When most people think about Parkinson's disease (PD), motor symptoms like tremors, stiffness, and balance problems usually come to mind first. But vision can be affected, too.
Researchers have increasingly recognized that PD may involve visual symptoms such as reduced contrast sensitivity, impaired color discrimination, and difficulty with visual processing.
These changes can affect everyday tasks, especially in environments with dim lighting, glare, or shifting illumination.
So with this in mind …
A recent study from researchers in Spain examined contrast sensitivity across six different lighting conditions.
The results showed that participants with PD performed worse than healthy controls across all tested lighting environments.
Why does contrast sensitivity matter in PD?
Contrast sensitivity measures how well someone can distinguish an object from its background when differences in brightness are subtle.
That is different from standard visual acuity testing. A patient may read high-contrast letters on an eye chart reasonably well but still struggle with low-contrast situations in real life—such as reading gray text, navigating stairs in dim lighting, or driving at night.
Why that distinction matters in PD: Visual symptoms may not always show up clearly during routine testing.
And if contrast sensitivity is reduced, patients may experience functional visual difficulties—even when their visual acuity appears relatively preserved.
So, what did the researchers actually test?
The researchers evaluated contrast sensitivity function using the Functional Acuity Contrast Test (FACT) and calculated an overall score known as the Index of Contrast Sensitivity (ICS).
Participants were tested under six illumination conditions:
- Photopic conditions without glare
- Mesopic conditions without glare
- Photopic conditions with mild glare
- Mesopic conditions with mild glare
- Photopic conditions with stronger glare
- Mesopic conditions with stronger glare
The goal was to see how PD affected visual performance under lighting conditions that better reflect real-world environments, not just standard bright-light testing.
Who participated in the study?
The study initially recruited 76 participants with PD and 96 healthy controls.
After age matching, the final analysis included:
- 61 participants with PD
- 61 healthy controls
The average age in both groups was approximately 66 years.
To note: Participants with PD had a neurologist-confirmed diagnosis and were classified according to the Hoehn and Yahr staging system. Patients in more advanced stages were excluded because the testing required participants to understand and complete the visual tasks.
Well, what did the researchers find?
The main finding was clear: Participants with PD had significantly poorer contrast sensitivity than healthy controls across all six lighting conditions.
The largest difference in overall contrast sensitivity appeared under mesopic conditions, meaning lower-light conditions.
Researchers also found that visual performance differences depended on both lighting and spatial frequency, with PD participants showing particular difficulty at mid-to-high spatial frequencies.
… and in practical terms?
The study suggested that PD's-related visual impairment may become more apparent when patients are tested under low-light or glare conditions rather than only under standard clinical lighting.
So why do these findings matter in daily life?
The findings may help explain why some people with PD report problems in visually demanding environments.
The study authors noted that reduced contrast sensitivity could affect tasks such as navigating stairs or curbs, driving at night, dealing with glare from headlights or streetlights, reading against low-contrast backgrounds, and adapting when moving between different lighting environments.
Why that matters: Because these are not abstract test results—they are the kinds of visual challenges that can affect mobility, independence, safety, and quality of life.
Did the disease stage make a difference?
At first, the data suggested that contrast sensitivity declined as PD stages advanced.
But after researchers adjusted for age, the relationship between disease stage and contrast sensitivity became less clear.
- What that means: Age appeared to account for at least part of the worsening visual performance seen in later-stage patients.
While the study still found formidable contrast sensitivity deficits in PD overall, the authors cautioned that stage-related differences should be interpreted carefully because age played an important role.
So what were the study's biggest limitations?
First: Although the full study included enough participants to compare PD patients with controls, the disease-stage subgroups were smaller. That made it harder to draw firm conclusions about how contrast sensitivity changes across disease stages.
Second: As this was cross-sectional in design, researchers measured participants at one point in time and could not determine how contrast sensitivity changed as the disease progressed.
The authors also noted that disease severity was classified using the Hoehn and Yahr scale, which focuses mainly on motor symptoms and may not fully capture the cognitive, visual, and non-motor features of PD.
And what did the researchers think this means clinically?
The authors believe contrast sensitivity testing in PD should not be limited to standard bright-light conditions.
Because deficits appeared across several lighting environments—and were especially meaningful under lower-light and glare conditions—testing under more realistic illumination may provide a better picture of how patients function in daily life.
They also suggested that contrast sensitivity testing could eventually help clinicians better characterize visual dysfunction in PD and potentially support future monitoring strategies.
Take home.
In a nutshell: This study found that PD patients had significantly poorer contrast sensitivity than healthy controls across bright-light, low-light, and glare conditions.
What this suggests: Standard visual acuity testing may not fully capture the real-world visual difficulties experienced by these patients.
- Plus, testing contrast sensitivity under varied lighting conditions may offer a more complete view of visual function—especially for those who report trouble with night driving, glare, dim environments, or low-contrast tasks.