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What Is Contrast Sensitivity?
Visual contrast sensitivity is the ability to distinguish between an object and the background behind it. However, contrast sensitivity is different from visual acuity, which measures how clear your vision is at a given distance.
High spatial frequencies make up detailed features such as sharp edges, facial features, and similar. Low spatial frequencies are more like coarse images, where you can see the overall shape of something but not detailed features.
Visual acuity is what your eye doctor measures when you read the eye chart during an exam. This is considered a high contrast test (black letters on a white background). You can have excellent visual acuity, but reduced contrast sensitivity and vice versa.
Low contrast sensitivity can also be a symptom of other eye conditions, such as glaucoma, cataracts, amblyopia, and age-related macular degeneration, among others.
What Does Contrast Sensitivity Do?
Contrast sensitivity is essential to have in many low contrast situations. Patients with reduced contrast sensitivity may have difficulty with:
- Driving at night or in other conditions with poor visibility such as rain or fog.
- Locating objects against a similarly colored background. Examples include finding black socks in a darkly colored drawer or pouring coffee into a black mug.
- Reading material with poor contrast, such as a newspaper.
- Stepping off curbs or steps.
- Distinguishing facial features on others.
How Does Good Contrast Sensitivity Help You When Driving?
Contrast sensitivity is an essential measure of visual function. It is necessary to have good contrast sensitivity for safety when driving in low-vision conditions, such as rain, low light, glare, or fog.
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What is a Contrast Sensitivity Test?
A routine eye exam does not include contrast sensitivity testing. If your eye doctor suspects you have a contrast sensitivity problem, the most common way to check for this is using a Pelli-Robson contrast sensitivity chart.
The chart features horizontal lines of uppercase letters in the same size. As you go from left to right, the contrast of each letter will decrease. The patient starts from the top of the chart and reads each row until they can no longer see any letters against the white background.
The Mars Letter contrast sensitivity test uses a similar chart as the Pelli-Robson test, except that the chart is smaller and viewed at a closer distance.
A more sophisticated way of testing contrast sensitivity uses sine-wave grating targets. Sine-wave gratings look like fuzzy lines of alternating light and dark shades of grey. Sometimes the lines are thicker, and sometimes they are thinner. The patient views multiple gratings on a computer screen or a wall chart.
This information tells the doctor how well you can see under low, medium, and high contrast situations. Your doctor can then plot a graph of these results, which is called contrast sensitivity function (CSF). CSF is more informative than visual acuity in determining an individual's spatial vision abilities.
A Snellen chart is different from a Pelli-Robson chart. Most ophthalmologists use the Snellen eye chart to measure visual acuity during an eye exam (visual acuity test). When you view the letter chart, you may notice that different letter sizes correspond to varying levels of visual acuity.
Then, they perform a refraction to see what your visual acuity is with your eyeglass prescription, which is your corrected visual acuity. Common refractive errors include astigmatism, myopia (nearsightedness), and hyperopia (farsightedness).
What is Normal Contrast Sensitivity?
The lower the score, the poorer the contrast sensitivity.
A score of 2.0 (units of measure are in log contrast) indicates normal contrast sensitivity.
A score of less than 1.5 is consistent with visual impairment (moderate loss).
A score of less than 1.0 represents visual disability (severe loss).
What Affects Contrast Sensitivity?
Many factors contribute to loss of spatial contrast sensitivity, including some eye diseases.
These eye conditions include:
- Cataracts. Cataracts are a common age-related eye condition. They occur when the natural lens inside your eye becomes cloudy and yellow. Many patients with cataracts have good visual acuity but still complain of poor quality vision due to decreased contrast sensitivity. These symptoms are amplified in situations where patients experience glare, such as oncoming car headlights when driving at night.
- Glaucoma. Glaucoma is a progressive eye disease that damages your optic nerves. Your optic nerves transmit visual signals to your brain, including information on color, brightness, and contrast. This is why damage to your optic nerves can affect contrast sensitivity.
- Optic neuropathies. Besides glaucoma, other diseases that can damage your optic nerves include multiple sclerosis, tumors, infections, aneurysms, or poor blood flow to the optic nerves.
- Macular degeneration. The macula is a small area in the retina, which is the tissue lining the back of your eye. Your macula is responsible for seeing fine details, such as reading text in a book. Macular degeneration is an age-related disease that damages the macula and causes vision loss. Patients may lose visual acuity, color vision, and contrast sensitivity. However, most people retain normal peripheral vision.
- Diabetes. Diabetic retinopathy involves damage and leaking of the blood vessels in the retina. Patients with this condition can experience a significant loss of contrast sensitivity. Studies also found that diabetic patients without retinopathy also had some decrease in contrast sensitivity.
- Eye trauma. Any eye injury, especially ones that damage your optic nerve, can potentially affect your contrast sensitivity.
- Laser eye surgery (LASIK and PRK). Conventional LASIK and PRK procedures can negatively affect contrast sensitivity. However, studies show that customized laser eye surgeries (such as wavefront-guided or wavefront-optimized LASIK and PRK) deliver better contrast sensitivity results versus conventional procedures.
How to Improve Contrast Sensitivity
If you suffer from poor contrast sensitivity due to underlying eye disease, talk to your eye doctor about treatment options.
Potential treatments for certain eye conditions/diseases include:
- Cataracts. Cataract surgery is a procedure that involves removing the cataract and replacing it with an artificial implant. Studies found that most patients regained normal levels of contrast sensitivity after cataract surgery.
- Glaucoma. There is no cure to reverse the effects of glaucoma. Instead, treatment focuses on slowing down the progression of the disease. The primary therapy is medicated eye drops that lower your eye pressure. Some research suggests that brimonidine, which is a glaucoma medication, may improve contrast sensitivity.
- Macular degeneration. There are two types of macular degeneration, dry and wet. There is no medical treatment for the dry form besides vitamin supplements. If the macular degeneration progresses from dry to wet, patients are more likely to experience vision problems. Wet macular degeneration is often treated with eye injections. One such medication is called aflibercept. Studies found that injections of aflibercept significantly improve contrast sensitivity.
- Diabetes. Diabetic macular edema is a complication of diabetic retinopathy. This condition involves fluid leakage underneath your macula. Aflibercept injections also treat diabetic macular edema. Researchers found that aflibercept treatment improved contrast sensitivity in patients with diabetic macular edema.
Besides treating the underlying disease, wearing corrective lenses can improve contrast sensitivity. Schedule an eye exam to make sure you are wearing the correct eyeglass prescription, which helps with your overall vision.
Why do Yellow-Tinted Glasses Improve Contrast Sensitivity?
Yellow-tinted lenses can help improve contrast sensitivity by increasing sharpness perception during the day. It is not recommended to wear yellow filtered lenses while driving at night because they provide no additional benefits.
Also, ask your eye doctor to recommend the appropriate type of lenses. An anti-reflective coating reduces glare and helps you see better in dim conditions. Yellow or copper tinted lenses enhance contrast and work in low light environments.
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