What if you had no sense of balance?

You can walk, run, hop and spin. You hardly ever think about balance.

But something inside you is working hard to keep you steady. What if it stopped?

Walking along a curb, riding a bike, or just standing on one foot all seem easy. But your body is busy the whole time, checking which way is up and which way you're moving, and making tiny fixes so you don't tip over.

So what if your sense of balance switched off? Could your eyes and feet do the job alone? To find out, we'll look inside your ear, discover why spinning makes you dizzy, and visit astronauts coming home from space.

The National Institute on Deafness and Other Communication Disorders (NIDCD) explains that balance relies on a series of signals to the brain from the eyes, the ears, and the muscles and touch sensors in the legs. The part of the ear involved is the vestibular system, or labyrinth.

This thought experiment removes the vestibular input while keeping vision and the body's other senses. It follows three threads: how the inner ear senses movement, why spinning confuses it, and what astronauts' return to Earth shows about the body relearning balance.

Deep in each ear are three tiny loops full of liquid. They tell your brain when your head turns.

Your eyes and feet help too. They all work together!

Deep inside each ear is a maze-like part called the labyrinth. In it are three semicircular canals: fluid-filled loops set roughly at right angles to each other. When you turn your head, the fluid inside moves and bends tiny hair cells, which send a signal telling your brain which way you turned.

Nearby are two little pouches, the utricle and the saccule. They contain tiny grains that gravity pulls on, so they tell your brain whether you're sitting up, leaning back or lying down. Your eyes and the touch sensors in your legs add their own clues.

Within the labyrinth, three fluid-filled semicircular canals form loops at roughly right angles. When the head rotates, the fluid moves and flexes a gelatin-like cupula, which NIDCD likens to a sail billowing in the wind, bending the stereocilia of hair cells and generating nerve signals about the direction of turning.

The utricle and saccule contain dense grains of calcium carbonate, called otoconia, that gravity pulls on, signaling the head's position relative to gravity and its linear movement. OpenStax notes that by comparing canals in different planes, the system can detect the direction of most head movements in three dimensions.

Now pretend your ears stop sending balance messages.

Your eyes and feet would have to work extra hard. Walking in the dark might feel very tricky!

If your inner ear stopped sending balance signals, your brain would lose its built-in sense of turning and tilting. It would have to rely on your eyes and the touch sensors in your legs and feet instead.

You might manage in a bright room on flat ground, but things could get tricky in the dark, on a bumpy path, or when turning your head quickly. NIDCD notes that the balance system also helps keep your vision steady when your body moves. These are ideas about a pretend body.

Removing vestibular input would leave vision and somatosensation, from the muscles and touch sensors of the legs. NIDCD explains that the vestibular organs normally work with these systems to control posture at rest and in motion, and help keep a stable visual focus when the body changes position.

It is reasonable to infer that, without vestibular signals, a person would lean more heavily on vision, and might find balance harder whenever vision is limited or the ground is uneven. How well people could compensate is not estimated here; the astronaut evidence below suggests the brain does reweight its senses.

When you spin, the liquid in your ears spins too.

When you stop, the liquid keeps swirling for a moment. So your brain thinks you are still spinning!

When you start spinning, the fluid in your semicircular canals lags behind your head, which bends the hair cells and tells your brain you're turning. Keep going round and round, and the fluid starts circling too.

Then you stop. Your head stops, but the fluid keeps moving for a moment, because it has inertia, the way water keeps swirling in a cup after you stop stirring. A balance scientist at Washington State University explains that your eyes now say you've stopped while the moving fluid says you're still turning. That mix-up is the dizzy feeling. It fades as the fluid settles.

OpenStax describes how, when the head rotates in a canal's plane, the endolymph lags, deflecting the cupula opposite to the head's motion. During a sustained spin the fluid comes to circulate with the head. When the spin stops, inertia keeps the fluid moving for a short time, as a Washington State University balance researcher explains on the university's Ask Dr. Universe site, so the canals go on reporting motion.

The brain then receives a rotation signal that conflicts with what the eyes report. NIDCD notes that when signals from any of the balance-related systems malfunction, dizziness or vertigo can result, and that everyone has a dizzy spell now and then.

In space, astronauts float. Their balance sense gets used to floating.

Back on Earth, they have to learn to walk steadily again. Their eyes help a lot!

In space, astronauts float, and their bodies adjust to a world without the usual pull of gravity. That's helpful up there, but a problem when they come home.

A NASA research summary from 2012 explains that astronauts must spend time readapting to Earth's gravity after long missions. During that time their walking can be unsteady, they often rely more on their eyes for balance, and many say they have to think carefully about movements that used to be automatic. Bit by bit, the body relearns.

A 2012 conference paper by Mulavara and colleagues at NASA's Johnson Space Center summarizes the problem: exposure to microgravity induces adaptive changes in sensorimotor function that suit space flight but are inappropriate for 1 g, so astronauts must readapt after returning to Earth.

During readaptation, gait is disturbed; astronauts often rely more on vision for postural and gait stability, and many report needing greater conscious supervision of actions that were previously automatic. The team studied how vestibular and body-load-sensing inputs contribute and how motor learning drives recovery: a real case of the brain reweighting its senses.