Watch the spinning spiral and see things expand!

A cream and green exhibit table and backboard with the title 'Watch the spinning disc and see things expand!. Two horizontal discs sit on the table top with black and white spiral patterns.

A motion after-effect makes things appear to ‘expand’ or twist, even though the things don’t actually change size or move.

How it works

Stare into a spinning disc, then quickly look at something else and notice a strange effect.

Things to try or ask around the exhibit

  • After you stare into the spinning spiral disc, look at your hand. What does your hand appear to do?
  • Try staring into the disc with one eye closed, then when you look at something, close that eye, open your closed eye and use it to look at your hand. Do you still get the wobbling after-effect? What does this mean about your eyes and your brain processing visual information?

Background

Just as your eyes adjust from bright light to dim light when you walk into a dark cinema, your visual system also needs to adjust from watching fast motion to something more stationary. Sometimes, when motion-sensitive cells in your visual cortex (brain) are overstimulated, this can cause a motion after-effect illusion.

There is debate over whether the motion-sensitive cells in your brain become fatigued or not, but continuous motion in a certain direction leaves us with a bias in the other direction—an after-effect. Different cells in your visual cortex respond to movement in different directions, but the motion after-effect works if you see movement in part of your visual field, rather than your whole visual field.

Finding the science in your world

If you stare into a large water fall, then look off to the side, you’ll probably notice surrounding bushes or rocks start to ‘wobble’ and ‘boil’. This is due to a motion after-effect.

When you are driving down the highway in your car, you see everything moving and you don’t get a motion after-effect when the car stops. When you’re riding in a train however, you only notice movement through a smaller window, while the carriage seats and walls seem to be still. When your train stops at a station, the platform can sometimes appear to move backwards as the motion after-effect in your brain continues while your motion cells are resetting.