A few days ago i was sent this amazing illusion (thanks Karen). The illustration below appears to show green and blue spirals…..
But in fact the apparent green and blue are exactly the same colour! Don’t believe me? Well, I put the image into photoshop and changed all of the other colours to black and you get this…..

The ball appears to swerve because our peripheral vision system cannot process all of its features independently. Instead, our brains combine the downward motion of the ball and its leftward spin to create the impression of a curve.