Now, neuroscientists have pinpointed the area of the brain that’s responsible for this weird effect. They showed the illusion, known as the ‘Kanizsa square’, to macaque monkeys. When the monkeys stared at the square, neurons in a region of their brain’s visual cortex known as 'V4' began to fire. When the four Pac-Men were rotated so that the illusion disappeared, however, the neurons went quiet.
"It's hallucinating without taking drugs," says Dr Alexander Maier at Vanderbilt University in the US.
So why does the brain see the square? It’s because it’s taking a shortcut. It thinks that the likelihood of the four Pac-Men lining up to create a square is low, while the likelihood of a white square partially obscuring the circles is comparatively high. So that’s what we end up seeing.
"Basically, the brain is acting like a detective," says Maier. "It is responding to cues in the environment and making its best guesses about how they fit together. In the case of these illusions, however, it comes to an incorrect conclusion.”
The illusion even works when the Pac-Men are moving, as Prof Bruce Hood memorably demonstrated in the Royal Institution Christmas Lectures 2011 (skip to 1:30).