In another famous illusion, an image can appear to be a duck or a rabbit, depending on which part of the picture you focus your eyes on.
"In this situation where an ambiguous stimulus is prone to multiple interpretations, your brain has to resolve it," Grill-Spector said. "There might be competition in your brain and it might be happening all the time. If you are walking in the fog and it's not clear what you're seeing; you have to make an inference."
Studying illusions can help scientists better understand how information travels from our eyes to specific areas of our brains and what goes wrong in people, who may, for example, have trouble recognizing faces.
Memory research offers similar insights. Just as it can be helpful for our brains to fill in information so that we "see" what isn't really there, it's adaptive for our memories to be less than perfectly accurate, said Sasha Cervantes, a human memory researcher at the University of Chicago.
"The brain requires energy to function and we couldn't consume enough sugar to retain and filter all the information we take in on a daily basis," Cervantes said. "It would be inefficient. It's not necessarily the best thing for us to remember everything with perfect clarity."