How Do We See in Color?
By: Stephanie Pappas, Life's Little Mysteries Contributor, Date: 29 April 2010 Time: 03:28 PM ET
Roses are red and violets are blue, but we only know that thanks
to specialized cells in our eyes called cones.
When light hits an object – say, a banana – the object absorbs
some of the light and reflects the rest of it. Which wavelengths
are reflected or absorbed depends on the properties of the
object. For a ripe banana, wavelengths of about 570-580
nanometers bounce back. These are the wavelengths of yellow
light.
When you look at a banana, the wavelengths of reflected light
determine what color you see. The light waves reflect off the
banana's peel and hit the light sensitive retina at the back of your
eye. That's where cones come in. Cones are one type of
photoreceptor, the tiny cells in the retina that respond to light.
Most of us have 6 to 7 million cones, and almost all of them are
concentrated on a 0.3 millimeter spot on the retina called the
fovea centralis.
Not all of these cones are alike. About 64 percent of them
respond most strongly to red light, while about a third are set off
the most by green light. Another 2 percent respond strongest to
blue light. When light from the banana hits the cones, it
stimulates them to varying degrees. The resulting signal is zapped
along the optic nerve to the visual cortex of the brain, which
processes the information and returns with a color: yellow.
Humans, with our three cone types, are better at discerning color than most mammals, but plenty of animals beat us out
in the color vision department. Many birds and fish have four types of cones, enabling them to see ultraviolet light, or
light with wavelengths shorter than what the human eye can perceive. Many insects can also see in ultraviolet, which
may help them see patterns on flowers that are completely invisible to us. To a bumblebee, those roses may not be so
red after all.
Your Color Red Really Could Be My Blue
By: Natalie Wolchover, Life's Little Mysteries Staff Writer, Date: 27 June 2012 Time: 12:16 PM ET
Anyone with normal color vision agrees that blood is roughly the same color as
strawberries, cardinals and the planet Mars. That is, they're all red. But could it be that what
you call "red" is someone else's "blue"? Could people's color wheels be rotated with respect
to one another's?
"That is the question we have all asked since grade school," said Jay Neitz, a color vision
scientist at the University of Washington. In the past, most scientists would have answered
that people with normal vision probably do all see the same colors. The thinking went that
our brains have a default way of processing the light that hits cells in our eyes, and our
perceptions of the light's color are tied to universal emotional responses. But recently, the answer has changed.
"I would say recent experiments lead us down a road to the idea that we don't all see the same colors," Neitz said.
Another color vision scientist, Joseph Carroll of the Medical College of Wisconsin, took it one step further: "I think we
can say for certain that people don't see the same colors," he told Life's Little Mysteries.
One person's red might be another person's blue and vice versa, the scientists said. You might really see blood as the
color someone else calls blue, and the sky as someone else's red. But our individual perceptions don't affect the way the
color of blood, or that of the sky, make us feel.
Some sort of perception
An experiment with monkeys suggests color perception happens in our brains in response to our experiences of the
outside world, but that this process does not happen in any specific pattern. Like color-blind people and most mammals,
male squirrel monkeys have only two types of color-sensitive cone cells in their eyes: green-sensitive cones and bluesensitive cones. Lacking the additional information that would be picked up by a third, red-sensitive cone, the monkeys
can only perceive the wavelengths of light we call "blue" and "yellow;" to them, "red" and "green" wavelengths appear
neutral, and the monkeys cannot distinguish between red and green dots amid a gray background.
In work published in the journal Nature in 2009, Neitz and several colleagues injected a virus into the monkeys' eyes that
randomly infected some of their green-sensitive cone cells. The virus inserted a gene into the DNA of the green cones it
infected that converted them into red cones. This gave the monkeys blue, green and red cones. Although their brains
were not wired for responding to signals from red cones, the monkeys soon made sense of the new information, and
were able to find green and red dots in a gray image.
The scientists have since been investigating whether the same gene therapy technique could be used to cure red-green
color blindness in humans, which affects 1 percent of American men. The work also suggests humans could one day be
given a fourth kind of cone cell, such as the UV cone found in some birds, potentially allowing us to see more colors.
But the monkey experiment had another profound implication: Even though neurons in the monkeys' brains were wired
to receive signals from green cones, the neurons immediately adapted to receiving signals from red cones instead,
somehow enabling the monkeys to perceive new colors. Neitz said, "The question is, what did the monkeys think the
new colors were?"
The result shows there are no predetermined perceptions ascribed to each wavelength, said Carroll, who was not
involved in the research. "The ability to discriminate certain wavelengths arose out of the blue, so to speak — with the
simple introduction of a new gene. Thus, the brain circuitry there simply takes in whatever information it has and then
creates some sort of perception."
When we're born, our brains most likely do the same thing, the scientists said. Our neurons aren't configured to respond
to color in a default way; we each develop a unique perception of color. "Color is a private sensation," Carroll said.
Emotional colors
Other research shows differences in the way we each perceive color don't change the universal emotional responses we
have to them. Regardless of what you actually see when you look at a clear sky, its shorter wavelengths (which we call
"blue") tend to make us calm, whereas longer wavelengths (yellow, orange and red) make us more alert. These
responses — which are present not just in humans, but in many creatures, from fish to single-celled organisms, are
thought to have evolved as a way of establishing the day and night cycle of living things.
Neitz and his colleagues found that changing the color of light has a much bigger impact on the day-night cycle of fish
than changing the intensity of that light, suggesting that the dominance of blue light at night really is why living things
feel more tired at that time (rather than the fact that it's dark), and the dominance of yellow light in the morning is why
we wake up then, rather than the fact that it's lighter.
But these evolved responses to color have nothing to do with cone cells, or our perceptions. In 1998, scientists
discovered a totally separate set of color-sensitive receptors in the human eye; these receptors, called melanopsin,
independently gauge the amount of blue or yellow incoming light, and route this information to parts of the brain
involved in emotions and the regulation of the circadian rhythm. Melanopsin probably evolved in life on Earth about a
billion years prior to cone cells, and the ancient color-detectors send signals along an independent pathway in the brain.
"The reason we feel happy when we see red, orange and yellow light is because we're stimulating this ancient blueyellow visual system," Neitz said. "But our conscious perception of blue and yellow comes from a completely different
circuitry — the cone cells. So the fact that we have similar emotional reactions to different lights doesn't mean our
perceptions of the color of the light are the same."
People with damage to parts of the brain involved in the perception of colors may not be able to perceive blue, red or
yellow, but they would still be expected to have the same emotional reaction to the light as everyone else, Neitz said.
Similarly, even if you perceive the sky as the color someone else would call "red," your blue sky still makes you feel calm.
6.4 IP: LIGHT READING 2: Eyes
Name___________________________ Class______ Date_____________ Honors? Yes No Score ________
1. Draw a basic picture of the eye. Label the 6 important parts of the eye for vision. You will need to know the job
of each part.
2. What are the two types of cells that make up the retina? What are the jobs of each?
a. _____________________________________________________________________________________
b. _____________________________________________________________________________________
3. Ask at least 3 friends, family, or teachers NOT in physics how many TOTAL color-sensing cells they think are in
our eyes. List the name and guess for each person. Then tell the actual number of color-sensing cells.
4.
5.
6.
7.
8.
Guess 1 Name: ________________________ Guess 1 Amount: ______________________________
Guess 2 Name: ________________________ Guess 2 Amount: ______________________________
Guess 3 Name: ________________________ Guess 3 Amount: ______________________________
Actual Amount: ______________________________
What kind of cone is missing in color blind monkeys (and most colorblind humans)?
______________________________
Use at least 2 complete sentences to describe how scientists have “fixed” color blindness in monkeys (and could
possibly do the same in humans).
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
What is the name of the more primal (earlier evolved) color receptors in our eyes? What colors were they
sensitive to?
___________________________________________________________________________________________
___________________________________________________________________________________________
Do you think that we all see colors similarly? Or do you think that your red could be my blue? Use at least 2
complete sentences to explain your answer.
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
Use letters a-d as examples. Complete letters e-h. Leave your answer in scientific notation:
a. (3 x 104) (5 x 108) = 15 x 1012
e. (4 x 105) (6 x 109) =
b. (1.3 x 107) (4.5 x 10-3) = 5.85 x 104
f.
(2.5 x 109) (1.9 x 10-6) =
c. (5.2 x 104) (6.7 x 10-8) = 34.84 x 10-4
g. (7.2 x 103) (3.7 x 10-5) =
d. (8.1 x 10-2) (9.7 x 10-9) = 78.57 x 10-11
h. (3.4 x 10-7) (7.9 x 10-6) =