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...

7 comments:

Anonymous said...

Do you like playing the game where you need to use priston tale Gold, when you do not have priston tale Money, you must borrow it from friends, or you buy priston tale Gold. If you getcheap priston tale Gold, you can continue this game.
Do you like playing the game where you need to use wow gold, when you do not have World of Warcraft Gold, you must borrow warcraft gold from friends, or you buy wow gold. If you get cheap wow gold, you can continue this game.

Unknown said...

The timepiece will represent watch number for the brand,
http://www.watchestoo.com
replica watches
rolex
rolex oyster perpetual datejust watch
rolex oyster perpetual datejust replica watch
rolex 18k
rolex stainless
AIR KING
DATEJUST
DAY DATE
DAYTONA
EXPLORER
GMT MASTER
JUBILEE DATEJUST LADIES
OYSTER PERPETUAL

Unknown said...

louis vuitton name is inspired from lv , a fashion museum in Paris that houses exhibitions and collections of French fashion and costume from the 18th century to the present day including clothes owned by Marie-Antoinette, Louis vuitton bags , etc. Damier Azur canvas is a fresh fabric, recalled us to the endless sea, the salty breeze, the dazzling sunshine, the golden beach, and the clear sky. louis vuitton handbags is definitely the epitome of Damier reinterpretation, definitely the ultimate summer getaway.

Unknown said...

The Breitling Bentley was first developed in 1969 - Chrono Superocean created the Caliber II, the World's for self-winding chronograph movement. Superocean was easily distinguishable by the placement of the crown on the left-hand-side of the casing. These Chronomat B01 still place the crown of the left-side.

A Coach Wallets can be quite a useful item for a young woman to carry. Most of Coach Ergo are of quite a large size. That makes them convenient for the amount of items they can help a person carry. In addition, they are quite well known for Coach Legacy sleek and attractive appearance. In fact, a Coach Luggage is a coveted and sought after item for many young women.

The copyright THE DREW CAREY SHOW laws are in place to ensure that the owner of the copyright is protected from any types of fraud and illegal activities, which includes THE DREW CAREY SHOW DVD duplication of any form. It is best to simply avoid making any duplicates of any THE DREW CAREY SHOW DVD COLLECTION movies to protect yourself and your friends. Purchasing the movie will help in support of your favorite actors and directors.

Anonymous said...



Feel free to suгf to my webpage: pure green coffee bean extract 800 mg

Leonie Daecher said...

How to bet on football this week: 50+ special hints for bettors | OddsDigger . Click this link for more ways to win money.

Anonymous said...

off white
golden goose sale
yeezy shoes
bape
nike kyrie 7