Pablo Picasso once said, "We all know that Art is not truth. Art is a lie that makes us realize truth, at least the truth that is given us to understand. The artist must know the manner whereby to convince others of the truthfulness of his lies."

If we didn't buy in to the "lie" of art, there would obviously be no galleries or exhibitions, no art history textbooks or curators; there would not have been cave paintings or Egyptian statues or Picasso himself. Yet, we seem to agree as a species that it's possible to recognize familiar things in art and that art can be pleasing.

To explain why, look no further than the brain.

The human brain is wired in such a way that we can make sense of lines, colors and patterns on a flat canvas. Artists throughout human history have figured out ways to create illusions such as depth and brightness that aren't actually there but make works of art seem somehow more real.

And while individual tastes are varied and have cultural influences, the brain also seems to respond especially strongly to certain artistic conventions that mimic what we see in nature.

What we recognize in art

It goes without saying that most paintings and drawings are, from an objective standpoint, two-dimensional. Yet our minds know immediately if there's a clear representation of familiar aspects of everyday life, such as people, animals, plants, food or places. And several elements of art that we take for granted trick our brains into interpreting meaning from the arbitrary.

Lines

For instance, when you look around the room in which you're sitting, there are no black lines outlining all of the objects in your view; yet, if someone were to present you with a line-drawing of your surroundings, you would probably be able to identify it.

This concept of line drawings probably dates back to a human ancestor tracing lines in the sand and realizing that they resembled an animal, said Patrick Cavanagh, professor at Universite Paris Descartes.

"For science, we're just fascinated by this process: Why things that are not real, like lines, would have that effect," Cavanagh said. "Artists do the discoveries, and we figure out why those tricks work."

That a line drawing of a face can be recognized as a face is not specific to any culture. Infants and monkeys can do it. Stone Age peoples did line drawings; the Egyptians outlined their figures, too.

It turns out that these outlines tap into the same neural processes as the edges of objects that we observe in the real world. The individual cells in the visual system that pick out light-dark edges also happen to respond to lines, Cavanagh said. We'll never know who was the first person to create the first "sketch," but he or she opened the avenue to our entire visual culture.

Faces

This brings us to modern-day emoticons; everyone can agree that this :-) is a sideways happy face, even though it doesn't look like any particular person and has only the bare minimum of facial features. Our brains have a special affinity for faces and for finding representations of them (some say they see the man in the moon, for instance). Even infants have been shown in several studies to prefer face-like patterns over patterns that don't resemble anything.

That makes sense from an evolutionary perspective: It benefits babies to establish a bond with their caregivers early on, notes Mark H. Johnson in a 2001 Nature Reviews Neuroscience article.

Our primitive human ancestors needed to be attuned to animals around them; those who were most aware of potential predators would have been more likely to survive and pass on their genes.

So our brains readily find faces in art, including in Impressionist paintings where faces are constructed from colored lines or discrete patches of color. This "coarse information" can trigger emotional responses, even without you bearing aware of it, Cavanagh and David Melcher write in the essay "Pictorial Cues in Art and in Visual Perception."

Patrik Vuilleumier at the University of Geneva and colleagues figured out that the amygdala, a part of the brain involved in emotions and the "flight or fight response," responds more to blurry photos of faces depicting fear than unaltered or sharply detailed images. At the same time, the part of our brain that recognizes faces is less engaged when the face is blurry.

Cavanagh explains that this may mean we are more emotionally engaged when the detail-oriented part of our visual system is distracted, such as in Impressionist works where faces are unrealistically colorful or patchy.

Color vs. luminance

Artists also play with the difference between color and luminance.

Most people have three kinds of cones in the eye's retina: red, blue and green. You know what color you're looking at because your brain compares the activities in two or three cones. A different phenomenon, called luminance, adds the activities from the cones together as a measure of how much light appears to be passing through a given area.

Usually when there is color contrast, there is also luminance contrast, but not always. In the research of Margaret Livingstone, professor of neurobiology at Harvard University, she explored the painting "Impression Sunrise" by Claude Monet, which features a shimmering sun over water. Although the orange sun appears bright, it objectively has the same luminance as the background, Livingstone found.

So why does it look so bright to the human eye?