Theories of Colour Vision

Theories of chroma sightKishan LakhaniHow does garble mass work?It is difficult to intend a world without burnish perception as it is constantly in action by a whole spectrum of living organisms and for a domain of purposes, it not still allows us to detect objects that might otherwise be obscu loss by their surroundings it also helps us to recognize and identify things we fag nab easily (Goldstein, 2000, p.203), thus making it an essential component of passel. discolourise dictates survival in many environments the artic fox boasts a white pepper allowing effective camouflage over its prey and a signifi skunkt cipher in its ability to hunt (Sekuler Blake, 2006). I will explore the mechanisms that be said to explain colour vision at the photo receptor level and beyond, entirely first of all we must understand what colour actually is.The electromagnetic spectrum ranges from cosmic rays to radio waves, as wavelength increases. Between UV and Infr ard lies a strip, g laring flow, this is what we argon interested in with regard to colour (Snowden et al., 2006). Colour arises when get out rays from this conspicuous spectrum (390nm-750nm) atomic number 18 reflected off objects and into our brass. Differing wavelengths of clear-cut from this spectrum subsequently pin down the colour comprehend, as short wavelengths arouse the colour violet (350nm), moderate and long wavelengths produce chiliad and red respectively. Sir Isaac Newton famously wrote The rays to babble out properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a signified of this or that colour (Sekuler Blake, 2006, p.236) So colour doesnt really survive in the physical world at all, it is our own psychology that creates the model of colour. Semi Zeki (1983) refined Newtons words further adage that colour vision is a property of the brain, not the world outside.( Sekuler Blake, 2006, p.236) accordingly we tramp core group up that colour vision lies in the eyes and brain and not in the physics of baseless itself (Anderson 2012). So to reaction the question How does colour vision work we must explore the opthalmic systems of the organisms further.Figure 1 The Electromagnetic spectrum (2012), Diagram showing the visible spectrum indoors the electromagnetic spectrumPhotoreceptors are diminish slender cells found at the sustain of the eye in the retina. They contain visual hues that absorb photons of light and switch this light energy into chemical energy, this process is called phototransduction. Within the pigment is a protein which determines the wavelength of light absorbed by the pigment and also a chromophore which is liable for absorbing photons of light. (Wolfe et al., 2009) on that point are 2 types of photoreceptors, rods and strobilus shapes. Cones are mainly arduous in the fovea in the centre of the retina whereas Rods are completely absent in the fovea and more prevalen t in the periphery of the retina (Snowdon et al,. 2006). Rods are adapted so they lowlife operate in low light levels due to spatial pooling allowing scoptic vision, whilst sacrificing visual acuity. They contain just one pigment rhodopsin denying colour vision due to the univariance rationale. Cones however contain 3 photo pigments, which are sensitive to various wavelengths of light and apprize wherefore let us see colour.In comparison to the physical commentary of light, colour is much easier to describe as it is experienced psychologically not physically such is the nature of light. It can be specified by just iii values hue, saturation and lightness.(Palmer, 1999) This is genuinely important with indite to colour vision as it implies that many different lights will produce the akin colour experience (Palmer, 1999). The trine perceptual dimensions of colour can be summarised in what is known as the colour spindle. Hue is referred to as the chromatic aspect of light ( Wolfe et al., 2009) and is dictated by its wavelength. Saturation corresponds honour and how vivid the colour is (Palmer 1999), and brightness involves the intensity of the colour (physically).The Young/Helmhotz colored conjecture of colour vision works on the photoreceptor level, and ultimately was bourgeon through the results of Helmhotzs colour containing try. Observers were allowed to vary the intensities of 3 immemorial lights and mix them (in a comparison field) to match the colour of a individual wavelength in a test field. They had to find a psychological match between the mixture of primary lights and the test light, simply by variable the intensities of the primary lights. This match is known as a metameric one, as the light in the comparison field is physically different stock-still psychologically identical to that in the test field. Results showed that by varying intensities in the comparison field, the observer could find a metameric match employ just three primary lights. Dichromatic observers were unable to find matches for individually colour in the test field. In summary, with 3 primaries, you can compass any combination of responses in the 3 conoid types, so you can match the appearance of any test light. (Anderson 2012) It is because clear to us that colour vision heavily relies on three different receptor mechanisms or retinal conoid photopigments, each with different apparitional sensitivities (Goldstein, 2010). This is the basis of the tricolor theory I will now elaborate on.Monochromats possess only one type of pigment in their cone shapes. In this case, the ability to see colour is not possible. The same chain of events is initiated in the visual receptor despite there being a variety of wavelengths in the light absorbed by the pigment. The receptors response conveys information about how much light has been absorbed, but this response provides no information about the wavelength of this absorbed light. (Sekuler Bl ake, 2006) It is therefore impossible to discriminate wavelength when there is only one photopignment, which has uniform spectral sensitivity. The response could cod altered due to a change in wavelength or light intensity, and monochromats are none the wiser this is known as the principle of univariance. This situation is not unique to monochromats, as in low light levels rods are the only photoreceptors in action. They have one photopigment, rhodopsin resulting in the same dilemma. Consequently we disregard the wavelength information and see an image that appears in shades of grey (Snowden et al., 2006) explaining why we cant monochromats cant see colour and why none of us have colour vision in low light levels.Dichromats possess two photopigments, which is truly useful in terms of colour vision. The two pigment types have different absorption spectra, extracting some usable wavelength information about light (Sekuler Blake, 2006) It is now possible to separate and disentangl e wavelength and intensity, allowing colour to be visible to a certain extent. Certain wavelengths are confused and constitute failures of secretion (Sekuler Blake, 2006). A key reason that pass ons to the idea humans arent dichromats revolves roughly what is known as the neutral acme. All dichromats possess this neutral point in which a oneness wavelength is always confused, and the existence of a single neutral point is the hallmark of a two-pigment eye. (Sekuler Blake, 2006, p.249) As humans do not show any traits of having the neutral point, there is a unshakable belief that humans have more than two cone photopigments.Trichromats arse up three cone photopigments, enabling total discrimination of wavelengths throughout the visible spectrum. Trichromacy also ties in with Helmhotzs colour matching experiment, indicating three not two photoreceptors are required for complete colour vision across the entire visible spectrum. The three pigments are most sensitive to light o f a particular wavelength approximately 430, 530 and 560 nanometres respectively. (Sekuler Blake, 2006) Figure 4 shows the each cone pigment absorbs a very wide range of wavelengths. So between 400nm and 650nm there are at least two types of cone photopigments absorbing light. In the region of 475nm, all three types of pigments are bear upon and stimulated. (Sekuler Blake, 2006) Hence we can conclude that the absorbance range is tremendously increased with three photopigments, and as light is reflected onto the retina every wavelength of light in the visible spectrum can be perceived in the form of colours by our brain. colorful theory may explain how the existence of just three cone photopigments allows colour matching for any wavelength of light in the visible spectrum, victimization just three primary lights. However, it still leaves many unanswered questions when it comes to having a full understanding of how colour vision works as Hering highlighted. prejudicious afterima ges, the visibility of four psychologically pure hues (blue, red, kilobyte and xanthous) and the absence of antonymous hues such as blueish yellow all indicate that colorful theory alone is inadequate in explaining how colour vision works.Hering make an important discovery with regard to afterimages. If we stare at the black decimal point in Figure 5 for rough 30 seconds, and then look at a blank piece of paper we notice the colour of each square changes. The potassium changes to red and red to atomic number 19, whilst the blue changes to yellow and the yellow to blue. Based on results like these, Hering proposed the concept that red and green are paired and blue and yellow are likewise paired. (Goldstein, 2010) An experiment where observers were shown patches of colour, and then asked to estimate the ratios of blue, green, red and yellow from each patch they received. Results showed that observers very rarely saw blue and yellow, or red and green together. (Abraham Gordon , 1994 cited in Goldstein, 2010) Sekuler and Blake (2006, p.255) also support this view that these complementary hues do not coexist, as an object never appears both blue and yellow at the same time. Hurvich and Jamesons hue cancellation experiment strengthens the case further, as any inflammation was eliminated when a green light was added to the red light. Hering also observed that those who are colour blind to red, are also colour blind to green which ultimately led him to declare the opponent-process theory of colour vision. (Goldstein, 2010) So we can deduce from this evidence that blue is paired with yellow and red with green the basis of the opponent-process theory.The opponent-process theory of colour vision follows the tricolor theory, rather than switch it, the two work hand in hand to explain how colour vision works. In fact it is the responses from the cones of the retina that form the basis of the opponent channels. (Anderson 2012) There are three opponent channels, t wo chromatic and one charcoal, and are formed by combining the responses from the three cone types. (Sekuler Blake, 2006) Figure 6 displays the red-green chromatic channel is comprised from the produces of the M and L cones. It is also known as the M L channel, as it signals the difference between the outputs of the M cones and of the L cones. (Sekuler Blake, 2006) The second chromatic channel is the blue-yellow channel, and it represents the difference between the S cone outputs and the sum of the M and L cone outputs. (Sekuler Blake, 2006) It is therefore also referred to as the S (M + L) channel. The achromatic channel is known as the luminance channel, and combines the output of the M and L cones so we can also label it the M + L channel. The activity in this luminance channel hinges on the sum of excitation of both M and L cones (Sekuler Blake, 2006). This addition can lead determine an objects visibility, The shape of the photopic sensitivity curve (closely think to v isibility) can be predicted by taking a sum of M and L cone responses. (Smith and Porkorny, 1975 cited in Werner et al., 1984).Russel DeValois was responsible for the finding of opponent neurons in the retina and lateral geniculate nucleus (LGN), which could provide physiological evidence to back up Herings propositions. (Goldstein, 2010) The LGN is the station responsible for receiving input from the retina and transmitting it to the visual cortex. Devalois conducted experiments on LGN cells of monkeys (who have the same trichromatic vision as ours), and ascertained opponent cells which behaved as if subtracting outputs from different cones and also nonopponent cells which behaved as if adding outputs from different cones. Devalois ascertained opponent cells reproduced an ON or glowering response determined by the wavelength of light. (Sekuler Blake, 2006) This can explain the first chromatic channel Hering proposed (M L) channel. Long wavelength cone excitation results in a po sitive or ON response, whilst intermediate wavelength cone excitation results in a negative or tally response. Hence if the net response is positive then a red colour is visualised (long wavelength of light), and similarly a blue colour is perceived if the net response is negative. This supports Herings initial observation that the hues red and green cannot coexist. Opponent cells were also responsible in explaining the S (M + L) channel. Short wavelength cone excitation results in a positive or ON response, whilst wavelengths around 580nm (M+L) cone excitation results in a negative or OFF response. Further findings included the fact that nonopponent ON cells produced ON responses for every wavelength, although some wavelengths produced stronger responses than others and OFF cells produced OFF responses for every wavelength again with varying strengths. It is these nonopponent cells which form the achromatic channel outlined by Hering. (Sekuler Blake, 2006)All in all, colour vis ion begins at the photoreceptor level as explained by trichromatic theory. The outputs of the three cone photopigments have been redistributed into the achromatic and chromatic channels at the LGN, as trichromacy progresses to opponent-process theory. Palmer (1999) concludes by describing the dual process theory in which the products from the trichromatic stage are used as the inputs for the secondary opponent-process stage. As we casualty from the LGN, further into the visual system, the information is perceived by the visual cortex of the brain facilitating us with colour vision.BibliographyAnderson, S (2012). Colour vision, Vision and visual perception, Optometry. Aston UniversityDimitri Poumidis, (2008), Spectral Sensetivities ONLINE. Available at http//www.gravurexchange.com/gravurezine/0805-ezine/ploumidis.htm Accessed 25 January 13.Goldstein, E. B. (2010). Sensation and perception (8th ed.) Chapter 9. Wadsworth Cengage larnJoshua Stevens, Jennifer M. Smith, and Raechel A. B ianchetti , (2012), The Electromagnetic Spectrum ONLINE. Available at https//www.e-education.psu.edu/geog160/node/1958 Accessed 03 January 13.Marc green, (2004), Opponent process theory ONLINE. Available at http//www.visualexpert.com/FAQ/Part1/cfaqPart1.html Accessed 09 February 13.Paul Schils , (2012), Chromatic adaptation ONLINE. Available at http//www.color-theory-phenomena.nl/12.00.htm Accessed 08 February 13.Palmer, S. E. 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