesions
- Longer P100 latency with greater variability compared to pattern VEP 9
Integration with Other Testing
VEPs are part of the broader category of evoked potentials that assess sensory pathway function 10. They complement other visual function tests and can be combined with pattern electroretinography (PERG) for simultaneous assessment of retinal and post-retinal function 1.
For assessment of chiasmal and post-chiasmal lesions, multi-channel VEP recordings with additional electrode placements beyond the standard midline occipital site are required 1, 2, 3.