Hazel is in the eye of the beholder; more memory
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These statistical findings are unpublished, and not yet accepted by peer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
ID.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:
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) 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. 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."
Further Reading:
How memory
works, part 1, WonderQuest
How memory
works, part 3, WonderQuest
The brain from top to bottom by Bruno Dubuc, Canadian
Institutes of
Neuroscience, Mental Health, and Addiction
Neuroscience for kids by Eric Chudler, University of Washington
Brain
Facts and Figures by Eric Chudler, University of Washington
(Answered March19, 2007)
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