New Light Shed On How Retina's Hardware Is Used in Color Vision

Biologists at New York University and the University of Würzburg have identified, in greater detail, how the Retina's cellular hardware is used in color preference. The findings, published in the latest issue of the Proceedings of the National Academy of Sciences (PNAS), enhance our understanding of how eyes and the brain process color.

Light can serve as an attractive or repulsive landmark for orientation -- we identify an object or a light source at a certain location in visual space, then approach it or retreat from it. This process, called phototaxis, was the focus of the PNAS study.
Conducted by biologists at New York University's Center for Developmental Genetics and the Department of Genetics and Neurobiology at the University of Würzburg in Germany, the research specifically examined the photoreceptor cells in the Retinas of the fruit fly Drosophila. Drosophila is a powerful model for studying the color vision process as it is amenable to very specific genetic manipulations, allowing researchers to analyze how its visual system functions when different elements of its Retina are affected.
The visual systems of most species contain photoreceptors with distinct spectral sensitivities that allow animals to distinguish lights by their spectral composition (i.e., color). In Drosophila, six of these (R1-R6) are responsible for motion detection and are sensitive to the brightness or dimness of a broad spectrum of light. Two others (R7 and R8) are used for color vision by comparing ultraviolet light (UV), detected by R7, with green or blue light detected by two types of R8. The NYU and University of Würzburg biologists investigated how photoreceptor types contribute to phototaxis by blocking the function of either R7 or R8, or a combination of a range of photoreceptors (R1-R6, R7 and/or R8).
In the study, they constructed two sets of "Y-shaped mazes" with two different types of light at the ends of each: UV and blue in one and blue and green in the other. Under this arrangement, the fly would show a preference for certain type of light (UV vs. blue in one maze; blue vs. green in the other) by moving toward it. The researchers could then link specific preferences to the make-up of each fly's visual system.
In a "UV vs. blue" choice, flies with only R1-R6 and flies with only R7/R8 photoreceptors preferred the blue to the UV light. This finding suggested that these two sets of photoreceptors (R1-R6 and R7/R8) function separately in phototaxis as flies with only one of these sets showed similar preferences. In addition, flies without a functioning R7 photoreceptor preferred the blue to the UV light, whereas flies without R8 preferred UV. In the "blue vs. green" maze, flies without a functioning blue R8 photoreceptor preferred green, whereas those with a defective for green R8 photoreceptor preferred blue. This shows that each subclass of photoreceptors [R1-R6, R7, R8 (blue), R8 (green)] is used by the fly to distinguish colors and setup its innate color preference. In a previous work, the same authors had shown that motion detection only involves R1-R6 and not R7 and R8, suggesting that there are two independent channels in the fly visual system -- one for motion and one for color.
"This simple insect can achieve sophisticated color discrimination and detect a broader spectrum of colors than we can, especially in the UV," said NYU biologist Claude Desplan, one of the study's authors. "It is a great model system to understand how the Retina and the brain process visual information.
The research was supported by a grant from the National Institutes of Health.
http://www.sciencedaily.com/releases/2010/03/100308151051.htm

The above story is reprinted from materials provided by New York University, via EurekAlert!, a service of AAAS.
Note: Materials may be edited for content and length. For further information, please contact the source cited above.

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Eye Allergies

 

Eye Allergies often are hereditary, and occur due to processes associated with other types of allergic responses.

When an allergic reaction takes place, your eyes may be overreacting to a substance perceived as harmful, even though it may not be. These substances are called allergens.

For example, dust that is harmless to most people can cause excessive production of tears and mucus in eyes of overly sensitive, allergic individuals.

Allergies can trigger other problems, such as conjunctivitis (pink eye) and asthma. Combined nasal and eye Allergies create a condition known as rhinoconjunctivitis.

About 30 percent to 50 percent of U.S. residents have allergy symptoms. And about 75 percent of those symptoms affect the eyes.*

Allergy Symptoms and Signs

Common signs of Allergies include:

• Red, swollen or itchy eyes
• Runny nose
• Sneezing and coughing
• Itchy nose, mouth or throat
• Headache from sinus congestion

Beyond more obvious symptoms, you also may feel fatigued and could suffer from lack of sleep.

What Causes Eye Allergies?

Many allergens are in the air, where they come in contact with your eyes and nose. Airborne allergens include pollen, mold, dust and pet dander.

Other causes of Allergies, such as certain foods or bee stings, do not typically affect the eyes the way airborne allergens do. Adverse reactions to certain cosmetics or drugs such as antibiotic eye drops also may cause eye Allergies.

Some people actually are allergic to the preservatives in eye drops such as those used to lubricate dry eyes. In this case, you may need to use a preservative-free brand.

General Eye Allergy Treatment

Avoidance. The most common "treatment" is to avoid what's causing your eye allergy. Itchy eyes? Keep your home free of pet dander and dust and keep pets off the furniture. Stay inside with the air conditioner on when a lot of pollen is in the air. Use high quality furnace filters that trap common allergens and replace the filters frequently.

Make sure you wear wraparound sunglasses to help shield your eyes from allergens, and drive with your windows closed during allergy season.

Medications. If you're not sure what's causing your eye Allergies, or you're not having any luck avoiding them, your next step probably will be medication to alleviate the symptoms.

Over-the-counter and prescription medications each have their advantages; for example, over-the-counter products often are less expensive, while prescription ones usually are stronger and may be more effective.

Eye drops are available as simple eye washes, or they may have one or more active ingredients such as antihistamines, decongestants or mast cell stabilizers that inhibit inflammation. Antihistamines relieve many symptoms caused by airborne allergens, such as itchy, watery eyes, runny nose and sneezing. Decongestants help shrink swollen nasal passages for easier breathing.

Relief for Watery, Itchy Eyes

Common causes of excessively watery eyes are allergies and dry eye syndrome — two very different problems.

With allergies, your body's release of histamine causes your eyes to water, just as it may cause your nose to run.

It may seem illogical that dry eye syndrome could cause watery eyes. But sometimes an underlying dry eye condition stimulates your tear glands to overproduce the watery component of your eye's tears as a protective response, leading to watery eyes.

Decongestants clear up redness. They contain vasoconstrictors, which make the blood vessels in your eyes smaller, lessening the apparent redness. They treat a symptom but not the cause of eye allergies.

In fact, with extended use, the blood vessels can become dependent on the vasoconstrictor to stay small. When you discontinue the eye drops, the vessels actually get bigger than they were in the beginning. This process is called rebound hyperemia, and the result is that your red eyes could worsen over time.

Some products have ingredients that act as mast cell stabilizers, which alleviate redness and swelling. Mast cell stabilizers are similar to antihistamines. But while antihistamines are known for their immediate relief, mast cell stabilizers are known for their long-lasting relief.

Eye Allergy News

Have Fall Allergies? You Might Not Want To Live in One of These Cities

October 2011 — Is your town one of the top 100 most challenging in the country for fall allergies? Check the new list compiled by the Asthma and Allergy Foundation of America.

Knoxville, Tenn. is number one this year, joining other pollen-ridden southern and midwestern cities. The next four on the list are: Dayton, Ohio; McAllen, Texas; Jackson, Miss.; and Oklahoma City.

It's not just pollen that's causing the problem. Indoor allergens such as dust mites, pet dander and mold also trigger allergic reactions in the fall.

Please click here for the top 100 U.S. cities for fall allergies.

Also, take a look at today's pollen count map.

Antihistamines, decongestants and mast cell stabilizers are available in pill form, but pills don't work as quickly as eye drops or gels to bring eye relief.

Nonsteroidal anti-inflammatory drug (NSAID) eye drops may be prescribed to decrease swelling, inflammation and other symptoms associated with seasonal allergic conjunctivitis, also called hay fever.

Prescription corticosteroid eye drops also may provide similar, quick relief. However, steroids have been associated with side effects such as increased inner eye pressure (intraocular pressure) leading to glaucoma and damage to optic nerve.

Steroids also have been known to cause the eye's natural lens to become cloudy, producing cataracts.

Check the product label or insert for a list of side effects of over-the-counter medications. For prescription medication, ask your doctor. In some cases, combinations of medications may be used.

You may benefit from immunotherapy, in which an allergy specialist injects you with small amounts of the allergen to help you gradually build up immunity.

Eye Allergies and Contact Lenses

Even if you are generally a successful contact lens wearer, allergy season can make your contacts uncomfortable. Airborne allergens can get on your lenses, causing discomfort. Allergens also can stimulate the excessive production of natural substances in your tears, which can bind to your contacts and cause blur and additional discomfort. Pollen maps can help you determine when allergens are present.

Ask your eye doctor about eye drops that can help relieve your symptoms and keep your contact lenses clean. Certain drops can discolor or damage certain lenses, so it makes sense to ask first before trying out a new brand.

Another alternative is daily disposable contact lenses, which are discarded nightly. Because you replace them so frequently, these types of lenses are unlikely to develop irritating deposits that can build up over time and cause or heighten allergy-related discomfort.

By Gina White, with updates by Marilyn Haddrill; reviewed by Vance Thompson, MD

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Eye Color

 

Eye Color often is the genetic trait that fascinates parents the most as a child develops. Will the child's eyes be black, brown, blue, gray, green, hazel or some combination of colors?

How a child looks depends on the genetic material each parent contributes to the child. But the parents' genes can mix and match in many different ways. The influences from each parent aren't known until — surprise — after the child is born!

How Eye Color Develops

The colored part of the eye is called the iris, which has pigmentation that determines our Eye Color.

Human Eye Color originates with three genes, two of which are well understood. These genes account for the most common colors — green, brown, and blue. Other colors, such as gray, hazel and multiple combinations are not fully understood or explainable at this time.

Her eyes may be blue only temporarily; babies' eyes can change color if the brown pigment melanin develops as they grow.

We used to think of brown being "dominant" and blue being "recessive." But modern science has shown that Eye Color is not at all that simple.

Also, Eye Colors don't come out as a blend of the parents' colors, as in mixing paint. Each parent has two pairs of genes on each chromosome. So multiple possibilities exist, depending on how the "Wheel of Fortune" spins.

Dutch researchers have announced they are working on ways to determine Eye Color of adults with sophisticated DNA analysis that can predict with 90 percent accuracy whether people have brown or blue eyes.

In May 2010, the same researchers said they were closing in on the ability to more accurately predict even variable Eye Colors via DNA analysis and new understanding of how genetics govern Eye Color.

Researchers said these discoveries also have implications for forensic investigations at crime scenes where recovered DNA may give clues about the actual appearance of suspects.

Eye Color Poll Results

Which Eye Color Do You Think Is Most Attractive? Here's the Answer

August 2011 — After collecting 6,468 responses to our eye color poll, we thought it was time to share the results with you:

Green is the winner among our site visitors, but responses were spread pretty well over most of the other colors, too.

To some, eye color isn't that significant. As someone said on our Facebook page, "All eye colors can be 'the most attractive.' ... Your eye color has to be about you and express the truth of you to others."

Another person posted this comment about eye health being much more important: "The most attractive color is the white of the eye when it is clear and bright, which makes any iris beautiful." — L.S.

Most babies are born with blue eyes that can darken in their first three years. Darkening occurs if melanin, a brown pigment usually not present at birth, develops with age.

Children can have completely different eye colors than either of their parents. But if both parents have brown eyes, it's most likely that their children also will have brown eyes.

The darker colors tend to dominate, so brown tends to win out over green, and green tends to win out over blue.

However, a brown/blue parent mix doesn't automatically produce a brown-eyed child.

Some children are born with irises that don't match in color. Usually this is caused by faulty developmental pigment transport, local trauma either in the womb or shortly after birth or a benign genetic disorder. Other causes can be inflammation, freckle (diffuse nevus) of the iris and Horner's syndrome.

Having an early eye exam is important to make sure nothing serious is going on — and "nothing serious" is the most common finding.

Changes in Eye Color

The iris is a muscle that expands and contracts to control pupil size. The pupil enlarges in dimmer lighting and grows smaller in brighter lighting. The pupil also shrinks when you focus on near objects, such as a book you are reading.

Eye Color News

Can We Change Eye Color With Surgery?

November 2011 — A company called Stroma Medical is developing a way to turn brown eyes blue permanently, using a laser.

How it works exactly is unclear, but it has been reported that the laser releases the brown pigments from the iris, and what's left is a blue color that develops over a couple of weeks. One downside may be that the iris color afterward can't be accurately predicted.

The procedure is not FDA-approved and will require more testing before U.S. availability is possible. A potential safety question is, what happens to the released pigments within the eye?

For more information, watch this video of a news report on KTLA.com. — L.S.

When the pupil size changes, the pigments in the iris compress or spread apart, changing the eye color a bit.

Certain emotions can change both the pupil size and the iris color. That's why some people say their eyes change colors when they're angry or loving.

Eye color also can change with age. This happens in 10 to 15 percent of the Caucasian population (people who generally have lighter eye colors).

For instance, my once very brown eyes are now hazel, a combination of brown and green. However, some hazel eyes actually get darker with age.

Note that if your adult eye color changes pretty dramatically, or if one eye changes from brown to green or blue to brown (called heterochromia), it's important to see your eye doctor. Eye color changes can be a warning sign of certain diseases, such as Fuch's heterochromic iridocyclitis, Horner's syndrome or pigmentary glaucoma.

Ultimately, if you don't like the eye color you inherited, you can always change it with colored contact lenses. But remember, even colored contact lenses are a prescription medical device and must be prescribed and monitored by an eye doctor. Don't buy them

over the Internet or get them from a friend without having an eye doctor's prescription!

By Burt Dubow, OD

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