Human Colour Vision: 1. Colour Mixture and Retino-geniculate Processing

Normal humans can match any wavelength of light with a combination of three other wavelengths. This capacity, called trichromacy, is due to the presence of three types of cone photopigment in the photoreceptors. These photopigments differ in their peak absorption at either short (S), middle (M), or long (L) wavelengths. A match between two fields of differing spectral distribution occurs when the relative intensities of lights in one field are adjusted so that the activity in each of the three classes of photoreceptor is identical for the two fields. This is possible because of the principle of univariance: light absorbed by a photopigment molecule produces the same response independent of wavelength. Bipolar cells separate the response of the photoreceptors, giving rise to parallel ON and OFF pathways from the retinal ganglion cells to cortex. The main projection site of the retinal ganglion cells for processing of chromatic signals is to the lateral geniculate nucleus (LGN). One class of retinal ganglion and LGN cell (parvocell) carries signals based on combinations of cone signals, L-M or S-(L+M). Although neural signals carrying colour information undergo additional transformations in the cortex, responses of these cells can account for many aspects of colour discrimination. 1. Trichromacy and its Representation Colour is a sensation resulting from the activity in the nervous system that is initiated by light. Classically, colour is described by three dimensions: hue (chromatic sensations such as red, green, yellow or blue), saturation (proportion of chromatic content relative to achromatic or lightness-darkness content) and brightness (our subjective impression of stimulus intensity). These are not the only perceptual qualities of colour, however, as one can also speak of a colour’s film or surface quality, its lustre, transparency and so forth. Most objects that we experience in nature reflect a broad band of the spectrum, but in the laboratory it is useful to use narrow spectral bands, or monochromatic lights. With such stimuli, Maxwell (1860) and Helmholtz (1867) were able to demonstrate one of the most important characteristics of the human visual system: that it is normally trichromatic. Figure 1 illustrates what is meant by trichromacy. If half of a bipartite field such as that shown in Figure 1 is filled with monochromatic light of variable visible wavelength (λv, 400-700 nm), or any other spectral composition, it is possible for an individual to adjust the amounts of three other monochromatic lights (λ1 + λ2 + λ3) placed in the other half of the bipartite field so that the combination matches λv perfectly in hue, saturation and brightness. These two fields are said to be metameric (i.e., the stimuli are physically different but perceptually identical). The matching stimuli (λ1 + λ2 + λ3) are called primaries and any three wavelengths may serve as primaries provided only that no one of the primaries can be matched by a combination of the other two. Thus, the three primaries are typically selected from the short-, middleand long-wavelength regions of the spectrum. An individual who requires three primaries to match any other light is known as a trichromat. Metameric matches can be described by a colour-mixture equation using the symbol “≡” to denote that the equality is perceptual, not physical. With appropriate specification of the units of the colour match, it is also possible to manipulate the match like an algebraic equation. Thus, for a trichromat, if one of the three lights in the mixture (e.g., λ1) in the bottom of the bipartite field is moved to the other half-field, the equation is written to express this by a negative sign: λv λ1 ≡ λ2 + λ3. In this case, the two half-fields would still match as implied by the ≡ symbol. This manipulation is necessary in normal colour matching when λv may take on any value in the visible spectrum. Two important characteristics of colour matching are described by Grassmann’s (1853) laws of scalar invariance and additivity, as stated in Figure 1. Scalar invariance means that if the intensity of one half of the field is increased or decreased by a factor k, the match will be restored by increasing or decreasing the lights in the other half of the field by the same factor. Additivity means that if a light λa is added to one half-field, the match will be restored by adding the same light to the other half field. While trichromacy characterises normal human vision, it should be noted that approximately 6% of males and 0.5% of females are anomalous trichromats. That is, they require three primaries for spectral colour-matching, but they combine them in proportions that differ systematically from those selected by most of the population. A more extreme departure from normal trichromacy is found in about 2% of males and about 0.03% of females. These individuals are said to be dichromatic because they require only two primaries for colour matching. Finally, while rare, some individuals are classified as cone monochromats because they can match any wavelength with any other simply by varying the intensity of one of the lights. Collectively, these departures from normal colour vision are referred to as (congenital) “colour vision deficiencies.” 2. The Eye and Retina Vision is initiated when light travels through the ocular media (cornea, anterior chamber, lens and vitreous humour) and retinal cells of the eye until it is absorbed by the photopigment contained in the photoreceptors (rods and cones). The left side of Figure 2 illustrates the human eye, while the right side shows an enlargement Fig. 1 Illustration of a bipartite stimulus used in a colour-mixture experiment. By varying the relative intensities of three primaries in the lower portion of the field, it is possible to obtain a metameric match (denoted by the symbol ≡) to the upper half of the bipartite field. The match can be specified by a colour-mixture equation and can be shown to obey the properties of scalar invariance and additivity. λv λ1 + λ2 + λ3 f λv ≡ λ1 + λ2 + λ3 (λv) ≡ k(λ1 + λ2 + λ3) (scalar invariance) λv+ λa ≡ λ1 + λ2 + λ3 + λa (additivity)