Vision changes after a concussion: when contrast sensitivity tells you something

You hit your head three weeks ago. The ER did a quick eye check, asked you to follow a finger, shone a light, and told you your vision was fine. A few days later the chart in the optometrist's office said 20/20. And yet your laptop screen feels weirdly hard to focus on, headlights at night are harsher than they used to be, your reading speed has dropped enough that you've started re-reading paragraphs, and a friend's face across the table in a dim restaurant is somehow more work to recognise than it should be.

You are not imagining this. There is a part of vision that the standard chart does not sample, and after a concussion it is a relatively common part to be affected. The measurement that picks it up is called contrast sensitivity, and the rest of this post is about what is and isn't known, where in the spatial-frequency curve concussion-related changes tend to show up, and what you can usefully do with that information today — between clinical visits, in your own room, on your own screen.

This isn't a replacement for seeing a clinician. It's one quiet measurement you can take, repeat, and bring with you.

What "passing" the standard eye exam actually means

Visual acuity — the 20/20 number from the Snellen chart — measures one specific thing: your ability to resolve very fine, very high-contrast detail at a fixed distance. It's a real test of a real capability and it catches a lot. What it does not do is sample most of the visual function that everyday tasks depend on. Faces, road edges, computer-screen text under indoor light, peripheral motion at the edge of your visual field — almost none of those live at the top-right corner of the visual world the eye chart was designed to probe.

The eye chart asks whether you can read the smallest line of black ink on white paper. It does not ask whether you can comfortably read black-grey text on a white screen for an hour, distinguish a face in evening light, or track a moving target without losing your place. Concussion can affect many of those abilities while leaving the chart-reading capability essentially untouched. A clean acuity result rules out the most catastrophic kinds of visual injury and rules in nothing else. The set of things it doesn't measure includes the set of things post-concussion vision symptoms most often live in.

Two reviews are worth knowing about. Ciuffreda and colleagues looked back at clinical records of 220 patients with acquired brain injury — most of them with traumatic brain injury (160 of the 220), the remainder after stroke — and found that roughly 90% of the TBI group had at least one demonstrable oculomotor (eye-movement-control) problem: accommodation (focusing), vergence (aiming the two eyes together), and saccadic or pursuit eye-movement control. Acuity-on-the-chart was not the abnormal finding in most cases (Ciuffreda et al., 2007).¹ A few years later, a comprehensive review by Greenwald, Kapoor and Singh tracked the visual sequelae of TBI through the first year post-injury and documented the same pattern in the broader literature: subtle, functional visual changes — spanning acuity, contrast sensitivity, oculomotor function, accommodation and visual field — that the standard exam does not register (Greenwald, Kapoor & Singh, 2012).² The point isn't that your acute care was wrong; it's that the part of vision you're noticing is a different part from the one the chart was built to certify.

What contrast sensitivity is, briefly

Contrast sensitivity is how faint a pattern your visual system can still see. It is measured across a range of pattern sizes — from coarse, broad patterns (a foggy outline of a tree) to fine, detailed ones (the texture of newsprint) — and the result is plotted as a curve called the contrast sensitivity function, or CSF. The curve is an inverted-U: low at the very-coarse end, peaking somewhere around 3 to 6 cycles per degree of visual angle (the band that covers most face- and edge-related work at conversational distance), and falling off again at fine detail. Visual acuity — 20/20 — sits at one specific point on the falling right-hand tail of that curve.

A CSF measurement, done well, returns the whole curve, which is why it can register changes that a single acuity number doesn't: it samples a richer range of what vision actually does. The longer treatment of what the measurement is and why it's worth caring about lives in the primer post; this one is about what the literature says about that curve after a concussion specifically.

What the research actually shows

Three honest summaries:

Many post-concussion patients show measurable contrast sensitivity changes — but the pattern isn't perfectly specific. Reduced contrast sensitivity is one of the visual changes documented after TBI in the review literature, with reductions in some studies reported in the mid spatial-frequency band (roughly 3 to 12 cpd) — the band most relevant for face and edge perception under normal lighting (Greenwald, Kapoor & Singh, 2012).² These changes can be present even when acuity is intact. They are associated with mTBI in published peer-reviewed work, not specific to it — many other things can produce similar patterns, which is why a CSF result is a screening signal rather than a diagnosis. Contrast sensitivity is not the only visual-processing measure that shifts subtly after a concussion: changes have also been documented in temporal processing (how quickly the visual system resolves flicker)³ and in visual evoked potentials (the electrical response the visual cortex makes to a patterned stimulus),⁴ which is part of why the broader picture is described as a set of subtle, multi-modality changes rather than a single signature.

Oculomotor and accommodative problems are the most common documented sequelae. Ciuffreda and colleagues' retrospective sample found that the most frequent post-TBI vision findings were not contrast or acuity changes but problems with how the eyes move and focus — convergence insufficiency (the eyes struggle to aim together for near work), accommodative dysfunction (trouble focusing at near), and saccadic and smooth-pursuit deficits (Ciuffreda et al., 2007).¹ A larger military sample studied by Capó-Aponte and colleagues found the same broad pattern across blast and non-blast injuries: near-vision binocular dysfunctions — reduced convergence, receded near point of convergence, and reduced accommodation among them — were the more common findings, with the overall profile differing little between injury mechanisms or across post-injury stages (Capó-Aponte et al., 2017).⁵ What this means for a contrast-sensitivity test: a normal CSF curve does not rule out the things that most often go wrong after a concussion. CSF measures one slice; the visual-rehab specialists measure the others.

Recovery is the usual trajectory, but it isn't guaranteed and it isn't always quick. Most patients with post-concussion visual symptoms improve over weeks to months. A meaningful minority don't, or improve only partially. Greenwald, Kapoor and Singh's year-one review documents both halves of this — the typical curve back toward baseline, and the population for whom symptoms persist past the textbook recovery window (Greenwald, Kapoor & Singh, 2012).² CSF changes appear to follow a similar pattern: often present during the symptomatic phase, often normalising as symptoms improve, but the day-to-day correlation between the curve and how you feel is loose enough that a single measurement on a single day is not a verdict either way.

The honest framing, then. Many post-concussion patients show CSF changes that may correlate with subjective symptoms. The measurement is informative as a trend over time on the same setup; less informative as a one-shot number. It does not diagnose concussion or persistent post-concussion symptoms — both of those are clinical determinations that depend on history, examination, and judgement that an online test cannot perform. And no result, normal or reduced, settles whether your symptoms are "real": they are real because you're experiencing them, regardless of what any particular instrument reads.

What you can do with it

A CSF curve is most useful when you treat it as a single thread in a longitudinal record, not a verdict in a single sitting. The practical version of that:

Take the curve today. Our free test takes about three minutes in the quick mode, longer for a full curve, runs in the browser, and keeps results on your device by default. Calibration happens at the start (sizing the screen, sizing the viewing distance, sizing the brightness response) so the numbers are comparable across sessions on the same setup.

Retake it. A common rhythm during early recovery is once a week for the first few weeks, then once a month as things stabilise. Try to test in similar lighting, same device, similar time of day — fatigue, ambient light, and eye strain all move the curve, and you want as much of the variance as possible to come from you, not from setup differences. If you have a clinic visit on the calendar, taking the test the day before and bringing the curve is more useful than a same-morning reading taken in an unfamiliar waiting room.

Bring it to someone who works on this. The right kind of clinician for vision-related concussion symptoms is a neuro-optometric rehabilitation specialist — an optometrist with additional training in post-injury and post-stroke visual rehabilitation. General optometry exams cover refraction and ocular health and are essential to rule out structural problems first; neuro-optometric rehabilitation covers binocular function, oculomotor control, accommodation, and vestibular-visual integration. The Neuro-Optometric Rehabilitation Association maintains a referral directory; the Optometric Vision Development & Rehabilitation Association (formerly COVD) is a related body with overlapping membership.

Keep a short journal alongside the curve. A few lines per session: contrast number (or screenshot of the curve), light level in the room, fatigue level, hours of screen time that day, headache frequency over the past 24 hours, reading speed if you've been timing yourself. The journal isn't the test result — it's the context the test result lives inside. When you bring something to a clinician, the journal makes the curve interpretable in a way the curve alone cannot be.

What it isn't

Note. A contrast sensitivity test is a screening and tracking measurement, not a diagnosis.

It does not diagnose concussion, persistent post-concussion symptoms, or any other condition.

It is not a substitute for a comprehensive neuro-optometric examination, which covers binocular vision, oculomotor function, accommodation, and vestibular-visual integration — none of which an online CSF test measures.

A single result is a snapshot. Test-retest variation is real, even with carefully validated clinical instruments — and an online test on a consumer screen is noisier than that. A trend over several weeks on the same setup is far more informative than any one session's number.

Vision changes can come from many sources. A reduced curve is consistent with concussion-related changes; it is also consistent with refractive error, fatigue, dry eye, certain medications, early cataract, glaucoma, and a long list of other things. The test cannot disambiguate. A clinician can.

A normal result does not rule out vision-related concussion sequelae. Many of the most common ones — convergence insufficiency, accommodative dysfunction, oculomotor control problems — do not show up cleanly on a CSF measurement. They need a neuro-optometric examination.

The framing we'd ask you to hold: this is one piece of objective data you can collect at home, repeat at intervals, and bring to the people who can actually examine you. It's not a substitute for that examination; it's a thread of evidence that the examination otherwise wouldn't have access to.

Where to go next

The first step is finding a clinician who works with post-concussion vision specifically. Two starting points:

• The Neuro-Optometric Rehabilitation Association (noravisionrehab.org) — an interdisciplinary group focused on rehabilitative vision care after brain injury and stroke; their site has a "Find a Doctor" directory you can filter by location.
• The Optometric Vision Development & Rehabilitation Association (covd.org, the rebranded former COVD) — a related body whose members include many vision-therapy and neuro-optometric specialists.

If you're not sure which to use, NORA is the more direct match for post-concussion symptoms specifically; the OVDRA directory has broader coverage but a wider range of practice focuses. Either is a better referral pathway for post-concussion vision symptoms than searching "optometrist near me" — the standard optometric exam is excellent at what it covers and does not cover the things that most often need attention here.

The second step, which can run in parallel: take the test, save the result, retake it at intervals that match your recovery rhythm. The curve you'll have built up by the time you're sitting in a neuro-optometrist's office is something the appointment can be built on.

If you want the methodology specifics — how the calibration works, what the adaptive procedure does, how we render contrasts faithfully on a consumer screen — the methodology page covers it. If you want the conceptual background on what a CSF curve is and what 20/20 leaves out, the primer goes through it carefully.

Take the test

Take the test now. Save the result. Re-take in a week and compare the curves. If a clinic visit is on the calendar, bring the trend.

Three minutes today, three minutes next week, three minutes a month from now. The first is a snapshot. The second is the start of a line.