Look at an optical illusion and you may think you're seeing things -- such as a curved line that's actually straight, or a moving object that's standing still. You wonder if your eyes are playing tricks on you.
It's not your eyes. An illusion is proof that you don't always see what you think you do -- because of the way your brain and your entire visual system perceive and interpret an image.
Visual illusions occur due to properties of the visual areas of the brain as they receive and process information. In other words, your perception of an illusion has more to do with how your brain works -- and less to do with the optics of your eye.
An illusion is "a mismatch between the immediate visual impression and the actual properties of the object," said Michael Bach, a vision scientist and professor of neurobiophysics at the University of Freiburg Eye Hospital in Freiburg, Germany, who studies illusions and has a large collection of them on a Web site.
Everything that enters the senses needs to be interpreted through the brain -- and these interpretations occasionally go wrong, Bach told ABCNews.com. Illusions, he said, may serve as a test bed to determine whether scientists understand vision correctly.
Susana Martinez-Conde, director of the laboratory of visual neuroscience at Barrow Neurological Institute in Phoenix, Ariz. offered a similar definition. "An illusion is a phenomenon in which our subjective perception doesn't match the physical reality of the world."
Although people popularly call some brain teasers "optical illusions," this might not be the best term for them, as scientists make a distinction between optical illusions and what they call visual illusions.
An optical illusion suggests that the illusion arises because of some properties of the eye, Bach pointed out.
But since optical illusions are rare, a better and more accurate term is "visual illusions," because this helps to explain why these perceptions happen.
A good example of an optical illusion – one that actually occurs inside the eye – is floaters.
Floaters are small specks, spots or shadowy shapes that seemingly float in your field of vision. To some, they look like a bright white snow or flashes of light.
Floaters are caused by tiny irregularities in the fluid that fills the eye. In other words, they're real. They become more common as one gets older.
But nearly every other illusion happens at the brain level, which is why scientists say they shouldn't be called "optical illusions," and why the term "visual illusions" is more appropriate.
Instead of thinking that you cannot trust your eyes when you see an illusion, you really should be saying, "I cannot always trust my visual system," said Bach. The visual system includes not only the eyes but the optic nerve, which links the eye to the brain; and the primary visual cortex, the area of the brain that processes visual information.
Another example of an illusion is when you "see stars" after a hard blow to the head.
According to Bach, seeing stars results from a mechanical stimulation and activation of the neurons in the eye, which your brain misinterprets as light. Light does not enter the eye when you hit your head, but your visual system perceives it that way.