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Incredible Illusions

Moving Lines Illusion

Why are they moving?

This amazing illusion really makes you feel like the lines are moving after you know they have stopped. This kind of illusion is called a motion after-effect and it can tell us a lot about how we see things move.

We see motion after-effects all the time. Next time you visit a waterfall, try staring at it for a couple of minutes. Your eyes will gradually get used to seeing the water fall. Then when you look away, everything you see will seem to float upwards! Trees and rocks appear to float up through the air even though you know they cannot.

The same thing may happen when you watch the closing credits of a video or movie. After watching all the credits roll up to the top of the screen, the whole TV may seem to start sinking through the floor. Next time you watch a film, try it!

When you see something, the visual image goes in through your eyes and passes into your brain. Along the way, all kinds of things happen to it to make it possible for you to understand what you see. Different parts of the brain look for different kinds of things in the image (detectors). Some look for colours, others look for shapes and the angles of lines. Still others look for movement.

Movement detectors

There are thousands and thousands of these movement detectors. Each one looks for movement in a different direction. When a movement detector sees something moving in the direction it is looking for, it tells other parts of the brain. The detector keeps telling other parts of the brain again and again until the movement stops. Even when there is no movement in the direction a detector is looking for, it occasionally sends a message by mistake.

Because the movement detector keeps passing the same message over and over, other parts of the brain gradually start ignoring these messages. When you watch the stripes moving left for a long time, after a while the brain ignores the movement detectors that tell it about the left motion of the stripes. Movement detectors for other directions are also sending messages about the lines moving (by mistake). Because they don't send their messages as often, the brain doesn't ignore them.

Then the motion of the stripes stops. The brain gets some mistake messages telling it the lines are moving right and it ignores the mistake messages it gets about the lines moving left. Because there are no messages balancing the mistake messages about the right movements, the brain thinks that the lines must be moving to the right. This is why we think the lines are moving when they are not — a motion after-effect.

Our eyes are not the only way we can feel motion after-effects. We can also feel them through our other senses. Have you ever noticed how when a train or bus stops moving, it can feel as if it is moving backwards. Once again you can be so used to feeling the train moving that you get a motion after-effect when it stops.