WonderQuest with April Holladay, Article printer-friendly version

The color of hazel eyes; How we remember, part 2

The Question of the Month, readers contribute answers:

Q: What exactly are hazel eyes, and what color are they? Rita Lichtenburg South Africa

A hazel eye: greenish gold, much like reader Christine Bourquin's eyes. Photo courtesy of NightWing and Wikipedia.

A: It seemed like a simple question, when I first read it. Then I went to various sources, and found no consensus.

Hmmm. This is a question for a world-renown expert.

"The question of Hazel eye colour has haunted the literature," muses geneticist Rick Sturm, principal research fellow at the University of Queensland in Australia. "The fact is eye color (like skin and hair color) is a continuous spectrum — from the lightest shades of blue to the darkest brown/black." See figure for a hint at the variation.

Moreover, it's subjective. "Visual impressions of colour can change as often as you change the lighting conditions."

Also, hazel eyes, like blue eyes and all light eyes, reflect colors around them, as a pool reflects the sky.

Variations in eye color. Photo courtesy of Rick Sturm.

Sturm and his team have studied adolescent twins to determine the genetic underpinnings of eye color. In the course of these studies, he's taken 1,937 eye photographs, and divided the group into three major eye-color definitions. Sixty-percent of the twins fall in the 'blue' category, 26% in 'green/hazel' and 14% 'brown.'

"Surprisingly," 74% of the green/hazel eyes had a brown ring around the pupil. "This major pattern may explain a lot of eye color that is commonly referred to as Hazel."

These statistical findings are unpublished, and not yet accepted by peer review; they "only represent my opinion at this stage" says Sturm.

How do such eyes occur? Certainly the simple model we learned in school about brown-eye color being dominant over blue falls short of an explanation. Indeed that one-gene theory is kaput. There is no single gene for eye color. Now, we know two major genes and other minor ones account for the tremendous variation of human eye color, says Sturm, part of the team making this discovery, reported in 2007.

The gene OCA2 produces a protein that allows the hair, skin and eyes to make pigment (called melanin) that colors these body parts. The more pigment in the eye, the darker it is. Much pigment results in brown eyes; little pigment causes blue eyes.

Furthermore, a change happens fairly frequently to the pigment protein under the control of the OCA2 gene. When the protein changes, its function changes. It makes a different pigment that then colors the eyes green or hazel. Sturm likens this process to "changing a light bulb from brown to green."

Further Reading:

The eyes have it on multiple gene question by Rick Sturm, University of Queensland, Australia, February 2007

Genetics of eye color unlocked by Paul Rincon, BBC News, December 2006

The genetics of hazel eyes by Barry Starr, Stanford University

Introduction to genetics, GlaxoSmithKline

D.L. Duffy, G.W. Montgomery, W. Chen, Z.Z. Zhao, L. Le, M.R. James, N.K. Hayward, N.G. Martin, R.A. Sturm. A three-SNP haplotype in the intron 1 of OCA2 explains most human eye color variation. American Journal of Human Genetics, 80: 241-252 (2007).

Readers' answers:

Hazel eyes are not one color. It is a name for a combination of colors: golds, greens and browns. My eyes have a dark green circle on the outer edge, going in with lighter greens and specks of gold into a starburst effect with a gold edge, inside that is a lighter brownish gold color with starburst lines inside to the pupil. The colors seem to change with hair and clothing color. The eye color becomes significantly greener with the salt of tears.
Christine Bourquin, Castro Valley, California
My Gramma's eyes are so hazel. They look greenish gold most of the time. But if she wears something that is really green they turn to green, then if she wears blue they almost look dark blue. But the majority of the time they are hazel. She always told us that wasn't a color. Her eyes are sort of green mixed with gold. I wish my eyes were hazel...
Brittney Martin, Roy, Utah
Hazel eyes are a mixture of many colors. I have Hazel eyes and mine are rimmed in blue and have flecks of gold, medium brown and green in them. When I wear certain colors my eyes change, kind of like a chameleon.
Kim, Mississippi

Q: How do we store memories in our brain? How do we recall memories? Rajeev, Bangalore, India

A neuron network in the cortex of a mouse. Photo courtesy of PLoS Biology.

Review

Last week we considered how information flows through the brain, and how the brain places a new short-term memory into long-term memory.

Our senses pick up information, and pass it to sensory memory, where it lasts a fraction of a second. Interesting stuff goes into short-term memory, but just a few items at a time, maybe seven. The info lasts for less than a minute. Finally information that may help us in the future (for instance, the smell of a saber-tooth tiger) goes into long-term memory, where it can last a lifetime.

A new short-term memory, for example, 'Delicious apple', gets into long-term memory by associating the concept with many key descriptive ideas: red color, tastes sweet, looks round, the sound of the crisp apple as I snap off a bite — and then such contextual items as 'I'm feeling good because it's a happy fall day and I'm picking apples.'

We use the hippocampus, an ancient evolutionary part of the cortex, to consolidate a new memory. An event creates temporary links among cortex neurons. For example, 'red' gets stored in the visual area of the cortex, and the sound of a bitten apple gets stored in the auditory area. When I remember the new fact, 'Delicious apple', the new memory data converge on the hippocampus, which sends them along a path (called the Papez circuit) several times to strengthen the links, and to pick up any emotional associations like 'happy fall day', and spatial connections like 'apple orchard'.

Neuron networks

That's the big picture. Now let's examine how neuron networks store and retrieve memories.

Special neuron networks exist that are pre-wired to link cortical neurons into a new network memory. One such network is the Papez circuit in the hippocampus we discussed earlier. The Delicious apple example illustrates how the Papez circuit entrenches temporary connections existing between visual (RED), hearing (BITE-SOUND) and limbic neurons (a HAPPY fall day) to form a new lasting memory: Delicious apple.

The RED part of the 'Delicious apple' network and memory. Each circle and its long tail(s) represents a neuron; this is a simplified representation. A typical neuron has 1,000 to 10,000 synapses. Drawing courtesy of Bruno Dubuc and http://thebrain.mcgill.ca/, modified by the author.

The RED part of the 'Delicious apple' network and memory. Each circle and its long tail(s) represent a simplified neuron. A typical neuron has 1,000 to 10,000 synapses. Drawing courtesy of Bruno Dubuc and http://thebrain.mcgill.ca/, modified by the author.

Consider, first, the RED part of the Delicious apple memory. It's a network in the visual area of the cortex that contains the sensation of the particular red color of a Delicious apple. This network (depicted in the drawing by solid dots) forms a path defined by its synapses. The RED neurons' synapses changed so their cellular membranes maintain a resting potential difference close to the outgoing neurons' firing threshold voltage. This makes it easy for the neurons along this path to fire, establishing a potentially conducting circuit. The path is the firing path for nerve impulses that stores and invokes the sensation RED in the Delicious apple memory.

Click here for an illuminating animation showing how a neuron fires, courtesy of Bruno Dubuc and here for a lucid look at the firing mechanism, including threshold voltages, courtesy of Eric Chudler.

The OVERALL network (shown in green) linking the RED, HAPPY and BITE-SOUND networks to form a DELICIOUS-APPLE MEMORY. Grey's Anatomy, modified by author.

The OVERALL network (shown in green) linking the RED, HAPPY and BITE-SOUND networks to form a DELICIOUS-APPLE MEMORY. Drawing from Gray's Anatomy, modified by author.

A similar situation exists for a network in the auditory area for the sound of the apple bite and in the limbic area for the memory of a happy fall day. Moreover, an OVERALL network (green lines in the figure) exists that connects each of these memory parts: RED, BITE-SOUND and HAPPY. The synapses of the OVERALL network changed in the same way to establish a preferred path linking each memory part. The structure of favored connections (OVERALL, RED, BITE-SOUND and HAPPY) all link to form the total DELICIOUS-APPLE MEMORY.

The brain retrieves the information by firing the DELICIOUS-APPLE MEMORY network, causing electrical signals to travel through the network that connects Delicious apple sensory data.

Next week I will tell the third part in this three-part story: "How synapse molecules change to define a network path and, hence, a pattern and a memory."