The Spark of Life: Electricity in the Human Body (26 page)

Seeing Red

Colour, like beauty, is in the eye of the beholder. It is not a property of light itself, as Thomas Young first recognized back in the early nineteenth century when he proposed that colour sensation was encoded by three different kinds of pigment. Colour is actually constructed by a collaboration between the eye and brain of the observer, but Young was correct about the three cone pigments. It is thought that humans may have evolved colour vision to enable us to see ripe, orange–yellow fruit on green trees and the yellowish colour of juicy young shoots, for which we need three types of cones. Most mammals, such as dogs and cats, have only two kinds of cone photopigment and so see only a limited range of colour: contrary to popular belief, bulls do not see red. Other creatures live in a world entirely without colour. But humans should not be too complacent, for we are far from having the best colour vision in the animal world and lag well behind the mantis shrimp, which enjoys ten or more different visual pigments. Even tropical fish possess four or five types of cone.

We can see light at wavelengths between roughly 400 and 700 nanometres, which corresponds to the blue and red ends of the visible light spectrum. Other creatures once more surpass us for they may see wavelengths far beyond this. Pit vipers, vampire bats and fire beetles, for example, sense infrared using specialized organs to detect heat. Most birds and insects possess an additional photopigment that enables them to detect ultraviolet light and flowers have evolved ultraviolet markings on their petals to guide butterflies and bees to their nectar stores. Male and female blue tits look similar to us, but not to their mates, for they carry bright flashes of ultraviolet reflective feathers on their crests. Bizarrely, urine stands out clearly in ultraviolet light, a fact exploited by birds of prey who track small rodents by the urine trails they use to mark their territory. Reindeer are also sensitive to near ultraviolet, which is thought to help them find food in a white-out, as in ultraviolet light the pale lichens on which they feed stand out starkly black against the white snow.

Through a Lens, Darkly

Even the colours you do see can be distorted. The lens of the eye starts out as crystal clear, but over time the transparent proteins of which it is composed may become damaged by continual exposure to ultraviolet light and clump together, so that the lens progressively becomes opaque and takes on a yellowish tint. As the cataract develops, the world gradually becomes blurred and hazy and its colours change: white becomes a dull yellow, greens become yellows, bright reds mutate into muddy pinks, blues and purples morph into red and yellow. These colour changes are very evident in Claude Monet’s later paintings. Soon after he was seventy, he began to develop cataracts in both his eyes and the lovely, delicate impressionistic nature of his painting may derive in part from the fact that he increasingly saw the world as blurred. But he was very frustrated that his poor eyesight meant he could no longer see colours with the same intensity he remembered and he was forced to carefully order the sequence of paints on his palette to aid their identification. After 1915, the emphasis on red and yellow colours in his work became particularly pronounced and the light-blue shades disappeared. He had particular difficulties with some of the Water Lily paintings he was working on, and having decided that he was no longer capable of painting anything beautiful, he destroyed several canvases. Finally, when he was in his early eighties and almost completely blind, he underwent surgery to remove the cataract in his right eye. Initially he was bitterly disappointed with the result, complaining of the changes in colour he perceived, but after a second operation to remove the cataract in his other eye he regained confidence and produced his wonderful late Water Lily canvases. These resemble his earlier paintings more closely than the ones he produced while suffering from cataracts.

Every year around 120,000 people in the UK have a cataract removed. Indeed, many of us will have this operation if we live long enough, for cataracts are a common side-effect of ageing. The operation is simple and transformative. Afterwards the world suddenly snaps into sharp focus and colours appear crystal clear. As my mother remarked, ‘the dirty yellowish shirts that I could never seem to wash clean were suddenly revealed to be a bright pristine white – it was like a washing powder advertisement’. Some people may also see the world in an entirely new light. The lens of our eye not only serves to focus light rays, it also cuts out ultraviolet light. Most people have an artificial lens implanted when their own is surgically removed in a cataract operation. But not all. Those who do not then see the world through new eyes for, like bees and butterflies, they become sensitive to ultraviolet light and everything appears brighter and bluer.
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The fact that Monet did not receive a new lens when his cataract was removed may have influenced the mauve and violet hues in his later paintings.

Extraordinary Facts Relating to the Vision of Colours

In 1798 the chemist John Dalton described his own colour blindness in a lecture held in Manchester, and published the first scientific account of the condition in an accompanying paper. In Dalton’s view, the colour of grass closely matched the paint-box red of sealing wax, which led him to conclude that he saw either red or green differently from other people. He also found it difficult to distinguish blue and pink, and to his considerable surprise, many colours appeared different by candlelight. He wrote that he ‘was never convinced of a peculiarity in my vision, till I accidentally observed the colour of the flower of the Geranium zonale by candle-light, in the Autumn of 1792. The flower was pink, but it appeared to me an almost exact sky-blue by day; in candlelight, however, it was astonishingly changed, not then having any blue in it, but being what I call red, a colour which forms a striking contrast to blue’.

Dalton interpreted these findings to indicate that the fluid in the cavity of his eye was tinted blue and so selectively absorbed the longer wavelengths of light, and he instructed that his eyes should be dissected after his death to see if this was the case. The gruesome experiment was duly performed the day after he died but the fluid was totally translucent. Two hundred years later, modern DNA technology was used to identify the cause of Dalton’s colour blindness, using fragments of Dalton’s eyes that had been carefully preserved by the Manchester Literary and Philosophical Society. It turned out he had an inherited condition known as deuteranopia.

Many men have impaired colour vision as a result of mutations in one of the three different kinds of cone visual pigments that detect yellow–green, green or blue–violet wavelengths of light. Most commonly, the yellow–green and green photopigments are affected. About 2 per cent of men completely lack one pigment, giving rise to a condition known as protanopia (absence of yellow–green photopigment) or deuteranopia (no green photopigment), and in 6 per cent of men the spectrum of one photopigment is shifted, so that colour is seen differently. In all cases, it is hard to distinguish between red and green, which both appear a muddy yellowish-brown. It is always worth remembering when preparing colour slides for a presentation that some of your audience may not be able to distinguish red and green colours easily. Similarly, it can be hard for the colour blind to distinguish between ripe and unripe fruit, and some foods appear the unappetizing colour of excrement. The genes for the yellow-green and green visual pigments are found on the X-chromosome, of which men have only a single copy, which explains why far more men than women are red–green colour blind. In women, the gene on the other X-chromosome can substitute for the defective one.

Some people, known as achromatopes, are born with a rare genetic condition that means they have only rod vision and cannot see colour at all. Such total colour blindness is very rare. In the general population, the incidence is about 1 in 30,000 people but as Oliver Sacks relates in his book
The Island of the Colour-blind
it is far more common on the Micronesian island of Pingelap, where 5 per cent of people are affected. It is thought they are all descended from a single individual, a carrier of the mutant gene, who was one of only twenty survivors of a typhoon that struck the island in the 1770s. Achromatopsia is due to a total lack of functioning cone cells. One of its main causes is a mutation in the cyclic GMP-gated channel of the cone cell, as is the case for the Pingelap islanders. Because a different gene codes for the rod channel, people with such mutations are not totally blind. They are, however, dazzled by bright lights and they find it hard to see in normal daylight because their rods cease to function at high light intensities. Even people with ‘normal’ colour vision may not see the world in exactly the same way. Variants in the DNA that codes for the visual pigments may produce subtle differences in our perception of colour. The red that I see may not be quite the same as that which you do.

On 15 November 1875, there was a terrible train crash at Lagerlunda in Sweden when two express trains travelling on the same single line track collided head-on. The driver of the late-running northbound train appeared to have ignored the red light the stationmaster had been waving at him to signal him to stop, and because he had died in the crash it was not possible to question him. The physiologist Professor Alarik Frithiof Holmgren investigated and concluded that the accident had been caused by the colour blindness of the driver.
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As a consequence, Sweden implemented a colour blindness test based on the ability to discriminate different coloured wools and other countries followed suit shortly afterwards. Today, people with colour blindness are banned from certain professions, including that of an airline pilot, and until very recently a few countries, like Romania, even denied people who are colour blind a driving licence.

Hear, Hear!

Our world is full of sounds. A Bach cantata, the roar of traffic, the swoosh of the seashore, the rustles of leaves, the chatter of children, the low throb of a generator, the high-pitched scream of a swift – all travel to our ears as pressure waves and are effortlessly converted by our brain into the sounds we hear. This process is extraordinarily sensitive for we can hear sounds quieter than the tinkle of a pin dropping and discriminate those that are separated by just one-thirtieth of a semi-tone, the smallest interval in Western music. How is it possible that we can distinguish such a range of sounds or pick out one soft voice against a background roar?

The outer and inner chambers of the ear, showing how displacement of the ear drum is transmitted via the three ear bones (malleus, incus and stapes) to the fluid-filled cochlea where the hair cells transduce sound vibrations into electrical impulses.

An elegant description of how we hear is given by Aldous Huxley in his novel
Point Counter Point
. ‘Pongileoni’s bowing and the scraping of the anonymous fiddlers had shaken the air in the great hall, had set the glass of the windows looking onto it vibrating; and this in turn had shaken the air in Lord Edward’s apartment on the further side. The shaking air rattled Lord Edwards’ membrana tympani; the interlocked malleus, incus and stirrup bones were set in motion so as to agitate the membrane of the oval window and raise an infinitesimal storm in the fluid of the labyrinth. The hairy endings of the auditory nerve shuddered like weeds in a rough sea; a vast number of obscure miracles were performed in the brain, and Lord Edwards ecstatically whispered “Bach!”’

As Huxley relates, sounds are simply pressure waves in the air that radiate outward from a sound source, much like the ripples in a pond.
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These are collected and filtered by our ears and funnelled towards the eardrum, which vibrates in response. In turn, this moves three delicate interlocking bones, the malleus (‘hammer’), incus (‘anvil’) and stapes (‘stirrup’), which are among the smallest bones in our body and no bigger than the size of a single letter of this print. These relay the vibrations to another membrane, the oval window. At this point the sound waves pass from air to the fluid-filled canals of the inner ear, where sensory cells convert the sound waves into electrical impulses. These are then forwarded via the auditory nerves to the brain, where they are interpreted.

The ear must register both the intensity and frequency (tone) of a sound. Nerve cells are not especially well suited to this, for their maximum rate of firing and the range of intensities they can signal are quite low. Yet the loudest sound we can hear may be 100,000 times greater than the quietest, and we can detect tones ranging from frequencies as low as 20 hertz (20 cycles per second) to as high as 20,000 hertz. So how do our ears do it?