Is 'red' the same to all creatures?

Article from CNN

Violets are blue and roses are red, but maybe those colors are all in your head.

What does it mean for an object to be "red"? Is the way you perceive blueness the same as your neighbor? Your cousin? What about your dog?

Many scientists believe that humans have color vision that is generally consistent across populations and cultures, and that there are evolutionary reasons behind that constancy.

"Color vision is all about emotions and moods, and it has much deeper and richer connections to the rest of our perceptual worlds," said Mark Changizi, a cognitive scientist at Rensselaer Polytechnic Institute in Troy, New York.

Color vision in humans and animals

But some people really don't see the color red in the way that most do. About 8 percent of men have trouble differentiating between certain colors; less than 0.5 percent of women have this problem, according to the American Academy of Ophthalmology.

Color vision is based on photoreceptors in the eye called cones, of which there are about 6 million to 7 million in the human retina. Humans normally have three types of cones, corresponding to short, medium and long wavelengths of light. Purplish blues are at the short end, and reds are at the long end. They eye also has about 120 million rods, which detect light but not color.

According to some estimates, the human eye can distinguish about 1 million to 10 million different colors. A small minority of women actually have four kinds of cones in their eyes -- meaning they could theoretically see even more colors -- but only a genetic test can determine who has extra cones, and it's unclear exactly how differently they may see.

In most cases of colorblindness, the cone systems for either medium or long wavelengths do not work properly, resulting in reds, greens and perhaps yellows appearing very similar. But different people experience this to varying degrees. In rarer cases, people have trouble telling blue and yellow apart; the rarest of all make people see the world in grayscale.

Dogs and cats are generally colorblind, somewhat like humans who have trouble with reds and greens, but only see pale shades of color. On the other hand, they see better at night and have better peripheral vision. Insects see through photoreceptor units numbering in the hundreds or the thousands, almost like viewing the world as a mosaic. Some animals actually have better color vision than humans. Pigeons and goldfish, for example, can see ultraviolet light, which is invisible to people.

"We're great for mammals but pretty mediocre by broader standards," said David Hilbert, associate professor of philosophy at the University of Illinois at Chicago.

Biologists believe that animals' visual systems have evolved over millions of years and that the particular structures around today have persisted because they carried some survival benefit to the animals.

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