The Senses

Everything you know about the world came in through your senses. Light, sound waves, molecules drifting in the air, pressure on the skin: none of these are experiences in themselves. They are physical events, and the senses are the instruments that detect them and translate them into electrical signals. The experience, the colour red, the sound of a voice, the smell of rain, is manufactured afterwards, in the brain. You do not see with your eyes. You see with your brain, using your eyes.

Sight

The eye is a camera. Light enters through the pupil, a hole whose size is adjusted by the coloured iris, is focused by the lens, and lands on the retina at the back, a sheet of light-sensitive cells. There are two kinds. About 120 million rods work in dim light and see only in shades of grey; six million cones, concentrated in the centre, work in bright light and come in three types sensitive to red, green and blue. Every colour you see is a mixture of three signals, which is why screens need only three colours of pixel.

The retina does some processing itself, detecting edges and movement, and sends about a million nerve fibres to the brain, which does the rest: judging distance from the slight difference between the two eyes' views, filling in the blind spot where the nerve leaves the retina, and recognising faces, words and objects in a fraction of a second. The image on the retina is upside down; the brain simply learns to interpret it. About a third of the cortex is involved in vision, more than for any other sense.

Hearing

Sound is vibration in the air. The outer ear funnels it down a canal to the eardrum, which vibrates, and three tiny bones, the smallest in the body, lever those vibrations into the fluid-filled cochlea, a spiral tube the size of a pea. Inside, thousands of hair cells sit on a membrane that vibrates at different places for different pitches, high notes near the entrance and low notes deep in the spiral, like a piano keyboard rolled up. When the hairs bend they fire nerve signals, and the brain reads the pattern as pitch and loudness.

The system is astonishingly sensitive: the faintest sounds you can hear move the eardrum less than the width of a hydrogen atom. It is also fragile. Hair cells do not regrow, and loud noise destroys them permanently, which is why hearing fades with age and with years of loud music.

Right beside the cochlea sit three looped tubes filled with fluid. As your head turns, the fluid lags and bends hairs inside them, telling the brain which way you are moving. This is the sense of balance, and when it disagrees with what your eyes report, as on a boat or in a car, the result is motion sickness.

Smell and taste

Smell is chemistry. Molecules in the air dissolve in mucus high in the nose and lock onto about 400 kinds of receptor on nerve cells there. Each smell activates a particular combination of receptors, and the brain learns the pattern, which is how a few hundred receptors can distinguish perhaps a trillion odours. Smell is the only sense wired almost directly into the brain's memory and emotion centres, which is why a scent can summon a childhood more vividly than any photograph.

Taste is simpler. Taste buds on the tongue respond to just five qualities: sweet, salty, sour, bitter and savoury. Everything else you think of as flavour, the difference between an apple and a pear, is smell, reaching the nose from the back of the mouth as you chew. Hold your nose and food goes bland. Sweet and savoury signal energy and protein; bitter warns of poisons, which is why children, who have more taste buds, dislike bitter vegetables.

Touch and beyond

The skin is the largest sense organ, packed with receptors for pressure, vibration, stretch, heat, cold and pain. Fingertips and lips have the most, and can feel textures a few millionths of a metre high. Pain is a sense in its own right, with its own receptors and nerves, and though it is unpleasant it is protective; the rare people born without it injure themselves constantly.

There are senses beyond the classic five. Receptors in muscles and joints tell the brain where every part of the body is without looking, which is why you can touch your nose with your eyes shut. Sensors in the arteries track blood pressure; others in the brain monitor temperature and the chemistry of the blood, producing thirst and hunger. None of these feel like senses, but all of them inform the brain about the world and the body.

Why it matters

The senses define the world you can know. They are also limited and selective: you cannot see ultraviolet, as bees can, hear the ultrasound a bat uses, or sense the magnetic field a bird follows. What feels like direct contact with reality is a model, built by the brain from a narrow band of signals, and remarkably good at keeping you alive.