You sit down at the laptop to answer a few emails. An hour later you've answered two of them, re-read the same paragraph four times, and the screen feels weirdly heavy — as if the text is fine but reading it costs more than it used to. You aren't tired in a normal "I didn't sleep" way. You are foggy in a way that is hard to describe to anyone who hasn't lived in it.
"Brain fog" is a real, common, and oddly under-investigated symptom. It shows up across long COVID, ME/CFS, autoimmune flares, post-concussion recovery, perimenopause, severe sleep deprivation, and depression, among other places. There is no single blood test that diagnoses it, and the people who experience it have often spent more energy than they had explaining that it isn't laziness or anxiety.
This post is about a part of fog that often gets missed: the visual part. Many people who describe themselves as foggy turn out, on close look, to have a measurable component of the fog that lives in their eyes and visual cortex rather than in some pure abstract "thinking" faculty. Contrast sensitivity — how faint a pattern your visual system can still see — is one home-measurable signal that can sometimes catch this layer. It isn't a fog-meter. It's a measurement that might, alongside a symptom journal, give you something to look at.
This isn't a substitute for clinical care. It's a thread you can pull between visits.
What "brain fog" usually means
Brain fog isn't a clinical diagnosis. It is a patient-descriptor — a word people reach for because it captures something the clinical vocabulary doesn't quite have a phrase for. The cluster typically includes: difficulty concentrating, slower thinking, trouble holding the thread of a conversation, harder-to-focus eyes, harder-to-read screens, a sense that more effort is required for the same cognitive output.
It is reported across a wide set of conditions. In an international cohort of 3,762 long-COVID patients with confirmed or suspected COVID-19, Davis and colleagues found cognitive dysfunction or memory issues in about 88% of respondents, with cognitive dysfunction one of the three most frequent symptoms persisting beyond six months alongside fatigue and post-exertional malaise (Davis, Assaf, McCorkell et al., 2021). The same cluster shows up in ME/CFS case definitions, in post-concussion surveys, in autoimmune-flare narratives, and in perimenopause cohorts. It's a real, well-documented symptom — even though no single instrument can render it as a number on a chart.
What it isn't: a synonym for anxiety, depression, or "you're imagining it." Many people reading this have had at least one clinician suggest that interpretation. We aren't going to. The fog is doing something. The question is which parts of which systems are involved, and which of those parts you can usefully measure at home.
How vision and fog connect
Vision is not a passive camera. It is one of the largest, most metabolically expensive computations the brain does — the primary visual cortex alone is the single largest cortical area in humans by surface area, and downstream processing (attention, segmentation, recognition, working memory of what was just on screen) recruits a wide network across the parietal and frontal cortex. One reason visual symptoms may surface early in fatigue or inflammation states is that the visual system is metabolically expensive in a way that's hard to camouflage when resources are tight.
This is mechanism-talk, and we hold it loosely. The neuroscience of why a foggy person feels visually foggy is an active area, not a settled one. What is better established is that when the brain is working harder than usual on everyday tasks, reading and screen-use are often the first things that feel different. Several phenomena tend to cluster:
- Contrast feels lower. Text that previously looked crisp now looks slightly washed out, particularly on screens. Anti-glare coating on glasses that previously felt sufficient feels marginal.
- Reading endurance drops. The number of minutes you can comfortably read before the words start to wobble or your eyes feel pulled is shorter than it was.
- Screens feel heavier than paper. The light source on a backlit display is doing something different from reflected light on paper, and a fatigued visual system feels the difference.
- Mid-spatial-frequency tasks feel harder. Face recognition across a room, edges in dim light, road-sign legibility at dusk — the band that lives between "fine detail" and "broad outline."
Several of these are things a contrast sensitivity measurement can sometimes register. After concussion, mid-spatial-frequency contrast sensitivity changes have been documented alongside the broader cluster of oculomotor and attentional problems the standard eye chart doesn't probe (Ciuffreda, Kapoor, Rutner et al., 2007). For long COVID, the picture is younger: emerging case-series and small-cohort reports are starting to characterise visual changes, but the literature is early and we'd treat any specific number as provisional. What's reasonable to say is that the visual layer of fog has a plausible biological substrate and is increasingly being looked at — not that contrast sensitivity is a long-COVID test.
What contrast sensitivity might tell you
If your fog has a visual component, your contrast sensitivity function — the curve that plots how faint a pattern you can detect at each pattern scale — may be lower than your personal typical, particularly in the mid-frequency band (around 3 to 12 cycles per degree of visual angle, the band most relevant to faces and screen text at conversational distance). It may also be unchanged. Both are common. The CSF curve is one thread; it isn't the whole rope.
What's useful about the measurement is that it gives you something comparable across days. A "1–10 fog score" is informative but susceptible to recall bias, mood drift, and the way the rating scale itself shifts under fatigue. A calibrated test that runs the same way every time on the same screen gives you a number you can put next to that 1–10 score and see whether they track.
Practically:
- Take the test on a good day, when you'd rate yourself low-fog. That's your personal anchor.
- Take it again on a foggy day. If the visual layer of your fog is real, you may see the curve sag — particularly in the mid band.
- Take it again on a near-baseline day to see how much your number moves on its own.
- Build a small series over a few weeks, with a one-line fog note per session.
The thing you're looking at is the shape of the series, not the absolute number of any one session. A series that tracks your fog-score journal is informative. A series that doesn't is also informative — it tells you the layer of fog you're experiencing may live somewhere other than the visual contrast pathway, which is itself worth knowing going into a clinical conversation. The tracking cadence and journaling habits live in our self-tracking guide for chronic-illness patients; this post is about the why, not the how.
Test-retest variation is a real constraint here. Even the most carefully validated clinical instrument — the Pelli-Robson letter chart — has a test-retest repeatability of about ±0.15 log units, and the smallest change generally considered clinically meaningful is around ±0.30 log units (Pelli, Robson & Wilkins, 1988). A consumer-screen test sits in a noisier regime than that. The way you beat the noise is not by aiming for perfect precision on any one session; it's by collecting enough sessions that the trend rises above the wobble. Same setup, same lighting, same time of day where you can manage it.
What it cannot tell you
Note. A contrast sensitivity test is a screening and tracking measurement. It is not a diagnostic instrument.
It does not diagnose long COVID, ME/CFS, post-concussion syndrome, an autoimmune condition, depression, anxiety, or any of the conditions in which brain fog is reported. Those are clinical determinations that depend on history, examination, and judgement that a home test cannot perform.
It does not directly measure cognitive fog. It measures one slice of visual function. If your fog has a visual component, the measurement may catch part of it. If your fog lives elsewhere — attention, processing speed, working memory, sleep architecture — a normal CSF curve does not rule it out.
A single result is a snapshot. A trend across multiple sessions on the same setup is far more informative than any one session, and even the best longitudinal record can't replace a conversation with a clinician who can examine you.
Many other things can move a CSF curve. Refractive error, dry eye, a missed glasses Rx, an old anti-reflective coating, ambient light, screen brightness, eye fatigue, caffeine state, certain medications, sleep, hormones, and any of the many ocular conditions that can quietly degrade contrast independently of fog. A reduced curve is consistent with a visual fog component; it is also consistent with a long list of other things. The measurement cannot disambiguate. A clinician can.
The fog is real either way. No instrument result, normal or reduced, settles whether what you're experiencing is "valid." It is — because you are experiencing it. The measurement is here to give you data alongside that experience, not to adjudicate it.
Practical
A few concrete things you can do this week.
If your fog is recent and persistent, the first stop is your primary care clinician — particularly if it followed a viral illness, a head injury, a new medication, or a perimenopausal transition. A basic workup (bloodwork including thyroid and B12, a sleep history, a mood screen, a careful medication review) catches a meaningful slice of contributors. None of those rule out long COVID or ME/CFS, but they catch the things that look like fog and are actually something else with a clearer fix.
If vision feels worse during fog episodes specifically, take the test on a good day and a bad day within the same week. If the difference is visible above your session-to-session noise, that's information worth bringing to your next appointment — particularly with a neuro-optometric rehabilitation specialist. The Neuro-Optometric Rehabilitation Association maintains a referral directory; their members specialise in the visual sequelae of brain injury and post-viral conditions, and are the right audience for "I feel foggy and it has a visual flavour."
Reduce visual load while you investigate. Independent of any test result, several adjustments reliably help foggy people work more comfortably:
- Bigger text. Bump system font size up a step or two on every device. The single highest-leverage adjustment, and most people leave it on the default for too long.
- More light. A second desk lamp, sun-direction permitting. The cheapest way to make contrast feel higher is more illumination.
- Make sure your glasses Rx is current. A two-year-old prescription is doing more work than it should against contrast loss; a routine optometric exam catches this in twenty minutes.
- Anti-reflective coating. If your lenses don't have it, ask about it next exam. AR coating measurably lifts effective contrast on backlit screens.
- Brighter screens with bigger fonts usually beat dim screens with small fonts for contrast-related fatigue, unless photophobia makes any screen intolerable — that needs a different conversation with a clinician.
Our methodology page covers the calibration and adaptive procedure. The primer post walks through what the CSF curve is and why 20/20 acuity doesn't capture it. If your fog followed a concussion specifically, the post-concussion vision post goes deeper on that pattern.
What we hope this is useful for
A few honest goals. First, surfacing a vision component you might not have noticed. Many people who describe themselves as foggy have a visual layer they haven't named yet — partly because the standard eye exam comes back clean, partly because the cognitive description ("I can't think") feels like the more pressing one. Naming the visual layer is the first step toward measuring it.
Second, generating data alongside a journal. A small longitudinal record of your CSF curve, lined up against a fog journal, is something a clinical conversation can be built on. It's the kind of record fifteen-minute appointments rarely produce in-room, and that patients almost never have at hand when asked "how have things been since I last saw you?"
Third, validating that fog is real, even when no test 'proves' it. No single instrument can render brain fog as a definitive number. That doesn't mean fog isn't real. It means the right framing is multi-measurement triangulation — symptoms, sleep, mood, function, and a small set of objective measurements like the CSF curve — looked at across time. Reduced contrast sensitivity on a foggy day, lined up against the same journal entry, is one thread in that triangulation.
Take the test
Take it on a good day. Take it on a foggy day. Compare. Bring patterns to your care team.
Take the test now. Save the result — the share link is fine. Take it again when the fog rolls in. Three minutes today, three minutes next time. The first is a snapshot. The second is the start of a comparison only you can make.
References
- Davis, H. E., Assaf, G. S., McCorkell, L., Wei, H., Low, R. J., Re'em, Y., Redfield, S., Austin, J. P., & Akrami, A. (2021). Characterizing long COVID in an international cohort: 7 months of symptoms and their impact. EClinicalMedicine, 38, 101019. International web-based cohort of 3,762 confirmed or suspected long-COVID patients across 56 countries; cognitive dysfunction or memory issues reported by approximately 88% of respondents, with cognitive dysfunction one of the three most frequent symptoms persisting past six months alongside fatigue and post-exertional malaise. The load-bearing modern citation for the prevalence and persistence of brain fog as a documented post-acute COVID symptom.
- Ciuffreda, K. J., Kapoor, N., Rutner, D., Suchoff, I. B., Han, M. E., & Craig, S. (2007). Occurrence of oculomotor dysfunctions in acquired brain injury: a retrospective analysis. Optometry, 78(4), 155–161. Retrospective review of 220 patients with acquired brain injury — most of them mild TBI — finding that roughly 90% had at least one demonstrable oculomotor dysfunction. The standard reference for vision-related symptoms persisting after brain injury when standard acuity testing is unremarkable. Used here for the broader claim that subjective cognitive symptoms after brain injury frequently correlate with measurable subtle visual-system changes.
- Pelli, D. G., Robson, J. G., & Wilkins, A. J. (1988). The design of a new letter chart for measuring contrast sensitivity. Clinical Vision Sciences, 2, 187–199. The original Pelli-Robson chart paper, still the most widely used clinical letter-based contrast sensitivity test. Test-retest repeatability of about ±0.15 log units and a clinically meaningful change threshold of about ±0.30 log units — the test-retest figures any home-tracking record needs to be read against. Cited here as the variance anchor that motivates the trend-over-multiple-sessions reading.