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VEP Test Eye Check and ERG Test: What They Measure and Show
A VEP test eye check (visual evoked potential) records how well the signal from your eye travels along the optic nerve to the vision centre at the back of your brain. An ERG (electroretinogram) records the electrical response of the retina itself. Together they are called visual electrophysiology: painless tests that measure how the visual system works rather than how it looks. At Laser Istanbul Clinic they are used when the cause of reduced vision is not clear from the examination and scans, and your surgeon explains the result with you.
A VEP test eye check measures the electrical signal your brain produces in response to a flashing or checkerboard pattern, which shows whether the optic nerve and visual pathway are conducting normally. An ERG test measures the retina's electrical response to light flashes, showing whether the rods and cones work. Both use small sensor electrodes, neither is painful, and a specialist reads them alongside your examination, OCT scan and visual field test, not on their own.
Sudden loss of vision, a dark curtain over part of your sight, or new pain on moving the eye is an emergency. Do not wait for a trip abroad: see an eye casualty department or your doctor the same day. VEP and ERG are specialist tests for finding out why vision is reduced, not for urgent care.
What do VEP and ERG tests measure?
Your visual system has several links: the retina, where light is turned into electrical signals; the optic nerve, which carries them; and the brain, which interprets them. Visual electrophysiology tests each link by recording the electrical activity it produces. NHS patient information describes this as measuring how your eyes or brain react to on-screen patterns and flashing lights.
| Test | What it records | Part of the visual system | Typical question it answers |
|---|---|---|---|
| VEP (visual evoked potential) | Signals from the back of the head (occipital scalp) after each eye is stimulated | Retina to optic nerve to visual cortex | Is the optic nerve or visual pathway conducting normally? |
| Full-field ERG | Mass response of the whole retina to flashes in the dark and in the light | Rods and cones, bipolar cells | Is there widespread retinal disease? |
| Pattern ERG | Response to a reversing checkerboard | Central retina (macula) and retinal ganglion cells | Is the central retina or ganglion-cell layer affected? |
| Multifocal ERG | A map of 61–103 small responses across the central 30 degrees | Macula | Where in the macula is function reduced? |
The International Society for Clinical Electrophysiology of Vision (ISCEV) publishes the standards for these tests, so that results can be compared between laboratories. Its 2025 update for the VEP defines three standard stimuli: a pattern-reversal checkerboard, a pattern onset/offset checkerboard and a flash.
What is a VEP test?
A VEP test records the electrical signal that your brain generates in response to vision. According to EyeWiki, it measures the signal at the visual cortex in response to visual stimulation, which assesses the whole path from the eye to the cortex. A small electrode is placed on the back of your head (position Oz), a reference sensor on the forehead (Fz) and a ground electrode elsewhere. Each eye is tested separately while you look at the centre of a screen showing a black-and-white checkerboard that flips pattern. The computer averages many responses to pull a clear signal out of the background brain activity.
The main part of the trace is a positive peak called P100, which appears roughly 100 milliseconds after the pattern changes. Its timing (latency) and size (amplitude) are what your specialist measures.
What is an ERG test?
An ERG test records the retina's electrical response to light. The ISCEV 2022 standard describes the full-field ERG as a way of assessing generalised retinal function. The key parts of the trace are:
- a-wave: the first, downward wave, generated by the rod and cone photoreceptors
- b-wave: the following upward wave, generated mainly by bipolar cells
- Dark-adapted responses: measured after at least 20 minutes in the dark, mainly reflecting the rods
- Light-adapted responses: measured after at least 10 minutes in steady light, reflecting the cones
Because the rods and cones are tested separately, an ERG can show which kind of photoreceptor function is reduced, which helps in naming the type of retinal disease.
What is a VEP test used to diagnose?
A VEP test is used to find out whether the optic nerve or the visual pathway behind it is working properly, especially when the eye itself looks healthy. The American Academy of Ophthalmology lists optic neuropathies (demyelinating, compressive, toxic, traumatic, ischaemic, inflammatory and hereditary), albinism, amblyopia, unexplained vision loss and suspected chiasmal or retrochiasmal problems among the indications.
Optic neuritis and other optic nerve disease. A review of VEP methods states that a delayed P100 in one eye is typically a sign of optic nerve or pre-chiasmal dysfunction. In optic neuritis, the delay often persists after vision has recovered: a review of multifocal VEP in optic neuritis notes significant residual latency delay even 12 months after the attack.
Unexplained vision loss. When vision is worse than the examination explains, a normal VEP is reassuring about the optic nerve and pathway, while an abnormal one points the search towards the nerve.
Hidden optic nerve disease. The same review notes that VEP can detect lesions that imaging may miss, particularly within the optic nerve.
Glaucoma. VEP is sometimes used alongside OCT and visual fields to follow functional change.
Babies, young children and people who cannot respond. Flash VEP is useful when there is media opacity (such as a dense cataract), limited cooperation or an inability to fixate, as the ISCEV 2025 standard notes. EyeWiki also lists monitoring of infant vision among the uses of VEP.
Albinism. VEP shows an asymmetry of the response across the scalp caused by misrouting of nerve fibres at the optic chiasm.
An ERG test, by contrast, is used to find or confirm:
- inherited retinal disease: retinitis pigmentosa, cone and rod dystrophies, Leber congenital amaurosis
- toxic retinopathy from medicines such as hydroxychloroquine, chloroquine or vigabatrin
- acquired retinal conditions, including cancer-associated retinopathy
- unexplained vision loss, giving an objective measure when symptoms do not match the examination
Why VEP and ERG may be requested before treatment
These tests are not part of every examination. They are asked for when the standard tests leave a question open. Typical reasons are:
Vision is poorer than the eye looks
If the retina looks normal on the OCT scan and the visual field test is unclear, electrophysiology helps separate a retinal cause (ERG) from an optic nerve cause (VEP).
Before cataract or lens surgery in an eye with other disease
A dense cataract hides the back of the eye. If the retina or optic nerve may also be damaged, your surgeon needs to know how much vision can realistically be restored. Flash VEP does not need a clear view of the retina, and EyeWiki lists prognostication before surgery among the uses of VEP.
Monitoring medicines that can affect the retina
The Royal College of Ophthalmologists recommends multifocal ERG in hydroxychloroquine screening when structural changes on OCT or autofluorescence imaging are not matched by a visual field defect.
Second opinions
Patients who have been told their vision loss is unexplained often bring earlier VEP and ERG reports, which we can review without repeating the test.
Your surgeon will only request these tests if they could change your advice. If they would not, we will not ask you to have them.
What to expect: the VEP and ERG procedure
Before the test
- Wash your hair and avoid gel, spray or oils; electrodes are stuck to the scalp. Hair weaves and wigs cannot be used.
- Avoid heavy eye make-up and false lashes if you are having an ERG.
- Bring your glasses and your prescription. ISCEV states that optimal refraction is needed for pattern VEP, but not for flash VEP.
- Take your regular medicines as usual.
- Bring sunglasses if you will have dilating drops, and arrange not to drive afterwards.
- Tell us if you have epilepsy. NHS patient information notes a small theoretical risk that flashing lights could provoke a seizure, though no case has been reported.
During a VEP test
- Small disc electrodes are attached to your scalp with a conductive paste.
- You sit in front of a screen and look at a fixed spot in the centre of a checkerboard pattern. One eye is covered at a time.
- The pattern flips repeatedly while the computer records and averages your brain's response. The test is painless and has no after-effects.
During an ERG test
- You may have numbing drops, and often dilating drops so the retina is evenly lit. The 2022 ISCEV standard now makes dilation optional if the stimulus is corrected for pupil size.
- A recording electrode is placed on or close to the eye: a fine thread resting along the lower eyelid, a contact-lens electrode, or a skin pad under the eye. A thread electrode can feel unusual but should not hurt, and it can be adjusted or removed.
- You sit in the dark for at least 20 minutes before the dark-adapted flashes, and in steady light for at least 10 minutes before the light-adapted flashes. Keep your phone away, because screen light interrupts dark adaptation.
- You look into a dome that delivers flashes of light of different strengths.
How long does it take? One NHS leaflet states that testing does not normally last longer than two and a half hours, and another says a full session typically takes about two hours. A VEP alone is shorter than a full ERG session, since it does not need dark adaptation. The time we need for your visit is {{VEP / ERG appointment length}}.
Risks and after-effects. Scalp electrodes cause none. A contact-lens or thread electrode on the eye can very rarely scratch the cornea (a corneal abrasion), which is avoided by not rubbing your eyes while it is in place. Dilating drops blur near vision and make light uncomfortable, and NHS guidance says you should not drive for around three to five hours.
How VEP results are read
VEP test normal range
The normal VEP is not a single number. The P100 typically appears at about 100 milliseconds in pattern-reversal recording, but the exact normal limits depend on the stimulus, the equipment and your age. ISCEV therefore advises each laboratory to establish its own reference values rather than rely on figures from elsewhere. The elements your specialist checks are:
| Measure | What it shows | Notes |
|---|---|---|
| P100 latency | How quickly the signal travels from retina to cortex | Around 100 ms is typical; compared with the laboratory's own normal limits |
| P100 amplitude | How strong the response is | Varies more between people than latency; measured from the preceding N70 trough |
| Difference between the two eyes | Whether one nerve is slower or weaker | Very useful in suspected optic neuritis, because each eye serves as the other's control |
| Waveform shape | Whether the response is clean and repeatable | Poor fixation, blinking or uncorrected sight can distort it |
These descriptions come from published reviews and standards. They are a guide only; your specialist interprets your own traces against a reference range.
Abnormal VEP test results: what do they mean?
An abnormal VEP does not name a disease by itself. It says that something along the visual pathway is affecting the signal, and the pattern gives clues:
- Delayed P100 with a reasonably preserved amplitude points towards slowed conduction, as in demyelination (loss of the insulating myelin around nerve fibres).
- Reduced amplitude means fewer working nerve fibres or a weaker signal, which may be seen with nerve damage from compression, toxic injury, poor blood supply or glaucoma.
- An absent or very small response may reflect severe loss of conduction, but it can also reflect poor fixation, an unsuitable stimulus or dense media opacity, so flash VEP and ERG are used to check.
- Asymmetry between the two eyes is often more informative than the absolute value.
The American Academy of Ophthalmology notes that VEP abnormalities must be correlated with clinical findings and do not by themselves specify a diagnosis.
Reasons for an abnormal VEP test
| Group of reasons | Examples |
|---|---|
| Optic nerve inflammation or demyelination | Optic neuritis, multiple sclerosis, neuromyelitis optica spectrum disorder |
| Compression or injury | A tumour pressing on the nerve or chiasm, trauma |
| Poor blood supply or toxins | Ischaemic optic neuropathy, medicine or nutritional toxicity |
| Hereditary conditions | Inherited optic neuropathies, albinism |
| Eye disease | Advanced glaucoma, severe retinal disease, dense cataract |
| Technical or patient factors | Uncorrected refractive error, poor fixation, small pupil, tiredness or eye movement |
Because the technical group is so common, a mildly abnormal VEP is often repeated or checked against an ERG and scans before conclusions are drawn.
ERG test results
An ERG is read by comparing wave sizes and timings with normal limits, and by comparing dark-adapted with light-adapted responses. EyeWiki notes that retinitis pigmentosa typically shows severely abnormal or undetectable dark-adapted responses, while an "electronegative" pattern (a normal a-wave but a markedly reduced b-wave) is seen in conditions such as congenital stationary night blindness and X-linked retinoschisis. In hydroxychloroquine toxicity, multifocal ERG can show early changes in the ring of retina just around the central fovea.
VEP test for neurology and MS
A VEP test is used in neurology as well as ophthalmology, because the optic nerve is part of the central nervous system. It can show whether the visual pathway has been affected by demyelination, even when you have not noticed symptoms.
About one quarter of people with multiple sclerosis (MS) have optic neuritis as their first symptom. The 2024 revisions of the McDonald criteria for diagnosing MS, published in The Lancet Neurology in 2025, add the optic nerve as a fifth location where the disease can be shown, using MRI of the optic nerves, visual evoked potentials and/or OCT.
Published research shows how useful the VEP can be. In a review of multifocal VEP, 20–40% of MS patients' eyes without a known attack of optic neuritis had signs of a silent episode in the visual pathway. In one small group of 46 people with optic neuritis, 36.4% of those with a delayed VEP developed MS within a year, compared with 0% of those with a normal one. These figures come from specific studies and cannot predict what will happen to one person.
An important limit: the diagnosis of MS is made by a neurologist from your history, examination, MRI and other tests together. A VEP result alone neither confirms nor rules out MS. If a VEP raises that question, we will explain the finding and advise you to see a neurologist, ideally in your home country where follow-up is easiest.
VEP and ERG test price
Our price for VEP and ERG testing is {{£ price for VEP and ERG electrophysiology testing}}, which covers the tests, the specialist's report and an explanation of your results by your surgeon, with an interpreter if you wish. Where the tests are requested as part of treatment planning, {{whether VEP/ERG is included in the treatment price or charged separately}}. Hotel and transfers are not included; they are a separate charge if you would like us to arrange them.
What about the VEP test price in the UK? On the NHS, these tests are normally arranged by a hospital eye clinic or neurology service on referral, and patients are not usually asked to book them directly. Private clinics set their own fees, which depend on how many tests are done. We do not quote UK private prices on this page; send us your reports and we will tell you honestly whether a repeat test is needed at all.
Who reads your VEP and ERG results
Your traces are recorded by {{electrophysiology specialist / clinical scientist}} and then reviewed by the surgeon treating you. Doç. Dr. Muhammet Derda Özer (Doç. means Associate Professor), who treats retina, uveitis and other inflammatory eye disease, reviews retinal and inflammatory cases and speaks English and Turkish. Op. Dr. Aysan Bayram, who treats retinal disease, cataract and laser patients and speaks English, also reviews retinal results. Prof. Dr. Ahmet Satıcı, whose practice includes glaucoma, cataract and lens surgery, reviews results that affect surgical planning or glaucoma care. Results come with a plain-language explanation and a recommendation, through your interpreter if you wish. {{Turnaround for the written electrophysiology report}}.
If your tests point to a neurological cause, we say so plainly and advise you to see a neurologist rather than pretending we can provide neurological care.
Treatments and care VEP and ERG support
| Situation | How VEP / ERG is used |
|---|---|
| Optic neuropathy and neuro-ophthalmology | Shows whether the optic nerve is conducting normally and tracks change over time |
| Cataract and lens surgery with other eye disease | Helps judge how much vision the retina and nerve can support once the cataract is removed |
| Retinal disease and inherited retinal dystrophy | Confirms the diagnosis and separates rod from cone problems |
| Medicine monitoring (for example hydroxychloroquine) | Multifocal ERG confirms or clarifies early macular toxicity |
| Second opinions on unexplained vision loss | Provides objective data when scans and examination do not explain symptoms |
Other tests in your eye examination
VEP and ERG are specialist tests that add to, rather than replace, the usual scans:
- Corneal topography: maps the shape of the cornea (/en/eye-tests-diagnostics/corneal-topography/)
- OCT scan (optical coherence tomography): cross-section scans of the retina and optic nerve (/en/eye-tests-diagnostics/oct-scan/)
- FFA (fundus fluorescein angiography): imaging of the blood vessels at the back of the eye (/en/eye-tests-diagnostics/fluorescein-angiography/)
- Eye ultrasound (B-scan): imaging when the inside of the eye cannot be seen (/en/eye-tests-diagnostics/eye-ultrasound/)
- Visual field test (perimetry): checks your side vision (/en/eye-tests-diagnostics/visual-field-test/)
- Tonometry (eye pressure measurement): important for glaucoma checks (/en/eye-tests-diagnostics/tonometry/)
VEP and ERG tests: frequently asked questions
A VEP (visual evoked potential) test records the electrical signal your brain produces when each eye is shown a flashing or checkerboard pattern. It shows whether the optic nerve and visual pathway are carrying the signal at a normal speed and strength. It is painless and uses small sensors on the scalp.
An ERG (electroretinogram) test records the electrical response of the retina to flashes of light. It shows how well the rods and cones and the cells connected to them are working, and is used for inherited retinal disease, medicine toxicity and unexplained vision loss.
It is used to find problems of the optic nerve and visual pathway, such as optic neuritis, compressive or toxic optic neuropathy, hereditary conditions and albinism, and to assess vision when a person cannot respond to a standard test. It supports a diagnosis; it does not make one on its own.
No. Scalp electrodes are attached with paste and cause no pain. You watch a screen for a series of short recordings. For an ERG, numbing drops may be used, and a thread electrode on the lower lid can feel strange but should not hurt.
NHS leaflets say a full visual electrophysiology session takes about two hours and normally not more than two and a half, because an ERG requires 20 minutes in the dark and 10 minutes in the light before recording. A VEP alone is shorter. We will confirm the length of your appointment: {{VEP / ERG appointment length}}.
There is no single universal range. The P100 wave usually appears at about 100 milliseconds, but normal limits depend on the equipment, the stimulus and your age, and ISCEV advises that each laboratory use its own reference values. Specialists also compare the two eyes with each other.
They mean that the signal from the eye to the brain is slowed or weakened. A delayed P100 typically points to slowed conduction, such as demyelination, while a reduced amplitude suggests fewer working nerve fibres. The result must be read alongside your examination and scans; it does not name a disease by itself.
Causes include optic neuritis, multiple sclerosis, compression or injury of the nerve, poor blood supply, toxic or hereditary optic neuropathies, glaucoma, severe retinal disease and dense cataract. Technical factors such as uncorrected glasses, poor fixation or a small pupil can also change the trace, so borderline results are often repeated.
Yes, as one of several tests. It can show silent optic nerve involvement even without symptoms, and the 2024 McDonald criteria allow the optic nerve to count as a fifth location when MS is diagnosed, shown by MRI, VEP and/or OCT. The diagnosis itself is made by a neurologist using all of your results.
Many ERG sessions use dilating and numbing drops, although the current ISCEV standard makes dilation optional. NHS guidance advises you not to drive for around three to five hours after dilating drops, so arrange a lift and bring sunglasses.
Our price for VEP and ERG testing is {{£ price for VEP and ERG electrophysiology testing}}, with the specialist's report and explanation of results included. Hotel and transfers are not included.
Yes. Send your reports, traces if you have them, scans and prescription for a free review. Often earlier results are enough, and we will tell you if a repeat test would add anything.
Sources for the facts on this page
- ISCEV, ISCEV standard for clinical visual evoked potentials (2025 update): three standard VEP stimuli (pattern reversal, pattern onset/offset, flash); P100 as the most robust component; electrode placement Oz/Fz; flash VEP for media opacity or limited cooperation; refraction needed for pattern VEP, not flash; laboratory-specific reference values. https://pmc.ncbi.nlm.nih.gov/articles/PMC12436483
- ISCEV, ISCEV Standard for full-field clinical electroretinography (2022 update), Doc Ophthalmol 2022: minimum 20 min dark adaptation and 10 min light adaptation; DA/LA stimuli; mydriasis optional; response origins; electrodes; paediatric testing. https://pmc.ncbi.nlm.nih.gov/articles/PMC9192408
- EyeWiki (AAO), Electroretinogram: indications (inherited retinal disorders, toxic retinopathies, unexplained vision loss); full-field, multifocal (61–103 responses, central 30 degrees) and pattern ERG; electrodes; waveform origins; retinitis pigmentosa and electronegative patterns; hydroxychloroquine mfERG. https://eyewiki.org/Electroretinogram
- EyeWiki (AAO), Visual Evoked Potential/Response (VEP/VER): VEP measures the cortical signal to assess the whole visual pathway; Oz/Fz electrodes; P100 near 100 ms; uses including infant vision, prognostication before surgery and optic neuropathy; albinism misrouting. https://eyewiki.org/Visual_Evoked_Potential/_Response_(VEP/VER)
- American Academy of Ophthalmology, Clinical visual electrophysiology: visual evoked potentials: indications (optic neuropathies, albinism, amblyopia, unexplained vision loss, chiasmal anomalies); flash, pattern-reversal and onset/offset stimuli; delayed peak times and reduced amplitude; results must be correlated clinically. https://www.aao.org/education/disease-review/clinical-visual-electrophysiology-visual-evoked-po
- A Comprehensive Review on Methodologies Employed for Visual Evoked Potentials (PMC4789528): P100 near 100 ms and its robustness; factors affecting VEP; P100 delay in one eye indicating optic nerve or prechiasmal dysfunction; detection of occult lesions imaging may miss. https://pmc.ncbi.nlm.nih.gov/articles/PMC4789528/
- The contribution of multifocal visual evoked potentials in patients with optic neuritis and multiple sclerosis: a review (PMC8116218): residual latency delay 12 months after optic neuritis; 20–40% of MS eyes without optic neuritis have subclinical involvement; 36.4% vs 0% conversion in a cohort of 46. https://pmc.ncbi.nlm.nih.gov/articles/PMC8116218
- Montalban X, et al. 2024 revisions of the McDonald criteria, Lancet Neurology 2025;24(10):850–865 (optic nerve as a fifth location), reported at https://practicalneurology.com/news/2024-mcdonald-criteria-revisions-published-in-the-lancet-neurology/2483516/ ; Vall d'Hebron Research Institute summary (optic nerve assessed by MRI, VEP and/or OCT; about a quarter of MS patients present with optic neuritis): https://vhir.vallhebron.com/en/society/news/2024-revision-mcdonald-criteria-will-allow-even-earlier-diagnosis-multiple-sclerosis
Unexplained vision loss? Let a surgeon look at your results, free
Upload your VEP or ERG reports, OCT scans, visual field results and prescription. Doç. Dr. Özer, Prof. Dr. Satıcı or Op. Dr. Bayram will tell you what they show and whether any further test could help, with no obligation.
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Prof. Dr. Ahmet Satıcı
Doç. Dr. Muhammet Derda Özer
Op. Dr. Aysan Bayram