Colors are our response to visual stimuli via complex processes that occur in our perceptual apparatus; this includes how our eyes register colors, classify the inputs and signal them to the brain, and the way the brain decodes the signals. Much of what we know about how we see colors at this time is only theory.
How the Human Eye Works
The cornea is the transparent outer covering of our eyeballs - light enters through here. Iris muscles regulate the amount of light let in by expanding or contracting. Three kinds of refractors focus light on the back surface of the eye; the aqueous humor, crystalline lens, and the vitreous humor refractor. On the back surface of the eye is the retina, which consists of many specialized cells arranged in layers. The light goes through layers of nerve cells before reaching the rods and cones, which are the layers most important to color perception, consisting of photoreceptors named for their shapes. Rods allow us to see forms in very dim light, but only in black and white, and function in light as well as darkness; cones are what allow us to see hues. However, only about 20% of the light that even reaches the retina is registered in the photoreceptive rods and cones; the rest we do not see. At the center back of the eye is an area about 1 mm in diameter called the forea. Light falling into this area gives the sharpest color definition because the forea contains only cones. When we look for details in images, we instinctively adjust ourselves so our focal point is centered on the forea. We only see about 2 degrees of our 360 degree vision with supreme accuracy. Each eye has approximately 100 million rods and 6 million cones, which communicated with the brain via the optic nerve. Electrochemical messages are sent by photoreceptors via the optic nerve across synapses within a complex network of optic nerve fibers. Signals from rods and cones are though to be gathered and passed on via bipolar and ganglion cells. Activities from the retina seem to be integrated by the horizontal and inner associative cells. Each cone is connected to one bipolar cell and one ganglion cell in the forea. On other parts of the retina, rods and cones are bundled together instead. Information from both eyes is transmitted from them to various areas on both sides of the brain, where the information is integrated to form a single image. Almost a third of the gray matter of the cerebral cortex is involved with this process.
Seeing Colors
Scientists have no definite proof of how cones work, but they know about rods. rods contain disks of a light-sensitive pigment called visual purple, or rhodopsin. When light strikes the visual purple, it bleaches it, which reduces the electrical signals for darkness that would otherwise be transmitted by the rods. Thus in the dark, rods have very large amounts of unbleached visual purple and we can still see forms with very little light present. Rods can function at light levels up to a thousand times weaker than cones. As for cones, English physicist Thomas Young and German physicist Hermann von Helmholtz both advanced and worked on a theory called Trichromatic Theory, which goes like this : Cones contain light-sensitive pigments called iodopsins, but the functions of these pigments is still relatively unknown. What is believed is that there are three different kinds of cone pigments - one for sensing long wavelengths (the red range), medium wavelengths (the green range), and one for short wavelengths (the blue-violet range). It is thought that these primary levels mix to form color. Red and green sensitive cones predominate the retina; there are few blue-violet sensitive cones. However, this theory only explains what happens in the photoreceptors under direct light stimulation. Electrochemical signals transferred from these receptors to the brain are handled somewhat differently and the process may have more than two stages, including a stage where colors may be discerned in pairs of opposing colors. This is called Opponent Theory - a response mechanism such as specialized cells in the visual cortex registers either green or red, or blue-violet or yellow. Only one kind of signal can be carried in each color pair at a time, while the other's signal remains inhibited. Another set of cells operating respond to variations between white and black and are thought to operate in a non-opponent fashion, yielding a wide range of hues. This theory has been supported by electrical analysis of nerve cells in various animals and helps explain why we don't perceive reddish green or blueish yellow.
After Images
Opponent Theory also explains after image effects, also known as successive contrast. After image effects are visual sensations of opposite colors that occur briefly after one color stimulus is removed. A theory for this is that when signalling for one color gets fatigued, it's opponent color is no longer inhibited.
Color Constancy
Color constancy, described by E. H. Land (developer of the Polaroid camera), is the phenomenon by which colors subjectively seem to remain the same under differing kinds of illumination. E. H. Land's Polaroid camera seemed to depict all colors using only red and green, and its system could also depict all colors using only red and a yellow. In addition to this is Retinex (retina + cortex) Theory, wherein a network of tiny peg-shaped blobs in the visual cortex seem to compare visual information from an object with it's immediate surroundings so that colors in the immediate vicinity seem more important than colors seen at a distance.
Variables in Color Perception
Color sensations occur as responses of our perceptual apparatus. Colors are not necessarily inherent properties in objects. Few animals other than humans seem to see the world through trichromatic color vision like we do.
Genetic and Cultural Differences
Minimal genetic differences between two people in terms of a particular amino acid can effect their cone cells and the way they see color. Color blindedness is mostly found in men and is only diagnosed if there are major variations from the norm. Approximately 7% of men cannot distinguish between red and green.
Design Factors
A large expanse of color will appear much brighter than a very small area of the same color. If a color field is far away, it will dull and lose the sharpness of it's edges. In some cases, a smaller area of color will appear darker than a larger area. This effect depends heavily on the color of the background. All perceptions of color is effected by the colors surrounding it; adjacent colors can change each other's apparent hues. An entire art movement was dedicated to these studies of color interactions, called the Op Art Movement.
Lighting
Fluorescent lighting leaves a blue cast, although there are now expensive "full spectrum" flourescent lights that are color corrected to approximate sunlight. Incandescent lights leave a reddish-yellow cast, redder than that of the natural midday sun. Sunlight's cast continually varies in color - in the morning it should appear blue to white and then red in the later afternoon. Our perception of this phenomenon is effected by other factors, however. Visual memory is what causes us to perceive colors as unchanging. In museums, strong lights may make colors appear brighter. Walls are being opened in museums across the globe to let more natural lighting in.
Surface Qualities
In the case of pigments, color perception depends on the characteristics of the surface from which it is reflected. A 3d surface will not reflect light uniformly - reflections from it's outermost contours will be strongest, causing this contours to appear very light. A rough or porous surface will reflect light in a diffused manner and without the extreme highlights of a glossy surface. Different materials will always have different reflective qualities.
Light may be reflected in many ways. Refraction is when a ray of light is bent (refracted) and then transmitted onward the same way it was going. Transparent surfaces may reflect some light, depending on surrounding and the angle at which it is seen. They also transmit light. If the surface is shiny, the light will be reflected, but not transmitted through. Translucent and semi-opaque surfaces transmit only a little light, some is reflected, and some is absorbed. In painting, is a translucent layer of pigment and medium is spread over a white ground, some light will be reflected back upward throughout the paint, which gives brilliance and luminosity to it's colors. The more opaque the paint, the less this effect occurs. A dark ground beneath a layer of translucent paint absorbs light rays so there is no appearance of brilliance or luminosity.
Emotions
Emotional states are likely to influence color perception. Depressed people see dimmer and darker colors and when we don't like something we see, our pupils narrow, allowing in less light. However, our pupils will widen when we look at something we do like, making it appear brighter.
Nonvisual Color Perception
The ability to feel colors is not uncommon in blind people and some people perceive colors when hearing sounds. This is so common that a body of research exists surrounding this phenomenon, called synthesia. Colors associated with sounds are subjective and change from person to person. However, rising pitch and faster tempos are often associated with lighter pitches, and sombre passages tend to be associated with dark colors. Females are much more likely to experience this than men. Synthesia has been known of for 300 years, but only studied seriously by scientists within the past two decades. fMRI scans and the internet have both greatly contributed to it's study. This ability is often associated with people possessing the ability of Absolute Pitch (knowing what note any sound is without any external reference). There are both tone-color and tone-space synthesias. Tone-color synthesia (TCS) elicits a color perception where as in tone-space synthesia (TSS) musical notes are organized explicitly in a well-defined spacial array. It is believed (and has been studied to be shown very true) that Absolute Pitch modulates the effects of Tone-Color Synthesia, but not Tone-Space Synthesia. Vincent Van Gosh was thrown out of piano lessons for insisting the musical notes had colors to them, and often describe colors as having personalities (another form of Synthesia), and also mentioned that the colors he painted sometimes seemed to have musical sound for him. There is great debate over whether Synthesia requires consciousness and many books published on Synthesia have the word "blue" in the title. (http://synesthesia.info/). About one in twenty-three people have some type of Synthesia, but people differ in the intensity of their perceptions. The most common type of Synthesia is colored weekdays, perceiving colored letter and numbers is most studied by scientists, and the perception of sound and color is statistically most studied by artists. Synthesia in accordance with the neuroscientific definition occurs in about 4% of the population. Our notion of common sense is derived from Ancient Greek Aristotelian thought on the unity of experience (synthesia). Johann Wolfgang von Goethe considere synthesia-like qualities an autonomous creative force of the human imagination. Unconscious body experience is essentially synesthetic according to philosopher Maurice Merleau-Ponty, and all sensory impressions correspond to each other on a preconscious level. Most people seem to be unaware of their synesthetic potential only because they unobservant of it - many synesthetics only became truly aware of themselves as synesthetics after reading an article which put a label on their abilities. (http://web.archive.org/web/20110628222645/http://www.pucsp.br/pos/tidd/teccogs/artigos/pdf/teccogs_edicao1_2009_artigo_CAMPEN.pdf)
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