Color perception

Color vision is the capacity of an organism or machine to distinguish objects based on the wavelength of the light they reflect or emit. A 'red' apple does not emit red light. Rather, it simply absorbs all the frequencies of light shining on it except the frequencies we call red, which are reflected. An apple is perceived to be red only because the human eye can distinguish between different wavelengths. Three things are needed to see color: a light source, a detector (e.g. the eye) and a sample to view.

In order for animals to respond accurately to their environments, their visual systems need to correctly interpret the form of objects around them. A major component of this is perception of colors.

Color perception
Perception of color is achieved in mammals through color receptors containing pigments with different spectral sensitivities. In most Old World primates there are three types of color receptors (known as cone cells). This confers trichromatic color vision, so these primates, like humans, are known as trichromats.

In the human eye, the cones are maximally receptive to short, medium, and long wavelengths of light and are therefore usually called S-, M-, and L-cones. L-cones are often referred to as the red receptor, but while the perception of red depends on this receptor, microspectrophotometry has shown that its peak sensitivity is in the yellow region of the spectrum.

Cone cells in the human eye
A particular frequency of light stimulates each of these receptor types to varying degrees. Yellow light, for example, stimulates L-cones strongly and M-cones to a moderate extent, but only stimulates S-cones weakly. Red light, on the other hand, stimulates almost exclusively L-cones, and blue light almost exclusively S-cones. The visual system combines the information from each type of receptor to give rise to different perceptions of different wavelengths of light.

The pigments present in the L- and M-cones are encoded on the X chromosome; defective encoding of these leads to the two most common forms of color blindness. The OPN1LW gene, which codes for the pigment that responds to red light, is highly polymorphic (a recent study by Verrelli and Tishkoff, 2004, found 85 variants in a sample of 236 men), so it is possible for a woman to have an extra type of color receptor, and thus a degree of tetrachromatic color vision. Variations in OPN1MW, which codes for the green pigment, appear to be rare, and the observed variants have no effect on spectral sensitivity.

Color processing occurs at a very early level in the visual system (even within the retina) through initial color opponent mechanisms. Opponent mechanisms refer to the opposing color effect of red-green, blue-yellow, and light-dark. Visual information is then sent back via the optic nerve to the optic chiasm: a point where the two optic nerves meet and information from the temporal (contralateral) visual field crosses to the other side of the brain. After the optic chiasm the visual fiber tracts are referred to as the optic tracts, which enter the thalamus to synapse at the lateral geniculate nucleus (LGN). The LGN is segregated into six layers: two magnocellular (large cell) achromatic layers (M cells) and four parvocellular (small cell) chromatic layers (P cells). Within the LGN P-cell layers there are two chromatic opponent types: red vs. green and blue vs. green/red.

After synapsing at the LGN, the visual tract continues on back toward the primary visual cortex (V1) located at the back of the brain within the occipital lobe. It is at this stage that color processing becomes much more complicated. Here the three-color segregation begins to break down, with cells responding specifically to its own, finely-tuned frequency and intensity stimulus. Within V1 there is a distinct banding (striation) of neurons into vertical columns that respond to specific visual stimulus orientations, colors, and visual field. Due to this banding, V1 is also referred to as "striate cortex", with any other cortical visual region referred to collectively as "extrastriate cortex". This columnar organization is ordered in groups to form hemifield- and orientation- segregated "hypercolumns". Each hypercolumn is a patch of cortex neurons that responds to stimuli from one visual hemifield and a full 0-180 degree orientation. Each hypercolumn also has a color representation field with a rough pinwheel shape, with the center of the pinwheel located near the center of the hypercolumn. This pinwheel represents a gradient of neurons that each represent different colors, with neighboring neurons responding to similar colors. This pinwheel organization is referred to as "chromatic blobs".

From V1, color information is sent to V2 in rough strips that represent the blobs from V1. V2 then synapses onto V4, the first color visual analysis region. From V4 color information is then sent to the inferior temporal lobe which is thought to integrate color information with shape and form. This pathway (V1 > V2 > V4 > inferior temporal) is known as the ventral stream or the "what pathway". This is separate from the dorsal stream ("where pathway") that is thought to analyze motion.

Other animals enjoying three, four or even five color vision systems include tropical fish and birds. In the latter case tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift the spectral sensitivity of the cell. (Some species of bird such as the pigeon in fact possess five distinct types of droplet and may thus be pentachromats.) Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromatic color vision; marine mammals have only a single cone type and are thus monochromats.

Color perception mechanisms are highly dependent on evolutionary factors, of which the most prominent is satisfactory recognition of food sources. In herbivorous primates, color perception is essential for finding proper (mature) leaves. In hummingbirds, particular flower types are often recognized by color as well. On the other hand, nocturnal mammals have less-developed color vision, since adequate light is needed for cones to function properly. There is evidence that ultraviolet light plays a part in color perception in many branches of the animal kingdom.

Chromatic adaptation
A given object may be viewed under various conditions. For example, it may illuminated by the sunlight, the light of a fire, or a harsh electric light. In all of these situations, the visual system indicates that the object has the same color: an apple always appears red, whether viewed at night or during the day. This feature of the visual system is called chromatic adaptation. Though this is generally true there are situations where the apparent brightness of a stimulus will appear reversed relative to its "background" when viewed at night. The petals of yellow flowers will appear dim compared to the green leaves. The opposite is true during the day. This is known as the Purkinje effect.

Chromatic adaptation is one of the more easily fooled aspects of vision, and is prone to some of the most spectacular optical illusions. This ability to maintain homeostatis of perception under considerable distortion may suggest support for a holographic model of information processing and storage.