Colour vision and colour blindness 621 Colour vision

In the normal human eye there are three types of cone, distinguished by the presence of one of three iodopsin pigments. The three pigments differ in their sensitivity to various wavelengths of light within the visible spectrum. Sensitivity implies that light of the corresponding wavelength is absorbed and has its energy converted to an electrical signal in a nerve.

Figure 6.5 shows that the peak sensitivities of the three types of cone are at wavelengths around 565,530 and 425 nm. The cones associated with the peak at the longest wavelength are usually

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Figure 6.5. Wavelength dependence of response of four types of photoreceptor. The response of rods, shown by the broken black line, has a maximum around 505 nm. The responses of the three types of cone are unequally spaced with maxima around 425 nm (violet), 530 nm (green) and 565 nm (yellow). Each curve is normalized and does not take into account the unequal numbers of photoreceptors or their relative sensitivities.

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Figure 6.5. Wavelength dependence of response of four types of photoreceptor. The response of rods, shown by the broken black line, has a maximum around 505 nm. The responses of the three types of cone are unequally spaced with maxima around 425 nm (violet), 530 nm (green) and 565 nm (yellow). Each curve is normalized and does not take into account the unequal numbers of photoreceptors or their relative sensitivities.

described as 'red-sensitive', even though the peak at 565 nm actually lies in a part of the spectrum generally accepted as yellow. (The familiar orange-yellow light from sodium-vapour street-lamps has a wavelength around 589 nm, which is 24 nm farther towards the red.) The peak at 420 nm is in the violet part of the spectrum near the short wavelength limit of vision, but the associated cones are usually called 'blue-sensitive'. Only the green-sensitive cones are accurately named. (In this section the traditional nomenclature is, however, retained for all three types.) The red-sensitive cones are the most numerous. The blue-sensitive cones comprise less than ten per cent of the total cone population in an average human eye and are even scarcer at the centre of the fovea, but compensate for their small numbers by having a higher sensitivity.

Each pigment has an appreciable response to wavelengths on either side of its peak sensitivity. For the cones with peak sensitivities at 565 or 530 nm, some response occurs across more than 70 per cent of the visible spectrum. The blue-sensitive cones have a response range that extends from the near UV to green. However, the shortest wavelength response is not exploited because the lens is opaque to UV radiation. The cumulative effect of UV absorption by the lens hastens the development of cataracts.

One function of the visual cortex is to interpret the cone responses in terms of hue. For light of wavelength around 650 nm, which is generally perceived as red, the stimulation of the green-sensitive cones is considerably smaller than the stimulation of the red-sensitive cones, but it is by no means negligible. The unequal spacing of the three peaks and the unequal numbers of the three cone types may benefit the overall performance of the human eye because clarity is probably more important than colour assessment. It may be advantageous to have few blue-sensitive cones in the fovea and to rely mainly on two types of cone having maximum responses at wavelengths only 35 nm apart, less than ten per cent of the total visible range. Because the refractive indices of the materials in the eyeball vary with wavelength, it is impossible to achieve perfect focus of the image on the retina for all wavelengths simultaneously. Relying mainly on a narrow band of wavelengths at the centre of the total visible range allows a sharper image to be conveyed from the retina to the brain. The wavelength-dependence of refractive index can be experienced during routine sight tests. It is a standard procedure for optometrists to present black letters on green and red backgrounds alternately in order to ascertain precisely which corrective lens produces the best compromise for the eye under test.

Another outcome of the unequal wavelength spacing of the three cone types is that the human eye can detect differences in the colours of two monochromatic sources much better in the middle of the visible spectrum. Yellowish greens stimulate the red-sensitive and the green-sensitive cones almost equally. In this part of the spectrum two monochromatic wavelengths only 1 nm apart can be distinguished. At the extremities of the spectrum only one type of cone is being stimulated significantly and so a wavelength difference less than 10 nm may be hard to detect. As either end of the visible range is approached, the colours appear to get darker but the hue hardly changes.

At 390 and 750 nm the total sensitivity of the retina is only around 0.01 per cent of the maximum value. These wavelengths may be regarded as the normal limits for human optical response, though the cut-off point depends on the light intensity and is not abrupt. The ratio between the longest and shortest wavelengths detectable by the eye is almost 2:1. In the realm of music or acoustics, such a wavelength ratio would be described as a range of one octave, which is tiny compared with the range of acoustic wavelengths detectable by human ears, which may extend over ten octaves - equivalent to a wavelength ratio of more than 1000:1. In passing, it is worth noting that the emission peaks of the three phosphors used to generate coloured pictures on TV and computer screens do not match the sensitivity peaks of the cones. These phosphors emit red, green and blue light, with wavelength peaks that are roughly equally spaced, for example at 610, 550 and 470 nm. Experiments have shown that such a combination of wavelengths produces a wide colour range.

6.2.2 Colour blindness

The ability to create a wide variety of colours by mixing three primary colours, and the existence of colour blindness, have long been recognized, but it was only in 1802 that a coherent explanation of colour blindness emerged. The British doctor Thomas Young was interested in the colour blindness of John Dalton, the chemist who pioneered the atomic theory. (Dalton's eyes are still preserved in Manchester.) Young correctly identified the cause as the absence of one of three colour-selective sensors normally present. ('Daltonism' is a term still sometimes used to describe colour blindness, especially the most common forms in which colours in the red-to-green part of the spectrum are confused.) Difficulty in distinguishing reds and greens occurs in about 7.7 per cent of Caucasian men (1 in 13), while the incidence in Caucasian women is about 0.6 per cent (1 in 132). This type of colour blindness is less common in other ethnic groups.

Although it was known at the time that red-green colour blindness was concentrated in certain families and was more common in men than women, it was not until the early years of the 20th century that the underlying genetic mechanisms became clear. The genes responsible for conveying the ability to create the red-sensitive and the green-sensitive cones are both located on the same chromosome, the X-chromosome. Furthermore, their DNA sequences have a large amount in common. The differentiation between these two types of gene appears to be recent in evolutionary terms, having developed less than 40 million years ago. The closeness of the locations of these two genes and the similarity in their structures increase the probability of errors during the formation of cells for sexual reproduction. As a result of such process errors, some X-chromosomes have acquired extra copies of the gene that produces green-sensitive cones at the expense of other X-chromosomes that possess none. Extra copies of this gene have little effect on human colour perception, but total absence of the gene for green-sensitive cones produces the most common form of colour blindness, known as deuteranopia. Figure 6.5 shows that in the absence of green-sensitive cones, light with wavelengths in the range 550 to 750 nm stimulates only one type of cone. This makes it virtually impossible to differentiate between colours in the green-to-red part of the spectrum. Monochromatic light with a wavelength around 485 nm stimulates both blue-sensitive and red-sensitive cones to a similar extent, making it difficult to distinguish from grey, though it would appear bluish green to people blessed with all three types of cone.

Within a Caucasian population the gene for green-sensitive cones is lacking in about 1 in 13 of the X-chromosomes. Males inherit an X-chromosome from their mother and a Y-chromosome from their father. With only one X-chromosome, the incidence of the deficient X-chromosome directly determines the incidence of deuteranopia in males. Females inherit two X chromosomes (one from each parent) and it is only when both X chromosomes lack the gene for green-sensitive cones that this form of colour blindness occurs. Consequently deuteranopia is much less common in females.

The lack of the gene for the red-sensitive cones is known as protanopia. It is less common than deuteranopia, but it is also associated with the X-chromosome and therefore more common in males than females. Colour blindness associated with the absence of blue-sensitive receptors is known as tritanopia, a very rare defect due to a missing gene on chromosome 7. Because this chromosome has no connection with sexual characteristics, the incidence of tritanopia is similar in males and females. The DNA sequence that codes for blue-sensitive receptors has developed separately for about 500 million years and has little in common with the sequences for red-sensitive or green-sensitive receptors.

A few people have no cone function at all and see only shades of grey. This condition is known as achromatopsia. The condition it is extremely rare worldwide, but it occurs in around one person in twelve on Pingelap, one of the Caroline Islands in the western Pacific. The gene responsible is present in almost 30 per cent of the population of this island, but because the gene is recessive, achromatopsia is manifest only in people inheriting the gene from both parents.

Where it may be important, colour schemes are sometimes devised with the commoner forms of colour blindness in mind. Many computer software packages, such as Word or Excel, use blue and yellow more liberally than red and green. Website designers are encouraged to use labels and variations in texture instead of relying on variations in colour. Traffic lights have red at the top and green at the bottom, so that their message can be deduced from position as well as from colour. The colour coding in mains cables has been chosen so that colour-blind people can identify the three leads correctly.

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