Your brain is tricked by using alternating points and colors into thinking that the lines you are seeing are not actually parallel. But they are. It’s true. In fact, you can check it yourself with a metric ruler to show that this is so.
But what is the reason for the illusion? According to scientists, this illusion has to do with the way in which the black and white edges interact in opposite directions all along the straight edge, thereby causing the brain to be tricked into believing that there is an inclination in the line.
So if you erase the image, the effect disappears, and it’s because you can’t resolve the tiny black-and-white interactions once they are blurred, so the brain is no longer led down that path.
This illusion was first seen in 1979 on the wall of a cafe in Bristol, UK. It became a sensation and since then, it has been modified in various ways to create the same gimmick, entertaining and continuing to confuse the minds of everyone ever since.