Wavelength is the physical property of light, whereas colour is a perceptual state, created by the brain (Isaac Newton, 1672). Colour is perceived by the visual system from the variations in the wavelengths of light that consists of electric and magnetic fields and can vary in luminance and intensity. The spectrum of light shows the light energy at each wavelength. Colour is processed together with information about luminance and visual form by the neural systems to create a unitary and robust representation of the visual world (Mollon, 1989). Colour is concerned with the reflectance spectrum of surfaces and less to do with lighting. Colour vision is particularly important for recovering object surface properties under variable illumination (Kelber, Vorobyev, & Osorio, 2003).

Colour vision helps the brain put things into context. An object can look similar under different lighting because of the information the brain stores from stimuli experiences. Colours don’t tend to change even when the lighting changes because contextual processing requires colour constancy. The visual system extracts invariance by setting information of an image into context (Toates, 2011). Colour constancy facilitates object perception and recognition and has an important role in scene segmentation and visual memory. Colour allows objects to be segmented from the background in a visual scene. Objects can be moved around and still identified because of their colour which does not change. The visual system is plastic with respect to being too able to extract information about spectral composition of a visual scene (Simunovic, 2010).

The perceived colour of an object depends on the mix of wavelengths that hit the eye when light is emitted or reflected from a surface or object within a scene. The lighting being reflected from the object into the eye remains the same unless the illumination changes. The brain is focused on the surfaces and objects and not necessarily the light reflected from them. If light is ambiguous, dramatic illusions of colour can exist as the visual system will decipher how to interpret the light. When lighting changes at a scene, the brain will work out the lighting by compensating for it, putting objects and surfaces into context using colour constancy.

The complex visual system is intricately storing information and extracting information to make sense of the world. It is the processing of such information that makes sense that the breast of a robin bird looks red because it is reflecting a large proportion of light of a particular wavelength, relative to the other objects that are simultaneously present, i.e. its wings (Zeki, 1993).

Through a process of adaptation, perceiving colour is not isolated to the wavelengths entering the eye. The cells adjust colour perception to many different aspects of the visual world and context. Colour appearance can be strongly affected by prior exposure to stimuli and the process of adaptation constantly adjusts visual sensitivity, demonstrated by afterimages such as when red sensitive cells that are being strongly stimulated become temporarily less sensitive, less able to signal the colour red, through a process of adaptation and instead an after image when the stimulus ends perceives the complementary colour of green.

Colour is encoded when the cornea and lens focus on objects and absorb light. The signals may be chromatic, with colour hues i.e. red, green, blue or achromatic, without colour hues, i.e. black or white. The signals are passed through to the retina and a network of cells to the sensory photoreceptors. The light stimulates the photoreceptors, passing the signal back through the retina through a number of neurons including bipolar and ganglion cells. The signals are passed through the optic nerve which sends the information to the brains visual cortex, resulting in perceived colour.

Colour is an aid, a useful tool that helps make sense of the world and perceiving objects and scenes less complicated. It can also be argued that colour adds complexity when considering object and scene perception. Objects that could be grouped together are not, because they have a difference in colour. Those with dichromatic vision or colour blindness in one eye can make a case for this, as they may benefit from both sides of this arguments.

Two types of photoreceptors exist, rods and cones. The rods are very sensitive, but they cannot detect fine detail. They are scotopic, working in dimmer light levels with a peaking sensitivity of approximately 500nm and do not contribute to colour vision. The cone photoreceptors are responsible for colour vision. Cones are photopic, managing day light and detect fine detail. There are three classes of cone photoreceptor, each sensitive to different wavelengths of light (Wolfe et al., 2015) .

The sensitivities of the cones are determined by the photopigment molecule. Each cone type has a different photopigment and chemicals that absorb different wavelengths and light energy. The S cones process ‘short’ wavelengths. The M cones process ‘medium’ wavelengths. The L cones process ‘long’ wavelengths. The human visual system can typically process a spectrum of light between wavelengths of ~400-700nm. The S cones have a peak sensitivity to wavelengths of ~430nm, M cones ~530nm and L cones ~560nm, with some overlap (Solomon & Lennie, 2007). The cones differ in numbers and retinal distribution, with only approximately 5% of S cones in total. S cones are typically missing from the fovea, the centre of the eye and subsequently less sensitive to longer wavelengths. The M and L cones are more similar in density and are therefore sensitive to all visible wavelengths.

Although not entirely accurate, each of the cones have a colour associated with them as they have each have a particular sensitivity to a range of wavelengths that encode certain colours. The S cone colloquially known as blue cone, the M cone colloquially known as green cone, the L cone colloquially known as red cone. Each class of cone plays a role in signalling colour. Lights that are physically different but perceptually identical are known as metamers.

The three cone photoreceptors represent the first stage of colour processing known as the trichromatic theory. Most humans and primates have normal trichromatic colour vision based on the sensitivity and responses of the three types of cone photoreceptor. First proposed by Thomas Young (1802), then Herman von Helmholtz (1857), then James Clerk Maxwell (1857) who further developed it.

Maxwell (1857) worked out colour-blindness and explained colour vision deficiency. Most humans and primates are trichromats with all three cone types. Humans with normal vision can distinguish between thousands of colours (Gegenfurtner, 2003). Many animals are dichromats and only have two cone types but are still described as being colour blind, albeit they can see some colour. Birds and fish have four cone types, so have a more complex colour vision. Studies suggest that colour vision has formed to adapt to the environment for the primate. For humans most forms of colour blindness are the cause of inherited defects in the L and M cones, which are the two genes located on the X chromosome. Change in the spectral sensitivity of one of the cone types is known as anomalous trichromacy. Humans and animals that cannot perceive colour at all, have achromatopsia vision. The cones are defective or parts of the brain cortex that processes colour is damaged. Colour blindness affects males than females because the photopigment genes are attached to the X chromosome, females have two X chromosomes so have double the chances to have all three cone types. Studies have suggested that colour blindness affects some ethnic groups more than others (Simunovic, 2010).

It is also clear from the studies of colour vision that humans can still survive without colour vision. Humans and primates with dichromatic vision or achromatic vision are still able to use other visual and sensory information to draw distinction between objects in a scene, such as identifying the edges of surfaces. The brain is plastic and will find ways to adjust when a deficiency exists.

The Hering’s opponent colour theory helped to further explain the second stage of colour processing. First proposed by Ewald Hering (1920), there are four colour hues that are perceptually unique: red, green, blue and yellow. They contain no mixture of any other hue. Hering’s opponent colour theory explained that the four unique hues exist as opponent colour pairs: the red-green pair and the blue-yellow pair. Herring paired these colours together after identifying that they were in opposition, as the activation of one of the colours inhibits the activity of the other in the pair. So, no hue would be reddish and greenish or blueish and yellow at the same time. The colours in the pairs are mutually exclusive. Any other combination of the colours such as red and blue or green and yellow is possible.

The colour opponent pairs were further extended to become colour opponent channels or colour systems, comparing the outputs of the three cones types, managed by the ganglion cells. The three opponent channels are: the red-green channel, the blue- yellow channel and the black-white (luminance) channel. Three opponent channels are said to exist as part of the visual systems way to identify the different wavelengths of light. Each cone on its own is colour blind. A single cone cannot distinguish a change in wavelength or intensity, creating a problem known as the ‘principle of univariance’.  The visual system resolves the problem by determining the wavelength by comparing the responses of all three cones. The ganglion cells find the difference between the responses of the different cone types. When intensity changes the visual system will process it and the colour will change as the responses from all three respond to the variations with light intensity and wavelength.  

Colour vision depends on the comparison of activity in different photoreceptors and of colour opponent neurons that compare the signals from the cones (Solomon & Lennie, 2007). Although trichromacy and opponency were once considered two conflicting theories, the two are different and each represent part of the visual process. Trichromacy demonstrates the requirement to have all three cones. Opponency reflects how the signals from the cones are compared by the ganglion cells to form different combinations. The corresponding combination will give the cells an excitatory response to the light for one region and an inhibitory response for the other (Wolfe et al., 2015).

The acceptance of the trichromatic theory and opponent colour channels has helped colour vision to evolve other the years. Knowledge of the photoreceptor sensitivities and models of the visual system have highlighted the benefits of colour vision making them more apparent and better explained.

Gegenfurtner, K. R. (2003). Cortical mechanisms of colour vision. Nature Reviews Neuroscience, 4(7), 563–572. https://doi.org/10.1038/nrn1138

Kelber, A., Vorobyev, M., & Osorio, D. (2003). Animal colour vision – Behavioural tests and physiological concepts. Biological Reviews of the Cambridge Philosophical Society, Vol. 78, pp. 81–118. https://doi.org/10.1017/S1464793102005985

Mollon, J. D. (1989). “THO’’ SHE KNEEL’D IN THAT PLACE WHERE THEY GREW…” THE USES AND ORIGINS OF PRIMATE COLOUR VISION.” In J. exp. Biol (Vol. 146).

Simunovic, M. P. (2010). Colour vision deficiency. Eye, Vol. 24, pp. 747–755. https://doi.org/10.1038/eye.2009.251

Solomon, S. G., & Lennie, P. (2007, April). The machinery of colour vision. Nature Reviews Neuroscience, Vol. 8, pp. 276–286. https://doi.org/10.1038/nrn2094

Toates, F. (2011). Biological psychology (3rd ed.). Biological Psychology (3rd Ed.).

Wolf, K. (2002). Visual ecology: Coloured fruit is what the eye sees best. Current Biology, Vol. 12. https://doi.org/10.1016/S0960-9822(02)00785-6

Wolfe, J. M., Kluender, K. R., Levi, D. M., Bartoshuk, L. M., Herz, R. S., Klatzky, R. L., & Lederman, S. J. (2015). Perceiving and Recognizing Objects. In Sensation and Perception.

Books:

Gegenfurtner, K. R. (2003). Cortical mechanisms of colour vision. Nature Reviews Neuroscience, 4(7), 563–572. https://doi.org/10.1038/nrn1138

Kelber, A., Vorobyev, M., & Osorio, D. (2003). Animal colour vision – Behavioural tests and physiological concepts. Biological Reviews of the Cambridge Philosophical Society, Vol. 78, pp. 81–118. https://doi.org/10.1017/S1464793102005985

Mollon, J. D. (1989). “THO’’ SHE KNEEL’D IN THAT PLACE WHERE THEY GREW…” THE USES AND ORIGINS OF PRIMATE COLOUR VISION.” In J. exp. Biol (Vol. 146).

Simunovic, M. P. (2010). Colour vision deficiency. Eye, Vol. 24, pp. 747–755. https://doi.org/10.1038/eye.2009.251

Solomon, S. G., & Lennie, P. (2007, April). The machinery of colour vision. Nature Reviews Neuroscience, Vol. 8, pp. 276–286. https://doi.org/10.1038/nrn2094

Toates, F. (2011). Biological psychology (3rd ed.). Biological Psychology (3rd Ed.).

Wolf, K. (2002). Visual ecology: Coloured fruit is what the eye sees best. Current Biology, Vol. 12. https://doi.org/10.1016/S0960-9822(02)00785-6

Wolfe, J. M., Kluender, K. R., Levi, D. M., Bartoshuk, L. M., Herz, R. S., Klatzky, R. L., & Lederman, S. J. (2015). Perceiving and Recognizing Objects. In Sensation and Perception.

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