Color vision

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Color vision is the capacity of an organism or machine to distinguish objects based on the wavelength (or frequency) 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.Image:Psychophysical.jpg

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.

Contents 1 Color perception 1.1 Cone cells in the human eye 2 Chromatic adaptation 3 An Empirical Explanation: Color Contrast and Constancy 4 References 5 See also 6 External links

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

Cone type	Name	Range	Peak sensitivity
S	β (Blue)	400..500 nm	440 nm
M	γ (Green)	450..630 nm	544 nm
L	ρ (Red)	500..700 nm	580 nm

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

An 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.

An Empirical Explanation: Color Contrast and Constancy

Evidently the colors we see are, like brightness, linked to the stimuli that generate them by the historical success and/or failure of the interactions of human observers and their predecessors with objects and illuminants (sources) in the world.

Given that the otherwise puzzling aspects of the sensation of light intensity (lightness/brightness) shown in DEMONSTRATIONS #02-09 [1] can be understood in terms of a wholly empirical conception of how the visual system generates percepts, it is only logical to ask whether the color sensations elicited by different spectra arise according to the same scheme. After all, the spectral qualities of a stimulus are ambiguous for exactly the same reasons as is spectral intensity, to wit the conflation of illumination, reflectance and transmittance in the spectral return (Figure 1)[2].Indeed, if the empirical theory outlined for the perception of luminance has merit, it should apply not only to color, but to all categories of visual sensation.

A useful starting point in any exploration of the genesis of color on an empirical basis is simultaneous color contrast, a phenomenon that shares many similarities with simultaneous brightness contrast. The standard stimulus for eliciting color contrast is two targets with the same spectral composition on differently chromatic backgrounds. As in DEMONSTRATIONS #02-09 [3], the two identical targets look different as a result of contextual differences; in the case of DEMONSTRATIONS #10,11,12,13 in SEE FOR YOURSELF [4], however, the perceptual distinctions are based on differences in the apparent hue and saturation of the targets rather than lightness/brightness alone (color sensations are generally described in terms of hue, saturation and brightness, all three of which are appreciably different in comparing the two targets).

The percepts elicited by the standard color contrast stimulus in DEMONSTRATION #10 [5] and similar stimuli in which the same spectral targets elicit different color sensations are usually ascribed to 'adaptation' of the color system to the average spectral content of the overall stimulus (typically at the input stages), and/or to computations of spectral ratios across chromatic contrast boundaries (e.g. Land, 1986) (see DEMONSTRATION #16[6]. Both these hypotheses, however, fail to account fully for all the perceptual consequences of such stimuli. Moreover, they are really mathematical descriptions rather than explanations, and provide only a limited biological rationale for color contrast (the truism usually provided is that it makes sense to see an object as having more or less the same color in different illuminants, and that color contrast anomalies are the price that must be paid for the supposed benefit of 'color constancy').

An explanation of color contrast (and constancy) can, however, be given in fully empirical terms. The sources of target and surround in the standard color contrast stimuli shown in DEMONSTRATION #09 [7] are, as all visual stimuli, profoundly ambiguous: much like the achromatic targets in DEMONSTRATIONS #02-05 [8], the same spectral patterns could have been generated by many combinations of reflectances, conditions of illumination and influences of transmittance (Figure 2)[9]. The visual system must nevertheless generate appropriate behavioral responses to the enormous variety of the spectral patterns returned to the eye. Accordingly, the visual system appears to solve this problem by using feedback from the success or failure of these responses to progressively instantiate patterns of neural connectivity that promote ever more appropriate reactions to the stimuli. In this phylogenetic and ontogenetic process, the neuronal activity elicited by spectral stimuli comes to link spectral profiles of inevitably uncertain provenance with what they typically turned out to be (i.e., with their empirical significance). In this scheme, then, the particular pattern of neuronal activity elicited in response to a given stimulus is ultimately dictated by the relative frequencies of occurrence of the real-world combinations of reflectances, illuminants and transmittances that have given rise to the spectral stimulus in the past. If perceptions of color are indeed generated in this wholly empirical way, then the same spectral target on two differently chromatic backgrounds should give rise to different chromatic sensations. The reason is that, in addition to requiring behaviors appropriate to the same reflectances in the same illuminant (i.e., the stimulus on the screen), such stimuli will in other instances have required behaviors appropriate to targets that arise from different reflectances in different illuminants. Consequently, the pattern of spectral returns elicits a pattern of neuronal activity that incorporates these possible underlying sources in proportion to their past occurrence in human experience with spectral stimuli. As in the case of brightness, these ideas have been confirmed by analysis of natural scenes statistics.

Evidently the colors we see are, like brightness, linked to the stimuli that generate them by the historical success and/or failure of the interactions of human observers and their predecessors with objects and illuminants (sources) in the world.

References

• Kandel E, Schwartz J, Jessel T. Principles of Neural Science. 4th ed. New York: McGraw-Hill; 2000. ISBN 0838577016
• Nolte J. The Human Brain: An Introduction to Its Functional Anatomy. 5th ed. St. Louis: Mosby, Inc.; 2002. ISBN 0323013201
• Verrelli, BC; Tishkoff, S (2004). "Color vision molecular variation." American Journal of Human Genetics. 75 (3), 363-375.
• Martin, Paul R (1998). "Colour processing in the primate retina: recent progress." Journal of Physiology. 513 (3), 631-638.
• Rowe, Michael H (2002). "Trichromatic color vision in primates." News in Physiological Sciences. 17 (3), 93-98.
• Land EH (1986) Recent advances in retinex theory. Vision Res 26:7-21.
• Long F, Purves D (2003) Natural scence statistics as the universal basis for color context effects. Proc Natl Acad Sci 100 (25): 15190-15193.
• Lotto RB, Purves D (2004) Perceiving color. Rev Prog Coloration 34:12-25.
• Lotto RB, Purves D (2002) The empirical basis of color perception. Consciousness and Cognition 11:609-629.
• Lotto RB, Purves D (2002) A rationale for the structure of color space. Trends Neurosci 2:84-88.
• Lotto RB, Purves D (2000) An empirical explanation of color contrast. Proc Natl Acad Sci 97:12834-12839.
• Lotto RB, Purves D (1999) The effects of color on brightness. Nature Neurosci 2:1010-1014.
• Purves D, Lotto B (2002) Why We See What We Do: An Empirical Theory of Vision. Sunderland, MA: Sinauer Associates.

See also

• Mantis shrimps are at least octachromats
• Simultaneous brightness contrast
• Visual Perception

External links

• "Evidence that men, women literally see the world differently: Study shows color vision may have been adaptive during evolution."
• Colour vision.
• Spectral Sensitivity of the Eye.
• Vision may not be what we thought.
• Overview of color vision.

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• Dale Purves Lab[10]