What influences perception of color?
How do we perceive colour? The first major theory of how we perceive colour, after Newton's discovery of the spectrum of colours making up white light, was put forward by Young (1802) and later Helmholtz (1867), who proposed that we have a trichromatic mechanism of colour vision. They proposed that every colour can be realised by a combination of three primary colours: Red, blue and green (RBG). Trichromacy is based on the property of colour to be able to be mixed. Mixing light is additive, when lights are mixed, new colours are produced, for example mixing green and red light produced a yellow light. Wald and Brown (1965) later supported this theory with the identification of three types of photopigments in retinal cones. The different photopigments' relative absorption rates of photons of light, favoured wavelengths corresponding to those needed to perceive the three colours, short (S), Medium (M) and Long (L). At long wavelengths, we typically perceive a red colour, medium wavelengths give a more green colour and at short wavelengths, blue. The opposing theory for over a century, was that proposed by Hering, (1878) focused on the complementary nature of certain colour combinations. He also identified yellow as a fourth primary colour alongside RBG. Whereas Helmholtz and colleagues focused on the input of sensory information, having an impact on how we perceive colour (bottom-up approach), the tetrachromatic view was that colour perception is a top down approach, where brain areas, such as the visual cortex and the basal ganglia, interact with the sensory information to inform perception. The existence of a trichromatic system, rather than a dichromatic system with cones just absorbing M and L wavelengths, or a monochromatic system with a single receptor, is a fairly new addition in evolutionary terms. Many animals see with dichromatic or monochromatic vision, which gives them a shorter spectrum of wavelengths, and the ability to see fewer colours, or in the case of monochromatic vision, only grey-scale. This is explained by the chemical properties of rod, which are more sensitive in dark environments. At high light intensities, rods are deactivated through the isomeration of rhodopsin, this results in the chemical splitting of the rhodopsin molecule, leading to the reduction of the inhibitory neurotransmitter glutamate at the synapses of rods and cones and in turn, increases the firing rate of amacrine cells. Therefore, in dim light, more rod cells are active, making it easier to see in dim light, in the transition between bright light and dark, there is a shift towards equilibrium to increase the amount of active rhodopsin. In a dichromatic system, colours are perceived due to the ratio of M and L wavelengths; S wavelengths, were a later addition following a mutation in cone cells, the fovea does not contain any S cells, with little impact on colour vision, suggesting that M and L cells are more important. For example, cone activations to red involve a higher absorption rate of L wavelengths than M wavelengths, whereas cone activations to green are the reverse, lower absorption of L than M wavelengths. Combined, these two absorptions equal the same as the single absorption rate in both L and M wavelengths which corresponds to seeing a yellow colour. This explains the additive property of green and red to produce yellow. However, in a dichromatic system, it is difficult then, to establish whether the same output (seeing a yellow light) is achieved due to adding the wavelengths of red and green light or a single wavelength combination to produce pure yellow light. This problem can be resolved when we add in S cone cells to provide a wider absorption spectrum. In an attempt to provide a model for the neural basis of colour perception, Yoder (2003), outlined a Relative Absorption Model (RAM). This model proposed that photoreceptors absorb S, M and L wavelengths in a certain ratio to create colours. Both excitatory and inhibitory inputs are received from the three classes of wavelength. In the second stage, the combination of inputs correlate with each of the colour cells, red, green, blue and yellow, and these identify which receptor type has the greatest absorption of photons and which has the least. Their response intensities correspond to the differences between those absorptions and the middle absorption. The trichromacy of colour has been established at the sensory level, however it is unable to account for complementary colours, and this is illustrated by after effects of colour. For instance, when we stare at a plain yellow square on a blue background for around 30 seconds, and then immediately look at the same yellow square on a white background, the second yellow square seems to display a blue tint. In addition, one can imagine a greenish-yellow, or a bluish-red but not a greenish-red. The perception of colour is a result of our brains ability to differentiate between the wavelengths of light that enters our eyes. when light hits an object changes are caused in the wave length of the light that bounces of it, our brain decodes this into colours.
