Friday, April 6, 2007

Vision primer

My trichromatic mouse post was way too long, so I decided to take all the background information on the visual system, which some of you may not need, and put it in a different post, thus breaking it into more digestible bits. This intro focuses on topics relevant to the article (photoreceptors and the initial processing stages of color vision), so although I would love to discuss physical nature of light, the complexity of the vertebrate eye, and higher visual processing, I'm leaving all that for another day!

Vision
Our visual system exploits light to generate a useful representation of our environment. This process involves a highly specialized division of labor, in which our eyes function as the light "detectors." They then convey this data in an organized fashion to the visual areas of the brain, which process this deluge of information such that we can make sense of our visual world.

This sounds basic, but I want to make sure I preclude the common misconception that our eyes are like tiny cameras, which project tiny inverted images of the world for the brain to "watch" like a movie on a screen. Instead, when light (with wavelengths between 700 billionths of a meter to 420 billionths of a meter) hits our eyes, it activates a certain pattern of sensory neurons in the retina (photoreceptors), and after some preliminary organization at the level of the retina, a modified pattern is conveyed to the brain, where it is decoded in the brain by a mechanism which is far from understood. So not only do our photoreceptors not relay "picture of a tree," they do not even convey information such as "green."

Color Vision
How, then, do we see "green?" Our eyes rely on a three-color system ("trichromacy"); that is, they contain three distinct classes of color photoreceptors (called cones), each of which respond differently to a given wavelength and intensity of light. The classes are generally called short-wave (S), medium-wave (M), and long-wave (L), in reference to their optimal responsiveness to either short, medium or longer waves of light. They are more commonly called Blue, Green, and Red cones, colors which correspond to their respective wavelength (although Red is somewhat a misnomer because the neurons actually respond optimally to Yellow).

However, these cones are pretty broadly tuned, meaning they are actually capable of responding to wide ranges of light; the functional difference between an optimal wavelength and a suboptimal wavelength for a given class is a difference in the firing rate of the cone. To add further confusion, the firing rates are dependent not only on the wavelength, but also to the amount (intensity) of light.

This explains why the magnitude of response of a single class of cones cannot tell us anything about the color of a light: a "green" cone will respond to a dim green light with the same firing rate as it will to a bright red light. In order to deduce the color of an object, it is thus necessary for the brain to compare the responses of representatives from each class of cones. With trichromatic vision, our brains can compare the firing rate of three classes of cells, all with different properties, giving us the ability to perceive and differentiate among a dazzling array of hues.

Most mammals have dichromatic vision, with only two cones (short and medium); seeing through their eyes would be akin to missing a cone, which is the case with the 8% of men and 0.5% of women who are colorblind. Many fish and reptiles, on the other hand, are tetrachromatic, and the retinas of birds and turtles may exhibit even more diversity.

To see what happens when you give a dichromatic animal trichromatic vision, read on...

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