Tom Cruise, Keira Knightly, Angelina Jolie: When viewed one at a time on a computer screen, photographs of gorgeous celebrities highlight pleasing features on generally symmetrical faces.
But if you look at the same images side-by-side while focusing on a cross between them, something bizarre happens. The faces begin to morph, becoming bulbous and grotesque, especially when one member of a pair has a face that is structured very differently from the other.
The optical illusion, as described in a 2011 research paper and illustrated in a disturbing YouTube video, is just one example of how our senses often distort reality -- a concept illustrated in a recent Dove ad that showed how other people tend to see us as more beautiful than we see ourselves. In the video, an FBI forensic artist drew women as they described their own faces and as others described them.
Through studies of illusions like these, along with research on vision, brain science and memory, researchers are beginning to understand the limits of our ability to interpret the endless reams of input that we absorb all day long. We often see things that aren't there and don't see things that are.
And while many details about the relevant brain processes remain mysterious, it seems that our innate tendency to distort reality can be a useful trait.
"One way we think about perception is that it's an interpretation of the world rather than a veridical representation of the world because it makes us function in the world," said Kalanit Grill-Spector, a neuroscientist at Stanford University in Palo Alto, Calif. "If you were representing exactly what was hitting your eyes, it might not be optimal for your behavior."
She offered a red tomato as an example. If you were to use a spectrometer to measure the wavelengths of light that bounce off of a tomato in different environments, the quality of the fruit's redness would appear to be wildly different when held outside than it would in the produce aisle of the supermarket. To our eyes, however, the same tomato looks equally red in both places, and that kind of color constancy allows our brains to understand that a tomato is a tomato, no matter where it is.
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."
From moment to moment, our biases and feelings influence the way we perceive the present and what we decide to pay attention to or consider important, she added. Later on, we can only remember the details we noticed in the first place.
Even then, every act of remembering involves a reconstruction of what happened, which makes memories malleable. Memories also change as we hear other people's accounts of them or as we pick certain memories to reinforce at the expense of others.
Understanding the flexibility of memories has become important in research on post-traumatic stress disorder. For people who have experienced trauma, intentionally morphing memories may be the key to recovery.
In ongoing research, Cervantes said, scientists are slowly piecing together a complicated picture of the brain regions involved in forming and reforming memories and how those regions communicate with each other.
For now, what's clear is that reality is a shifting concept that is colored by a person's beliefs and past experiences, which explains why two people can have opposite opinions about whether a third person is attractive or not, among other differences in perception. The way we experience reality also changes over time.
"You have a constantly evolving perception of the world around you," Cervantes said. "If you've ever read an article or book and then reread it 10 years later, you take it in in a completely different way. You've got a whole ever-changing set of knowledge you're going to embed and integrate" into your experiences.