Visual evoked potential (VEP/VER) is an objective test that records electrical signals of a few to tens of microvolts generated in the occipital visual cortex in response to visual stimuli, using electrodes placed on the scalp. The visual cortex is mainly activated by the central visual field, and the occipital lobe has a large projection area for the macula.
VEP depends on the integrity of the entire visual pathway, including the eye, optic nerve, optic chiasm, optic tract, optic radiation, and cerebral cortex. It reflects photopic function from the macular cones to the visual cortex, so lesions localized to the peripheral retina are not evaluated.
In ophthalmology, the three main electrophysiological tests are electroretinography (ERG), VEP, and electrooculography (EOG). VEP has unique value in detecting functional disorders of the upper visual pathway that cannot be detected by ERG, and in assessing visual function in patients who cannot undergo subjective testing.
The International Society for Clinical Electrophysiology of Vision (ISCEV) revised and published a standard protocol in 2016, and it is recommended to record according to this protocol to reduce inter-institutional variability.
VEP is useful when objective assessment of visual function is needed. Specific indications include infants and young children who cannot cooperate with visual acuity testing, cases where the fundus is not visible due to cataract or vitreous hemorrhage, suspected psychogenic visual disturbance or malingering, evaluation of optic nerve diseases, and unexplained visual loss.
VEP is not a test based on the patient’s subjective symptoms, but an objective test that measures visual pathway function. It is used to evaluate patients presenting with the following subjective symptoms.
VEP waveforms vary depending on the stimulation method. The main waveform components are shown below.
Pattern Reversal VEP
Waveform components: Three components: N75 (75 ms), P100 (100 ms), and N135 (135 ms).
Amplitude measurement: Measured as the potential difference from the N75 peak to the P100 peak.
Normal P100 latency: Approximately 90–120 ms (varies with age).
Characteristics: Low inter-individual variability and high reliability. In cases where an image can be formed on the retina, pattern VEP is generally selected.
Flash VEP
Waveform components: Evaluated using N70 (approximately 70 ms) and P100 (approximately 100 ms). Amplitude is measured as the size between N70 and P100.
Normal P100 latency: Approximately 90–120 ms (varies with age).
Characteristics: Since there is large individual variation, evaluation based on left-right differences is common. It is applied in cases with opacity of the ocular media or visual acuity of 0.1 or less.
Amplitude in children: It is about 1.5 to 2.0 times that of adults, and becomes almost the same as adults by 7 to 8 years of age.
Pattern VEP is broadly divided into transient VEP (t-VEP) and steady-state VEP (s-VEP). When the stimulation frequency is approximately 2 Hz or less, it is called t-VEP, and when it is 4 Hz or more (steady state), it is called s-VEP. t-VEP can evaluate spatial frequency characteristics by changing the check size, and since it correlates with visual acuity, it is widely used for objective visual acuity estimation. s-VEP can be measured in a short time, but it is difficult to evaluate latency prolongation with only amplitude information.
Abnormal VEP findings are broadly classified into three types.
Prolongation of P100 latency is most prominent in demyelinating diseases such as multiple sclerosis, and is highly valuable as a diagnostic aid. It also prolongs in optic neuritis and other optic nerve disorders. Latency prolongation is also observed in severe visual acuity loss (0.1 or less) due to macular disorders, but not as extreme as in optic neuritis. For details, refer to the section “Diagnosis and Examination Methods”.
Since VEP is an “examination method” rather than a specific “disease”, this section describes the main indicated diseases and risk factors (causes of visual pathway disorders).
The main indications for VEP are as follows.
Standard preparation for VEP recording is described below.
Patient preparation
Electrode placement (based on the International 10-20 system)
Electrodes are EEG dish electrodes (silver-silver chloride or gold) approximately 8 mm in diameter, fixed with conductive paste. Inter-electrode impedance should be 5 kΩ or less.
The three types of stimulation methods defined by ISCEV are as follows.
| Stimulation method | Stimulation conditions | Main features |
|---|---|---|
| Pattern reversal | Check 1° and 0.25°, reversal 2 rps | Low individual variability, high reliability |
| Pattern onset/offset | Onset 200 ms, offset 400 ms | Useful for malingering and nystagmus |
| Flash | 1 Hz, 3 cd·s/m² | Applicable to media opacities and low vision |
Recording conditions: The amplification factor of the biological amplifier is 20,000 to 50,000 times, the bandpass filter has a high pass filter (low cut) of 1 Hz or less, and a low pass filter (high cut) of 100 Hz or more. The number of averages depends on the S/N ratio, but at least 64 averages are required. The analysis time is 250 ms or more, and a pre-trigger time of approximately 20 to 50 ms is set.
Selection criteria for stimulation method are as follows:
Multi-channel VEP recording is required, with active electrodes placed at Oz (midline) as well as O1 and O2 (lateral).
In the differentiation of psychogenic visual disturbance, VEP is recorded using pattern stimulation regardless of the level of visual acuity. Basically, amplitude and latency are normal without left-right differences, but psychogenic patients may show better results than normal subjects because they are cooperative and intently watch the stimulus target. In suspected malingering, it is important to confirm whether the patient is fixating, and pattern onset/offset VEP is particularly useful.
In infants with excessive body movement, sedatives may be used, but better VEP waveforms are obtained in the awake state if possible. Chloral hydrate suppositories (30–50 mg/kg) or triclofos sodium solution (0.8–1.0 mL/kg) are used as sedatives. Recording under sleep requires interpretation considering sleep depth because sleep EEG is mixed in. Phenobarbital and other brainstem hypnotics are said to stabilize VEP waveforms, but caution is needed due to the risk of respiratory depression.
When there is opacity of the ocular media such as cataract, flash VEP can be used before surgery to estimate the function of the posterior pole and optic nerve, helping to predict postoperative visual prognosis. Abnormal flash VEP suggests the presence of visual pathway damage and serves as a reference for predicting poor postoperative visual acuity.
By performing VEP monitoring during surgery for skull base tumors or pituitary tumors, real-time detection of damage to the visual pathway and modification of the surgical approach become possible.
Conventional flash VEP intraoperative monitoring has been problematic due to instability and poor reproducibility under general anesthesia.
Foo et al. (2025) reported a case of skull base meningioma surgery in which, despite no intraoperative change in flash (on-response) VEP, the off-response VEP showed a 40% amplitude increase (from 2.8V to 4.0V) after resection of the tumor around the optic nerve, and postoperative visual acuity in the right eye markedly improved from 0.1 to 0.5 (Landolt C) 1). Off-response VEP independently records the potential generated at the end of light stimulation, and may provide more stable waveforms than conventional flash VEP, with higher sensitivity for detecting visual function improvement.
Pattern VEP (pVEP) is useful for evaluating amblyopic eyes as an index of suprathreshold visual processing. Prolongation of P100 latency reflects slower visual information processing speed in the amblyopic eye.
Blavakis et al. (2023) reported a series of three cases of strabismic amblyopia in which pVEP was evaluated before and after 20 hours (2–4 times per week) of dichoptic game training using a virtual reality (VR) system 2). In all three cases, P100 latency in the amblyopic eye improved (e.g., Case 1: from 145 ms to 136 ms with 10 arcmin stimulus; Case 2: from 147 ms to 139 ms), and stereopsis also markedly improved (e.g., Case 1: from 100 arcsec to 50 arcsec). It was suggested that improvement in visual processing speed assessed by VEP may precede visual acuity improvement.
In amblyopic eyes, prolongation of P100 latency is often observed compared to the healthy eye. This reflects slower visual information processing speed in the amblyopic eye. Improvement in P100 latency with treatments such as dichoptic training has been reported 2), and pVEP may be a useful indicator for monitoring treatment effects in amblyopia.
VEP records potentials evoked in the primary visual cortex (V1) of the occipital lobe in response to visual stimuli. The P100 component is recognized as an electrical correlate corresponding to the activity of the primary visual cortex.
The outline of signal transmission along the visual pathway is as follows:
Pattern VEP reflects foveal function more strongly than flash VEP and is suitable for evaluating central visual acuity. Flash VEP assesses the entire visual pathway from the retinal ganglion cell layer to the visual cortex, but has large individual variability.
In multiple sclerosis, demyelination damages the myelin sheath, reducing nerve axon conduction velocity and markedly prolonging P100 latency. Even after demyelination improves, latency prolongation may persist for a long time, and its ability to detect traces of asymptomatic optic neuritis makes it highly valuable as a diagnostic aid.
Amplitude reduction often reflects loss of nerve axons themselves (axonal damage). While latency prolongation alone suggests relatively good recovery, amplitude reduction tends to indicate a poorer prognosis.
In pediatric cortical visual impairment (CVI), flash VEP and pattern VEP have been applied for diagnosis and prognosis assessment. However, there are limitations in interpreting VEP in children with CVI, and reports on the diagnostic utility of VEP are conflicting.
Clark et al. (44 infants) reported that 85% (11/13) of infants with normal flash VEP responses experienced significant visual improvement, compared to only 55% (17/31) in the abnormal VEP group3). On the other hand, some reports indicate that normal flash VEP responses do not correlate with visual outcomes, and differences in the VEP paradigm used (flash vs. pattern), subject age, follow-up period, and definition of visual improvement are thought to contribute to the discrepancies in results3).
Sweep VEP is a method that uses pattern stimuli with gradually changing spatial frequencies to quantitatively assess visual thresholds, and is expected to be a more objective visual acuity measurement than flash VEP. Studies in children with CVI have confirmed the reliability and validity of sweep VEP grating acuity compared to clinical visual acuity assessment3). However, difficulties in electrode placement due to structural brain abnormalities and the influence of epileptic seizures or antiepileptic drugs are cited as limitations in interpretation3).
Multifocal VEP: Using a similar device as multifocal electroretinography, it is expected as an objective visual field measurement method to detect visual pathway disorders above the retina. Its application for objective evaluation of glaucomatous visual field defects is being studied, but because the response to macular stimulation is large and peripheral responses are small, there are still challenges for its widespread use as a general clinical test.
Event-Related Potentials (ERP): Electrodes are placed on the vertex, and the P300 component appearing around 300 ms is evaluated. It is related to information processing and cognitive activity, and in ophthalmology, it is applied for diagnosis and pathophysiological elucidation in some cases of psychogenic visual impairment.
There is a single case report where off-response VEP detected improvement in visual function with high sensitivity even when conventional flash VEP (on-response) failed to capture intraoperative changes 1). This method records on- and off-responses separately by prolonging the duration of light stimulation, and is expected to provide more stable waveforms and improved sensitivity. Currently, it remains a single case report, and the minimum threshold for significant VEP amplitude increase is not yet established, so further accumulation of multicenter data is necessary 1).
Sweep VEP continues to be studied as an objective visual acuity measurement method for difficult-to-evaluate cases, including children with CVI. Sweep VEP grating acuity is less sensitive than vernier acuity but consistently shows higher values than behavioral visual acuity (FPL method) 3). Future expansion of its application to pediatric diseases other than CVI is expected.
pVEP is used to evaluate the effects of dichoptic game training using VR headsets. Improvement in visual processing speed (P100 latency) assessed by pVEP may precede visual acuity improvement 2), and future large-scale randomized controlled trials are expected to verify this. Recurrence of amblyopia occurs in up to 25% within one year after treatment discontinuation, and the relationship between VEP changes during long-term follow-up and recurrence remains a challenge 2).
Foo MX, Hardian RF, Kanaya K, Abe D, Kitamura S, Sato Y, et al. Postoperative Improvement of Visual Function Following Amplitude Increase in Intraoperative Off-Response Visual Evoked Potential (VEP) Monitoring During a Skull Base Meningioma Surgery. Cureus. 2025;17(4):e82563. doi:10.7759/cureus.82563. PMID:40390717; PMCID:PMC12088698.
Blavakis E, Spaho J, Chatzea M, Gleni A, Plainis S. Dichoptic Game Training in Strabismic Amblyopia Improves the Visual Evoked Response. Cureus. 2023;15(9):e45395. doi:10.7759/cureus.45395. PMID:37854740; PMCID:PMC10579841.
Chang MY, Borchert MS. Advances in the evaluation and management of cortical/cerebral visual impairment in children. Survey of ophthalmology. 2020;65(6):708-724. doi:10.1016/j.survophthal.2020.03.001. PMID:32199940.
