Light, sound, pressure and chemicals are not thoughts. Somewhere at the edge of the body, energy from the world has to be turned into the electrical language of neurons, and at the other end, electrical commands have to become muscle force. This level follows both journeys, inward through the senses and outward through movement.

Translating the world

Every sense begins with transduction, the conversion of a physical stimulus into nerve signals. Each sense has specialized receptor cells for the job: cells in the eye that react to light, cells in the ear that bend with vibration, nerve endings in the skin that respond to stretch, heat or damage.

In the 1830s the German physiologist Johannes Müller noticed something odd. Press gently on your closed eyelid and you see light, though no light is present. He proposed what he called the law of specific nerve energies: the kind of sensation depends on which nerve is stimulated, not on what stimulates it. The brain does not receive light or sound; it receives spikes, and it interprets them according to where they come from.

Vision

The retina at the back of the eye contains two main kinds of light receptor. Rods work in dim light but do not distinguish colors, which is why a moonlit garden looks gray. Cones need brighter light and come in three types, most sensitive to long, medium and short wavelengths; the brain compares their signals to produce color.

Signals travel through the thalamus to the primary visual cortex at the back of the brain. In 1959 David Hubel and Torsten Wiesel recorded from single cells there in cats and found something unexpected: many cells in the visual cortex respond best to lines or edges at a particular angle. One cell might fire for a vertical bar, its neighbor for a slightly tilted one. Vision, it seemed, is built up from simple features into shapes and then objects. They shared a Nobel Prize in 1981.

Beyond this first stage, visual processing splits into two broad streams. A lower stream running into the temporal lobe helps identify what an object is; an upper stream running into the parietal lobe helps locate it and guide action toward it. The clearest evidence came from a woman known as D.F., studied by the psychologists David Milner and Melvyn Goodale after carbon monoxide poisoning damaged her lower stream. D.F. could not describe the angle of a slot, yet she could post a card through it smoothly, turning her hand correctly as she reached. Seeing to know and seeing to act turned out to be partly separate.

Hearing and pain

In the inner ear, the snail-shaped cochlea holds rows of hair cells that bend as sound waves ripple through fluid. High pitches stimulate cells near the cochlea's base and low pitches cells near its tip, so pitch is mapped by place. Loud noise can destroy hair cells, and in humans they do not grow back, which is why noise-induced hearing loss is permanent.

Pain is not a simple alarm wire. In 1965 Ronald Melzack and Patrick Wall proposed the gate control theory: signals from touch fibers can partly close a "gate" in the spinal cord against pain signals. That is why rubbing a bumped elbow can dampen the pain signals reaching the brain. The brain also opens and closes the gate from above, so fear, attention and expectation change how much something hurts.

Movement

In the 1930s to 1950s the Canadian neurosurgeon Wilder Penfield stimulated the exposed brains of awake patients during epilepsy surgery and recorded what moved or what they felt. He mapped a strip of motor cortex in the frontal lobe, drawn as a distorted little figure called a homunculus. The hands, lips and tongue take up a large share of the motor map, far more than the back or legs, because they make the finest movements.

Not every movement needs the brain. When a doctor taps below your kneecap, your leg kicks. The knee-jerk reflex is a loop through the spinal cord that does not wait for the brain: a stretch sensor signals a spinal neuron, which signals the thigh muscle directly. Meanwhile the cerebellum compares the movement you intended with the one you made and corrects the difference, and sensors in muscles and joints, the sense called proprioception, tell you where your limbs are with your eyes closed.

Worked example: the hot pan

A cook grabs the handle of a pan that has been sitting in the oven. Her hand jerks away before she feels any pain, and then, a moment later, the burn starts to throb. Why in that order?

Heat-sensing nerve endings transduce the heat into spikes. Some of those signals reach the spinal cord and trigger a withdrawal reflex straight back to the arm muscles, a short loop that takes a small fraction of a second. Other signals climb to the brain through the thalamus to the cortex, where they become a conscious experience of pain, and some travel along slower, thinner fibers, which is why the throbbing comes later. The reflex saved her skin; the brain then made the experience that will teach her to use an oven glove.