Tag Archives: Iris

eye-color-thumb

Why eyes have different colors: a science-based look

eye-color-thumb

Credit: LittleThings.

It is said that the eyes are the doorway to the heart; a reflection of our inner self and emotions with countless poets, writers, and artists praising this unique quality. Whether or not this is the case, eyes are deeply fascinating and have been so since time immemorial. But for all this, they’d certainly be far less interesting, and perhaps even frightening, if they weren’t colored.

Actually, all the magic happens within the colored part of the eye: the iris.

What gives eyes their color

The iris color is determined by the amount of melanin pigmentation. The more pigment there is, the darker the iris will be. Blue, gray, and green eyes are lighter because there is less melanin inside the iris.

By far, the most common eye color in the world is brown, with over 55% of the population falling into this category. Depending on where a person is born, eye color demographics can vary wildly. For instance, nearly all persons of African and Asian ancestry have brown eyes. It’s believed up to 10,000 years ago, all humans had brown eyes only. Then a mutation turned off the pigmentation on the front of the iris.

Hazel eyes are similar to brown eyes, the distinction being these are lighter. A defining trait of hazel eyes is their multi-coloured appearance that can vary from copper to green depending on the lighting. Hazel eyes have a higher concentration of melanin around the iris’ border. Estimates suggest 5 to 8 percent of the world’s population is hazle-eyed (both green and brown).

The next most common eye colors are blue, gray, and green in this order. It’s commonly quoted that only 2% of the world’s population has green eyes.

There are also so-called ‘amber’ eyes. which are even rarer than green eyes. Amber eyes or ‘wolf eyes’ as they’re sometimes called are completely solid and have a strong yellowish, golden, or russet and coppery tint. They can also contain a small amount of gold-ish gray. It’s not clear how amber eyes form but some suggest it’s due to the increased presence of a pigment called lipochrome (also known as pheomelanin).

Lastly, people with albinism, a condition that causes a complete lack of or very low levels of pigment in the skin, hair, and eyes, sometimes appear to have violet or red eyes. Because albino people essentially lack pigment in the iris, light simply bounces off the back of the eye. Albino eyes may appear red because the light reflected first off blood vessels at the back of the retina. This is the same reason why you sometimes appear red-eyed in photos. The eyes can appear violet in certain lighting conditions when the red color mixes with the bluish color resulting from light-scattering effects — basically for the same reason why the sky is blue.

The table below shows how melanin content influences eye color.

[panel style=”panel-info” title=”Iris pigmentation and coloring” footer=””]

 

Eye color
 

Melanin Presence (front layer)
 

Melanin Presence (back layer) 
 

Dominant Pigment Type
 

Brown
 

Heavy
 

Normal
 

Eumelanin
 

Blue
 

Light
 

Normal
 

Eumelanin
 

Gray
 

Even less than blue
 

Normal
 

Eumelanin
 

Green
 

More than blue eyes, less than brown
 

Normal
 

Pheomelanin
 

Hazel
 

More than green, less than brown
 

Normal
 

Pheomelanin and Eumelanin
 

Amber
 

Heavy
 

Normal
 

Pheomelanin
 

Red or Violet
 

None or extremely little
 

None or extremely little
 

n/a

 

[/panel]

Besides making eyes colored, the pigment melanin also serves the vital function of protecting them from the sun’s UV rays. It follows that darker eyes, which have the most melanin, are less sensitive to the sun’s harmful rays than lighter eyes like blue.

When we come into this world, our eyes are blue or almost colorless. This is true for all babies, no matter their ancestry. In time, the concentration of melanin increases and by age three, the eyes will have darkened to their true, final resting color. Or almost final. Much later in life, our eyes can change color once more. Disease and trauma can also inflict changes in iris coloring. More on that later.

How eye color is inherited

Note that this chart only takes into account parents’ eye colors. Because it only factors in the phenotype (i.e. what color the eyes appear) and not the genes themselves, it is not going to be 100% accurate in every case. Credit: SittingAround.

Note that this chart only takes into account parents’ eye colors. Because it only factors in the phenotype (i.e. what color the eyes appear) and not the genes themselves, it is not going to be 100% accurate in every case. Credit: SittingAround.

If both parents have blue eyes, there’s a good chance their offspring has blue eyes as well. It follows that iris coloring is governed by genetics. However, a baby’s eye color doesn’t come out as a blend of the parent’s eye, as if you mixed paint. Up until not too long ago, even doctors used to think used to think that eye color was determined by a single gene and followed a simple inheritance pattern in which brown eyes were dominant to blue eyes. The thinking was that if two parents both had blue eyes, they couldn’t make a baby with brown eyes. Imagine what sort of problems this caused back at home. Alas, this is wrong.

Instead, many various possibilities exist since each parent has two pairs of genes on each chromosome. In other words, eye color is a polygenic trait, meaning it is determined by multiple genes.

The eye color genes

Among the genes that affect eye color, OCA2 and HERC2 stand out. Both of these genes are found in the human chromosome 15. The OCA2 gene produces a cell membrane transporter of tyrosine, a precursor of melanin. Mutations in OCA2 result in oculocutaneous albinism, a condition associated with vision problems such as reduced sharpness and increased sensitivity to light. HERC2 regulates the OCA2 genes’ expression. In the European population, a common polymorphism in HERC2 gene is responsible for the blue eye phenotype. A person who has two copies of C allele at HERC2 rs1293832 will likely have blue eyes while homozygous TT predicts likely brown eyes.

[panel style=”panel-success” title=”Eye color inheritance ” footer=””]

rs12913832 TT ALLELE CC ALLELE TC ALLELE
Likelihood of eye color for people of European descent 85% chance of brown eyes;
14% chance of green eyes;
1% chance of blue eyes.
72% chance of blue eyes;
27% chance of green eyes;
1% chance of brown eyes.
56% chance of brown eyes;
37% chance of green eyes;
7% chance of blue eyes.

 

[/panel]

Several other genes play smaller roles in determining eye color. Some of these genes are also involved in skin and hair coloring. Genes with reported roles in eye color include ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1. The effects of these genes likely combine with those of OCA2 and HERC2 to produce a continuum of eye colors in different people.

Nowadays, many DNA tests are reliable enough to determine a person’s eye color from a hair sample alone, some with over 90% accuracy. Such analyses are now becoming more and more common in the field of forensic investigations.

How eye color changes during lifetime

The iris, the colored bit of the eyes, is essentially a muscle. Its role is to control pupil size so we can see better under varying lighting conditions. When there’s dim light, the pupils enlarges and, conversely, grows smaller in bright lighting. Pupils also change size depending on focused objects. For instance, reading a book requires your pupils to shrink in order to focus on the near objects, i.e. words inked on the paper.

When the pupil’s size changes, the melanin pigment is compressed or spread apart, slightly changing the eye color. The effect is minuscule but it’s there.

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.

When the pupil size changes, the pigments in the iris can become compressed or spread apart, changing the eye color a bit. Some have suggested that mood can also change eye color. While it’s true certain emotions like anger or love can influence pupil size, the iris doesn’t really change color. When a  person’s eyes have red, dilated blood vessels from anger, his eyes may appear to be greener because of the contrast, that’s all.

Eye color can permanently change over the course of one’s life, though. For 10 to 15 percent of the Caucasian population, iris color changes with age. For instance, hazel eyes can get dark with age. And if such a change in eye color happens fast and dramatically, like from brown to green or from blue to brown, this may be a sign of concern making a doctor’s appointment urgently required.  Eye color changes can be a warning sign of certain diseases, such as Fuch’s heterochromic iridocyclitis, Horner’s syndrome or pigmentary glaucoma.

The eye color infographic

How the eye works

 

Image via flickr. 

Doing some light reading

Touch interprets changes of pressure, texture and heat in the objects we come in contact with. Hearing picks up on pressure waves, and taste and smell read chemical markers. Sight is the only sense that allows us to make heads and tails of some of the electromagnetic waves that zip all around us — in other words, seeing requires light.

Apart from fire (and other incandescent materials), bioluminiscent sources and man-made objects (such as the screen you’re reading this on) our environment generally doesn’t emit light for our eyes to pick up on. Instead, objects become visible when part of the light from other sources reflects off of them.

Let’s take an apple tree as an example. Light travels in a (relatively) straight line from the sun to the tree, where different wavelengths are absorbed by the leaves, bark and apples themselves. What isn’t absorbed bounces back and is met with the first layer of our eyes, the thin surface of liquid tears that protects and lubricates the organ. Under it lies the cornea, a thin sheet of innervated transparent cells.

Behind them, there’s a body of liquid named the aqueous humor. This clear fluid keeps a constant pressure applied to the cornea so it doesn’t wrinkle and maintains its shape. This is a pretty important role, as that layer provides two-thirds of the eye’s optical power.

Anatomy of the eye.
Image via flikr

The light is then directed through the pupil. No, there’s no schoolkids in your eye; the pupil is the central, circular opening of the iris, the pretty-colored part of our eyes. The iris contracts or relaxes to allow an optimal amount of light to enter deeper into our eyes. Without it working to regulate exposure our eyes would be burned when it got bright and would struggle to see anything when it got dark.

The final part of our eye’s focusing mechanism is called the crystalline lens. It only has half the focusing power of the cornea but its most important function is that it can change how it does this. The crystalline is attached to a ring of fibrous tissue on its equator, that pull on the lens to change its shape (a process known as accommodation), allowing the eye to focus on objects at various distances.

Fun fact: You can actually observe how the lens changes shape. Looking at your monitor, hold your up hands some 5-10 centimeters (2-4 inches) in front of your eyes and look at them till the count of ten. Then put them down; those blurry images during the first few moments and the weird feeling you get in your eyes are the crystalline stretching to adapt to the different focal vision.
Science at its finest.

After going through the lens, light passes through a second (but more jello-like) body of fluid and falls on an area known as the retina. The retina lines the back of the eye and is the area that actually processes the light. There are a lot of different parts of the retina working together to keep our sight crispy clear, but three of them are important in understanding how we see.

  • First, the macula. This is the “bull’s eye” of the retina. At the center of the macula there’s a slight dip named the fovea centralis (fovea is latin for pit). As it lies at the focal point of the eye, the fovea is jam-packed with light sensitive nerve endings called photoreceptors.
  • Photoreceptors. These differentiate in two categories: rods and cones. They’re structurally and functionally different, but both serve to encode light as electro-chemical signals.
  • Retinal pigment epithelium. The REP is a layer of dark tissue whose cells absorb excess light to improve the accuracy of our photoreceptors’ readings. It also delivers nutrients to and clears waste from the retina’s cells.

So far you’ve learned about the internal structure of your eyes, how they capture electromagnetic light, focus it and translate it into electro-chemical signals. They’re wonderfully complex systems, and you have two of them. Enjoy!

There’s still something I have to tell you about seeing, however. Don’t be alarmed but….

The images are all in your head

While eyes focus and encode light into the electrical signals our nervous system uses to communicate, they don’t see per se. Information is carried by the optical nerves to the back of the brain for processing and interpretation. This all takes place in an area of our brain known as the visual cortex.

Brain shown from the side, facing left. Above: view from outside, below: cut through the middle. Orange = Brodmann area 17 (primary visual cortex)
Image via wikipedia

Because they’re wedged in your skull a short distance apart from each other, each of your eyes feeds a slightly different picture to your brain. These little discrepancies are deliberate; by comparing the two, the brain can tell how far an object is. This is the mechanism that ‘magic eye’ or autostereogram pictures attempt to trick, causing 2D images to appear three dimensional.  Other clues like shadows, textures and prior knowledge also help us to judge depth and distance.

[YOU SHOULD  ALSO READ] The peculiar case of a woman who could only see in 2-D for 48 years, and the amazing procedure that gave her stereo-vision

The neurons work together to reconstruct the image based on the raw information the eyes feed them. Many of these cells respond specifically to edges orientated in a certain direction. From here, the brain builds up the shape of an object. Information about color and shading are also used as further clues to compare what we’re seeing with the data stored in our memory to understand what we’re looking at. Objects are recognized mostly by their edges, and faces by their surface features.

Brain damage can lead to conditions that impair object recognition (an inability to recognize the objects one is seeing) such as agnosia.  A man suffering from agnosia was asked to look at a rose and described it as ‘about six inches in length, a convoluted red form with a linear green attachment’. He described a glove as ‘a continuous surface infolded on itself, it appears to have five outpouchings’. His brain had lost its ability to either name the objects he was seeing or recognize what they were used for, even though he knew what a rose or a glove was. Occasionally, agnosia is limited to failure to recognize faces or an inability to comprehend spoken words despite intact hearing, speech production and reading ability.

The brain also handles recognition of movement in images. Akinetopsia, a movement-recognition impairing condition is caused by lesions in the posterior side of the visual cortex. People suffering from it stop seeing objects as moving, even though their sight is otherwise normal. One woman, who suffered such damage following a stroke, described that when she poured a cup of tea the liquid appeared frozen in mid-air, like ice. When walking down the street, she saw cars and trams change position, but not actually move.