FACTS VCS, contrast sensitivity, spatial frequency, VCS testing
FACTS-Based VCS Testing: The 5 Spatial Frequencies Explained
FACTS-based VCS testing maps contrast sensitivity across five spatial frequencies. Here's what each one reveals.
The FACT (Functional Acuity Contrast Test) is the contrast-sensitivity standard used in biotoxin research. It measures how dim a pattern can get before your eyes can no longer tell it's there — and it does so across a range of pattern sizes, not just one. That range is what makes a FACT-based VCS test far more informative than a single contrast number.
What "spatial frequency" means in a VCS test
Spatial frequency refers to how fine a pattern is — how many cycles (one light bar plus one dark bar) fit into a degree of your visual angle. It's measured in cycles per degree (cpd):
- Low spatial frequency = large, chunky bars (coarse patterns)
- High spatial frequency = fine, closely spaced bars (detailed patterns)
Your eyes don't process all patterns equally. Most people are most sensitive to mid-range frequencies (around 3–6 cpd), where vision is sharpest, and less sensitive at the coarse and fine extremes.
The five spatial frequencies in a FACT VCS test
A FACT-based VCS test uses five spatial frequencies:
- 1.5 cpd — large, low-frequency gratings
- 3 cpd — coarse-to-medium
- 6 cpd — mid-range, where vision is usually sharpest
- 12 cpd — fine
- 18 cpd — very fine, high-frequency gratings
Each frequency is shown at nine logarithmically spaced contrast levels, from easy to see down to invisible. Your threshold at each frequency maps out a contrast sensitivity curve — a fingerprint of how your retina and visual pathway are performing.
Why five frequencies matter
A single contrast number can't tell you where vision is breaking down. Low-frequency deficits point to one set of causes (often optical or retinal); high-frequency loss points to another (often neural or capillary). Neurotoxin exposure tends to erode sensitivity in specific parts of the curve, so a five-frequency chart gives far more signal than a one-number screen.
For example, a person exposed to mold mycotoxins might show a deficit at mid-to-high frequencies while their low-frequency vision looks fine. That pattern is invisible if you only measure one contrast value.
How the online VCS screen works
This site's VCS screen follows the FACT structure:
- One eye at a time, to isolate each eye's performance
- Five frequencies, each at nine contrast levels
- Adaptive contrast levels that home in on your true threshold
- An "Unsure" button so you report honestly instead of guessing
You get a per-eye pass/fail verdict for free, and with the advanced report a full contrast sensitivity curve for each eye — showing exactly which frequencies are affected and how severe the deficit is. Take the free VCS test to see yours, or browse the blog for more on interpretation.
What your contrast curve can reveal
Your contrast sensitivity curve is most useful in context:
- A normal curve across all five frequencies — reassuring, but not a clean bill of health on its own.
- A deficit at specific frequencies — a pattern that, combined with symptoms and exposure, can justify deeper workup.
- A curve that recovers over time — a sign that treatment or remediation is working, which is why repeat testing is valuable.
Because the curve is per-eye, it can also reveal asymmetries — one eye more affected than the other — that a single number would hide.
Keeping the test reliable
A VCS test is only as good as its setup. For results that mean anything:
- Test in a dim, consistent room — bright light or glare washes out faint gratings.
- Use a calibrated screen at a steady brightness setting.
- Sit at the correct distance — the test's spatial frequencies depend on viewing angle.
- Test when rested — fatigue, migraine, and screen strain can all lower your score.
These caveats are exactly why a VCS screen is a screen and not a diagnosis: many temporary factors can nudge your curve. The value is in pairing a properly-run test with your history and repeating it under the same conditions over time.
Frequently asked questions
What does a "deficit" on a FACT VCS test mean? It means your contrast sensitivity at one or more frequencies fell below the expected range. On its own it's not a diagnosis, but combined with symptoms and exposure history it's a signal worth investigating with a clinician.
Why test each eye separately? Because neurotoxin-related vision change can affect one eye more than the other. Testing both eyes together would average out and hide that asymmetry. (More on contrast vs. acuity.)
Why is the "Unsure" button important? Forced-choice tests push you to guess, which inflates scores with luck. Honest "unsure" responses let the adaptive test find your true threshold — accuracy matters more than optimism.
How is this different from a regular contrast test at the optometrist? A standard contrast test often uses a single chart or a single frequency. The FACT uses five frequencies at nine contrast levels each, which is the resolution needed to detect the subtle, frequency-specific changes linked to biotoxin exposure.
If you have questions about mold, mycotoxins, or what your results might mean, you can also ask an AI trained on mold and mycotoxin questions for fast, plain-language answers to follow up on.
Written by
Matt Kelly
Certified Detoxification Specialist · Functional Mold Specialist · Indoor Air Consultant (IAC2)
Matt has trained in detoxification and functional medicine since 2017, specializing in mold toxicity and the conditions that travel with it. After mold illness, Lyme, and related chronic conditions affected his own family, he built VCScheck to make early, affordable biotoxin screening accessible to every family.