What Makes The World Spin When You're Dizzy?

[telaquapacky]

Try this. Get on one of those merry-go-round thingies at the city park, the rotating platform with railings for kids (not adults like you) to play on. Better yet, just remember what it was like the last time you made use of one to really spin yourself silly, until you were so dizzy you could hardly walk, but had to lay down on the grass.

You'll recall that the world keeps spinning after you have stopped. What is strange is that it keeps spinning in only one direction, and yet never seems to go anywhere. What I mean is, if you had a movie camera on a pivot and made it pan in circles, the world would spin- but it would give you a 360-degree view. But your dizzy spinning spins, freely, smoothly- yet you still see only the scene in front of you. Howzzat?

If someone were looking at you while your world is spinning, they would see your eyes wag back and fourth. This is a phenomenon called nystagmus. Another way to stimulate nystagmus is to sit by the rails and try to watch a train as it passes. Your eyes will lock on various features of the passing cars, and follow them- a movement we call a "smooth pursuit," followed by a fast backward movement we call a "saccade, " as your eyes move back to catch another feature, farther back on the train. That kind of nystagmus is called optokinetic nystagmus.

What's different about dizzy nystagmus (rotatory-induced nystagmus) is that you only "see" the smooth pursuit. You never see the backward saccade. What's happening is that by spinning long enough to get roaring dizzy, you overstimulated your semicircular canals in your middle ear. The semicircular canals drive your extraocular muscles to keep your eyes tracking on the world as you spin, like image stabilization in a video camera or expensive binoculars. When you stop, the semicircular canals keep sending the message to your brain that you're spinning, even though you've stopped, because they're overstimulated. Your brain keeps sending the command to your extraocular muscles to keep up the smooth pursuits and backward saccades, until the overstimulation of the semicircular canals wears off.

But why do you see movement in only one direction, never going anywhere? The reason is that there is something called "registered" eye movement, and "non-registered" eye movement. This is very important to your sense of equilibrium and your awareness of the space around you.

If you take a video camera and move it around here and there quickly, and you watch the picture on a screen, what you see on the screen is nonsense and disorienting to you. But your eyes do this all the time, and it isn't disorienting at all. This is because your brain knows when your eyes turn- because it is telling them to turn. When you dart your eyes quickly up to the left, the world doesn't suddenly drop down and to the right, because your brain expects the movement and compensates for it in your three-dimensional concept of the world around you. That's called a registered eye movement. A video camera moving around is like a non-registered eye movement.

Registered eye movements create no apparent movement in the world. But non-registered eye movements make the world appear to move. You can make your own non-registered eye movement by pressing on your eyeball with your fingertip through your lower eyelid. See the world move?

The reason that when you're dizzy and the world is spinning, but never goes anywhere, is because the smooth pursuit movement part of the nystagmus is a non-registered eye movement. The back-saccade part of the nystagmus is registered. So the smooth pursuit produces the appearance of motion, whereas the back-saccade doesn't.

[Sheryl]

That was really interesting. I wondered why that happens after spinning.

[OpenMind]

Phew, I feel giddy just reading that! Interestingly informative, though. Like trying to roll a ball to someone opposite you on a roundabout.