Color Vision

Color Vision is the ability of an organism to distinguish objects based on the wavelengths of light the object reflects, emits, or transmits. The eye picks up varying wavelengths (all within the visible spectrum) using photoreceptors, namely rod cells (rods) and cone cells (cones), and sends the information to the brain for processing. Because Color is frequently used to convey geographic information, especially in Remote sensing and Cartographic design, it is important to be aware of how color is perceived by the users of maps and images.

Rods and Cones
Rods are cylindrically-shaped photoreceptors which can function in low light and are almost entirely responsible for our night vision. Rods are also located in the retina of the eye and number 125 million in the human eye. Rods can respond to a single photon of light and are 100 times more sensitive to that photon than are cones. While rods play the primary role in night vision, motion detection and peripheral vision they play little to no role in color detection.

Cones are conically-shaped photoreceptors that function in brighter light. Cones are located in the retina of the eye and number around 6 to 7 million. There are three types of cones: S (short wavelength, also known as "blue cones") M (medium wavelength, also known as "green cones") and L (long wavelength, also known as "red cones"). Human perception of color comes from the brain processing the responses of all three cones, which is referred to as trichromatic vision. Cones have a faster response time to stimuli thus allowing them to perceive finer detail and rapid changes in the image.

Cone and Rod Distribution
The distribution of rods and cones, within the retina, is not even. Cone cells are focused towards the center of the retina. This yellow area, called the macula, contains a region called the fovea centralis. This region measures 0.3 mm in diameter and contains no rod cells. As you move away from the center of the retina, the number of cone cells greatly decreases while conversely rod cells increase. With rod cells outnumbering cone cells by roughly 20:1, the majority of the retina is covered with rod cells. It is because of this that rod cells allow for greater peripheral vision and motion detection.

Opponent Color Theory
Ewald Hering was the father of the opponent color theory (or opponent processing theory). This theory states that there are certain pairs of colors one does not see together at the same place and at the same time. For example, one does not see reddish greens or yellowish blues. However, we do see yellowish greens, bluish reds, yellowish reds, and so on. Scientific studies have shown that this is caused by the way the mind interprets the responses of the rods and cones in the human eye. There is some overlap between the colors that the visual system can interpret. Those overlaps are not reported to the brain as the exact colors observed by the visual system but rather as a signal indicating the differences in wavelengths.

Hering also stated that there was a distinct pattern to the color of images after we see them. For example, if one looks at a unique color patch for a few seconds and then switches their gaze to a white area they will see a greenish patch in that white area.

Hering hypothesized that trichromatic signals from the cones fed into subsequent neural stages and exhibited two major opponent classes of processing. This theory was later supported by greater evidence from scientific studies.
 * 1) Spectrally opponent processes which were red vs. green and yellow vs. blue.
 * 2) Spectrally non-opponent processes which were black vs. white.

Psychologists Leo M. Hurvich and Dorothea Jameson published observations in the 1950s and 1960s that solidified the opponent processes theory as a viable theory of how we view color. The discovery of electrophysiological responses that emulated opponent processing also gave the theory more clout. Consequently, with the quantitative data provided by the psychophysics and direct neurophysiological responses provided by electrophysiology, opponent processing is no longer questioned.

Models of mechanism of color vision
We also use "color model" to indicate a model or mechanism of color vision for explaining how color signals are processed from visual cones to ganglion cells. For simplicity, we call these models color mechanism models. The classical color mechanism models are Young-Helmholtz's trichromatic model and Hering's opponent-process model. Though these two theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for color opponency receive signals from the three types of cones and process them at a more complex level.

Color Blindness
Main article: Color Blindness

Color blindness is a condition that causes some people to have difficulties discerning between different colors. This problem occurs when pigments in the nerve cells of the eye called cones malfunction. When this malfunction occurs in one pigment, it becomes difficult to tell the difference between two different colors, such as shades of red and green. If another pigment is missing or not working properly, the person could have problems viewing blue and yellow shades as well as red and green. This defect in the eye is passed down genetically and there is no known cure. Color Blindness Knowing this, the color models can be used in mapping, cartography, etc. to accommodate people with such difficulties, allowing them to easily understand the product.

Vertebrate evolution of color vision
Vertebrate animals were primitively tetrachromatic. They possessed short, mid, long wavelength cones, and ultraviolet sensitive cones. Today, fish, reptiles and birds are all tetrachromatic. Placental mammals lost both the short and mid wavelength cones. Thus, most mammals do not have complex color vision but they are sensitive to ultraviolet light. Human trichromatic color vision is a recent evolutionary novelty that first evolved in the common ancestor of the Old World Primates. Our trichromatic color vision evolved by duplication of the long wavelength sensitive opsin, found on the X chromosome. One of these copies evolved to be sensitive to green light and constitutes our mid wavelength opsin. At the same time, our short wavelength opsin evolved from the ultraviolet opsin of our vertebrate and mammalian ancestors.

Human red-green color blindness occurs because the two copies of the red and green opsin genes remain in close proximity on the X chromosome. Because of frequent recombination during meiosis, these gene pairs can get easily rearranged, creating versions of the genes that do not have distinct spectral sensitivities.