Deficiencies of colour may result from congenital or acquired factors e.g. diseases/toxins (Hurvich, 1981). Some individuals (dichromats) may only be able to see shades of yellow, blue, black, and white (Hardin and Maffi, 1997). Thus, individuals are unable to discriminate easily between yellow-reds and yellow-greens, as they cannot detect red and green. Deficiencies in confusions between blues and yellows is less frequent (Hurvich, 1981). It is suggested that colour blindness may result from alterations in photopigments within individual’s receptors (Barker, et al., 1999). The visual field of a normally sighted individual may be divided into 'Zones' (Hurvich, 1981) (fig. 2). In the centre of the eye all colours can be seen, however, the eye's sensitivity to green and red hues is lost as one moves to either side. Thus, shades that looked red-yellow may now appear yellow at the sides. As progression to the periphery of the eye increases, the central hues will gradually appear colourless (white, grey shades).
An objects colour appearance is perceived to be stable, although the surrounding illumination performs spectral changes. This is known as colour constancy (Pokorny, Sheevell and Smith, 1991; cited in Nieves, Garcia-Beltran and Romero, 2000). For example, a carpet's colour will appear the same colour throughout the day, despite changes in the wavelengths reflected from it. Nieves et al, (2000), suggested, that colour constancy can be supported by the adaptation of L and M wavelength cones to illumination, whereas illumination changes were found to influence S-cones. However, Arend (1993) and Brainard, Brunt and Speigle (1997) found that an error in the visual systems’ estimation of illuminant match might contribute to colour consistency.
Colour within the visual spectrum, can be matched, by three different wavelengths of light (primary colours), being mixed in different proportions (Hardin and Maffi, 1997). Primary colours are defined on the basis that one colour cannot be matched by a combination of the other two (Pinel, 2000 pg182) [see Theory stage]. Colour matching experiments, for example the ‘spilt half’ design (fig.3), can be used to show this effect, where individuals alter a combination of three lights to different degrees, to match a presented colour (Ref). This process of colour matching involves the additive mixing of light (i.e. lights are added to make a new light). Red, green and blue are the three primary colours used in additive colour mixing, as they are most capable of producing any other colour (Levine and Shefnes, 2000). For example, by summating red, green and blue light, white light is produced, however the summation of yellow and blue light also produces white light, both of which summations are indistinguishable, the name given to this is a metameric match (Levine and Shefnes, 2000).
Helmholtz and Young's theory (Component or Trichromatic theory) suggests there are three types of colour receptors or cones which are tuned differently according to the photopigments they posses, giving each a different spectral sensitivity (Reber, 1995; Pinel, 2000; Eysenck and Keane, 2000). Each cone receptor is either more sensitive to short (S-), (blues) medium (M), (greens) or long (L-) (reds) wavelengths (Eysenk and Keane, 2000). It is suggested, that a particular stimulus’s colour is encoded by the ratio of activity in the three receptors (Setrular and Blaker, 1994). The theory was formulated through the observation of colour mixing i.e., three different wavelengths can match all colours in the visible spectrum. This supports the proposal that three primary colours exist. Though all three-cone types facilitate our perception of colour, the M and L cones do most of the work, as the S- cones are relatively rare (Hardin and Maffi, 1997). Thus, the majority of people have three-cone types, however some have only two, and are said to be colour deficient (Eysenk, REF).
Dichromate, are still, enable to see other colours thus are not totally colour blind. The deficiency (of red-green) is proposed to occur as a result of, M- and L- cones being missing or damaged (Sekuler and Blake, 1994; cited in Eysenck and Keane, 2000). A blue-yellow deficiency results if S- cones are missing or damaged. Therefore, a dichromat system only uses two wavelengths to match any given stimuli, however a trichromatic visual system uses three. Thus, the trichromatic theory does not completely account for colour deficiency, as it cannot explain why dichromats can see yellow and yet confuse red and green (Hardin and Maffi, 1997). To perceive yellow both L- (red), and M- (green) cones must be stimulated simultaneously by light to summate their inhibitory effects. If individual’s either posses a M- or L- cone a summation would not be possible. In addition, this model does not easily explain the other key phenomena such as negative after image and colour contrast, due to similar difficulties in explaining the neural mechanisms, (see below).
Hering (1878) proposed that three opponent processes exist in the visual system. One process responds in one direction (e.g. decreased firing) to e.g. red and in the opposite direction (e.g. increased firing) to the complementary colour e.g. green (Pinel, 2000; Hurvich, 1981). This is also true for cells signalling blue and yellow in the same opponent fashion and for black and white signals (Cited in Eysenck and Keane, 2000). Hering’s theory was based on several observations, e.g., that complementary colours cannot exist together (see earlier discussion on after-effects). Thus, opponent processing supports the existence of colour pairs (Hurvich, 1981; Pinel, 2000). In addition, the theory explains negative afterimages and colour deficiency, by suggesting that the cell that responds to red and green are independent of those responsible for the other complementary colours. Negative afterimage can be explained in terms of adaptation (Chichilnisky, Brain and Wandella, 1998; Shevell, 1982), e.g. if one stares at a colour, the input produces an excitatory response, and neuronal fatigue results (Pinel, 2000). Thus, the opponent colour is no longer inhibited so is seen when vision is transferred to a neutral background. However, the theory accounts for all the phenomena, that the trichromatic theory has difficulties explaining.
Though, both theories are correct, a combination of the two seems compelling, i.e. three cone receptors send signals to the opponent cells, which are recorded. For example, L- cones; send inhibitory signals to the red-green opponent cells and S wavelengths send excitatory signals (Hardin and Maffi, 1997). If the inhibitory signals are greater than the excitatory ones, yellow is seen, and if this is reversed, blue would be seen. The combination of these theories thus provides a stronger explanation for the phenomena. However, this combinational approach does not account for colour constancy. The Retinex theory (Land, 1986) was proposed instead. It proposes three visual systems that are separate from each other; each responding to different wavelengths i.e. S, M-, and L- wavelengths (Reber, 1995). Land, (1977) conducted experiments using Mondrians. He found that unitary colours viewed in Mondrians all appeared their own colour even when they all reflected the same mixture of white light However, when viewed not as part of a Mondrian, they appeared white under the same illumination (Pinel, 2000). Land's theory therefore suggests that, when objects can be compared, colour constancy is enabled. However, this theory is unable to account for colour constancy, when objects are viewed in isolation, as no comparison can be made (Jackobsson, Bergstrom, Gustafsson and Fedorovskaya, 1997). The theory also does not explain the role of colour-opponent cells or similar neurones in colour perception. In addition, the other phenomena cannot be explained by this theory therefore, it is subsequently disregarded as an explanatory model for colour phenomena.
In conclusion, a combination of the Opponent and Trichromatic theories would best explain the key phenomenon of colour, as although there is an overlap in that they can both account for colour mixing, other phenomena are exclusively explained by one of other of these theories . E.g. colour deficiency and after-images are neatly accounted for by the Opponent Theory, but not by and the Trichromatic Theory. The author suggests that dichromatic deficiencies might additional be explained in terms of regional problems in the eye, as red/green confusion only occurs in the peripheral regions (Hurvich, 1981).
The closest explanation for colour constancy was provided by the Retinex theory, however, neither this or the other two theories, provided a clear explanation for this phenomenon. The author therefore suggests that further investigation is needed, proposing that there may be a three-stage model, where the cone receptors send signals to the opponent cells, which in turn are categorised into different visual systems.
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A display created by the illumination of various proportions of three different waves. The name comes from a Dutch painter.
