What we experience shapes our reality. “Seeing is believing,” right? Not necessarily. Visual illusions can distort our perception so that what we “see” does not correspond with what is physically there. This special edition of Scientific American Mind explores the world of sensory illusions and delves into how they fool the brain.

The word “illusion” derives from the Latin illudere, “to mock,” whereas the word itself has roots in 14th-century Anglo-French, meaning an act of deception. That's because our brain—not our eyes—is the final arbiter of “truth.” We are wired to analyze the constant flood of information from our senses and organize that input into a rational interpretation of our world. Much of the time our brain decodes those signals correctly. But illusions derail the process—although our sensations may seem to be accurate, our perceptions are not. Some illusory adaptations help us to survive, such as being able to recognize the same object outside in bright sun or in a dimly lit room, despite the difference in brilliance or wavelength of the light source. Others just trick us. Illusions are both intriguing and fun; our deep fascination with them dates back to the ancient Greeks. For the past century, however, they have also played an important role in brain research.

Illusions help neuroscientists unravel the mysteries of how the human brain creates “reality.” In the opening article of this special issue, Susana Martinez-Conde and Stephen L. Macknik write, “It is a fact of neuroscience that everything we experience is a figment of our imagination.” Understanding how the brain “misperceives” sensory input and influences perception is at the heart of their research in their respective roles as directors of the Laboratory of Visual Neuroscience and of the Laboratory of Behavioral Neurophysiology at the Barrow Neurological Institute in Phoenix. In the articles that fill these pages, Martinez-Conde and Macknik will show you how your brain constructs 3-D images, how context can alter the way you gauge size and perceive color, perspective or pain—and share illusions that make objects appear, disappear or move.

If you want more eye candy, click through the past winners of the Best Illusion of the Year Contest. Martinez-Conde and Macknik run the Neural Correlate Society, which hosts the annual competition (illusionoftheyear.com). This year's event, sponsored by the Mind Science Foundation and Scientific American, was held in May at the Philharmonic Center for the Arts in Naples, Fla. And don't forget to notice the biggest illusion of all: the world around you.