Structure from Motion

Click the picture above to see the flash illusion and change the number of dots and their speed to intensify the illusion. In which direction is the ball moving?

The movement of the dots left and right inside the circle creates the illusion that the ball is moving.

The brain interprets the movement of the dots as an indicator of depth, and it makes the two-dimensional image appear to be three-dimensional. The ball seems to be moving clockwise at times and at other times counter-clockwise.

Speeding up and multiplying the dots intensifies the illusion, but the effect disappears completely when the movement stops. In other words, it's the movement of the dots, rather than the dots themselves that gives the hint of depth.

Our perception of the direction of movement changes alternately, just as it does in a static ambiguous illusion, since the exact same number of dots moves left and right. Sometimes we perceive the movement to the right as "forward" and other times as "backward." We can influence our perception of the direction by concentrating on one or the other.