An electroretinogram (ERG) measures the electrical activity of your retina in response to light. Your results report will show waveforms with two key measurements for each: amplitude (how tall the wave is) and implicit time (how quickly the wave peaks after the flash). Lower-than-expected amplitudes suggest fewer functioning retinal cells, while delayed implicit times point to cells that are working slowly. Understanding which waves are affected, and under which testing conditions, tells your doctor exactly which layer of the retina is involved.
The Two Waves on Your Report
Every ERG trace has two main components. The a-wave is the first downward dip on the graph, generated by your photoreceptors (the rods and cones in the outer retina) as they respond to a flash of light. The b-wave is the larger upward peak that follows, produced primarily by bipolar cells in the inner retina, with some contribution from supportive cells called Müller cells. Together, these two waves give a layered picture of retinal health: the a-wave reflects the light-sensing cells themselves, and the b-wave reflects the next set of cells in the signaling chain.
Amplitude and Implicit Time
These are the two numbers you’ll see reported for each wave, and they tell different stories.
Amplitude is measured from the baseline (or from the a-wave trough to the b-wave peak) in microvolts. It corresponds directly to the number of functioning retinal cells. A smaller amplitude means fewer cells are contributing to the signal. The extent of retinal damage correlates with the degree of amplitude decrease, so a mildly reduced amplitude is a different conversation than a severely reduced or absent one.
Implicit time is the interval, in milliseconds, from the moment the flash fires to the moment the wave reaches its peak. A delayed implicit time means the retinal cells are responding more slowly than expected. This distinction matters clinically. A localized problem like a retinal detachment might reduce amplitude without much change in timing, because the surviving cells still function at normal speed. A widespread condition like a rod-cone dystrophy typically causes both reduced amplitude and delayed timing, because the remaining cells are impaired across the board.
Scotopic vs. Photopic Results
Your report will separate results into dark-adapted (scotopic) and light-adapted (photopic) sections. This separation is how the test isolates rod function from cone function.
For scotopic testing, you sit in complete darkness for at least 20 minutes before the flashes begin. This lets your rods fully activate, so the responses recorded are dominated by rod cell activity. Several flash strengths are used: a dim flash that isolates rods alone, and a brighter “standard flash” that captures a mixed rod-cone response. If your scotopic amplitudes are reduced but your photopic results look normal, the problem is primarily in your rod cells.
For photopic testing, the room lights come on for about 10 minutes to suppress rod activity, then flashes are delivered against a bright background. This isolates cone-driven responses. The photopic section also includes a 30 Hz flicker test, where rapid, repeated flashes confirm cone function (rods can’t keep up with that speed). Reduced photopic amplitudes with normal scotopic results point to a cone-specific problem.
A third component sometimes appears in the photopic section: the photopic negative response (PhNR), a downward dip after the b-wave. This reflects ganglion cell activity in the innermost retinal layer and can be relevant in conditions affecting the optic nerve.
Types of ERG and What Each Reveals
Not all ERG tests are the same, and your report may come from one of three main versions.
- Full-field ERG (ffERG) measures the summed electrical response of the entire retina at once. It’s excellent for detecting widespread retinal diseases but relatively insensitive to small, localized defects. If you have a problem affecting only a small patch of retina, a full-field ERG may look normal.
- Multifocal ERG (mfERG) stimulates many small retinal areas simultaneously and records separate responses from each region. This produces a topographic map of retinal function, making it possible to pinpoint localized damage that full-field testing would miss. It’s commonly used to monitor for medication-related retinal toxicity, where early damage often appears as a ring of depressed signals around the central retina.
- Pattern ERG (PERG) uses an alternating checkerboard stimulus rather than flashes. It specifically reflects the health of retinal ganglion cells and their connection to the optic nerve, which makes it useful for detecting early glaucoma. The standard PERG uses checks about 0.8 degrees wide on a 15-degree field, with the pattern reversing about 4 times per second.
How Age Affects Normal Values
ERG amplitudes naturally decrease with age, and implicit times naturally lengthen. This is especially true for rod-mediated responses. Studies show that people over 50 have noticeably longer implicit times across most testing conditions compared to younger adults, reflecting a gradual loss of retinal responsiveness driven largely by slower rod cell function. B-wave amplitudes show the most age-related decline.
This matters for interpretation because your results are compared against reference ranges, and those ranges should be age-matched. A b-wave amplitude that looks mildly reduced for a 30-year-old might be perfectly normal for a 65-year-old. If your report doesn’t specify the reference population, it’s worth asking whether age was factored in.
Common Patterns and What They Suggest
Certain diseases produce recognizable ERG signatures. In retinitis pigmentosa, one of the most common inherited retinal diseases, scotopic responses are typically the first to deteriorate. Early stages may show reduced rod b-wave amplitude with delayed timing, while cone responses remain relatively preserved. As the disease progresses, both scotopic and photopic responses diminish, and in advanced cases, the ERG can become completely flat, or “non-recordable.” A non-recordable ERG doesn’t necessarily mean zero vision remains, but it does indicate that the retina’s electrical output has fallen below the test’s detection threshold.
For people taking hydroxychloroquine (a common medication for lupus and rheumatoid arthritis), multifocal ERG is used to catch early retinal toxicity. The characteristic finding is decreased signal strength in a ring around the center of the macula. On a color-coded mfERG map, this appears as blunted peaks in the central and paracentral zones of both eyes. Catching this pattern early, before visible damage appears on imaging, can prevent permanent vision loss.
In conditions that primarily affect blood supply, like central retinal artery occlusion, the b-wave is often disproportionately reduced compared to the a-wave. This makes the b-wave to a-wave ratio (b/a ratio) a useful marker. A normal retina produces a b-wave that’s larger than the a-wave. When the inner retina loses blood flow, the b-wave shrinks while the a-wave (generated by photoreceptors, which have a separate blood supply) may be less affected.
What a Flat or Severely Reduced ERG Means
A non-recordable ERG, where no measurable waveform appears, indicates severe loss of retinal function. This finding is associated with advanced stages of several conditions: retinitis pigmentosa, Leber congenital amaurosis, cone and cone-rod dystrophies, and certain rare metabolic conditions like Bietti crystalline dystrophy. It can also occur in some inflammatory conditions such as birdshot retinochoroidopathy. The clinical significance depends entirely on context. In a child being evaluated for poor vision from birth, a flat ERG points toward a congenital retinal dystrophy. In an adult with a known progressive condition, it marks an advanced stage.
Factors That Can Affect Accuracy
ERG results are sensitive to testing conditions. Incomplete dark adaptation is one of the most common sources of error: if you weren’t in darkness for the full 20 minutes before scotopic testing, rod responses will appear artificially reduced. Some protocols extend dark adaptation to 30 or even 45 minutes depending on what’s being measured. Light adaptation for photopic testing requires about 10 minutes of bright-light exposure.
Pupil dilation is essential for full-field ERG because the flash needs to reach as much of the retina as possible. Poor electrode contact, excessive blinking, and eye movements can all introduce noise that reduces signal quality. Your technician will typically run each test at least twice to confirm the results are reproducible. If two traces don’t match well, the recording is repeated. When reviewing your report, the presence of two overlapping, consistent waveforms for each condition is a good sign that the data is reliable.