Issue Number: 94
Edmund Fawcett sees the light in a fascinating study of the age-old puzzle about the workings of human vision
The Eye: A Natural History (cover) by Simon Ings The Eye: A Natural History (cover) by Simon Ings The Eye: A Natural History (cover) by Simon IngsIf you want to know how things look, ask a painter. They know more about visual appearances than just about anybody. They expect red light to cast green shadows and distant scenes to appear blue. They know that the eye reads an uptilted ground plane as a cue for distance, and that there is no such thing as brown light. If you want to know why, ask vision scientists.
It might sound as if there are separate topics at issue here. But, as Simon Ings’s fascinating book on the origin and workings of the human eye shows, vision science confirms what artists have long understood: the simplest-looking scene is a most elaborate construction. Vision may feel unified, smooth and effortless. After all, it wouldn’t be much use if it didn’t. However, the more that scientists learn, the more compartmentalised and intricate vision becomes.
Our eyes tell us more about our surroundings than any other sense. Through them we gauge the shape, size, distance and brightness of objects at a glance; we recognise faces on sight and we are instantly able to place countless other things in their proper categories in our minds; we also continuously picture an unbroken three-dimensional space in which we move about without collisions.
Yet in the minute or two that it has taken you to read so far, you have been functionally blind for about one-tenth of the time. Every third of a second, your overtaxed eyes have darted aimlessly about for an instant, as if for a breather, before returning to the arduous job of seeing.
Now try this small experiment. Hold out an arm, turn up your thumb like a painter and look at it. The thumb will just about cover the centre of your visual field, a circle roughly two degrees wide. Within it, your eyes are sharp. Outside it, acuity drops away and you no longer know what you are seeing. Nor do you recognise colour. The illusion of a coherent scene arises only because we are able to move our eyes.
But within that disc of attention we see more acutely than any other land-based vertebrate. Doing close-up work, such as tool-making or writing, for example, our eyes are unbeatable.
‘Eyes like a cat’ turns out to be a left-handed compliment. Galileo and his fellow scientists in the seventeenth-century Roman academy nicknamed themselves ‘lynxes’ after the feline predators whose eyes pierce the dark. Yet, like other cats whose ancestors scanned horizons for prey, even during the night lynxes see only in a narrow band, above and below which objects blur.
Ings opens with the origin of the eye, which appears to have evolved about 40 separate times in different animal species. Though interesting, much of this early story is speculative and hard to follow. But when he turns his attention to human vision, things get clearer and more exciting.
Vision science is abuzz –neuroanatomists are finding that sight involves many brain faculties, not just one. Visual-memory studies suggest the eye is no passive light-window, but actively interrogates its surroundings. Artificial-intelligence theorists propose how the brain ‘computes’ smooth, three-dimensional representations from bitty, two- dimensional patterns of retinal light. Behavioural psychologists have conducted entertaining experiments that suggest we are poorer observers than we like to think.
For anyone interested in art, the most gripping sections will probably be those on colour vision. It is also where expert knowledge seems firmest. Colour vision depends on a balance of arousal and inhibition among photosensitive cells at the back of the eye. Grasping the details requires a pencil, paper and a hot towel for your head. But it’s worth it. At the end, you will know why, to give one example, blue light is always slightly out of focus for us – which explains why blue is recessive and sits back from the painting surface.
Finding a unifying thread in The Eye can feel a lot like trying to spot the pattern in random dots. But Ings is not really to blame. Nobody yet has an overarching theory of vision, let alone of the brain or mind. If you can bear to skip the emergence of eyes in trilobites, start with the chapter on the theories of vision since the Greeks. It underlines just how far we are still from understanding sight. Then read on, to learn how rapidly vision science is, nevertheless, helping to close the gap.