It is a fact of neuroscience that everything we experience is a figment of our imagination. Although our sensations feel accurate and truthful, they do not necessarily reproduce the physical reality of the outside world. Of course, many experiences in daily life reflect the physical stimuli that send signals to the brain. But the same neural machinery that interprets inputs from our eyes, ears and other sensory organs is also responsible for our dreams, delusions and failings of memory. In other words, the real and the imagined share a physical source in the brain. So take a lesson from Socrates: “All I know is that I know nothing.”

One of the most important tools used by neuroscientists to understand how the brain creates its sense of reality is the visual illusion. Historically, artists as well as researchers have used illusions to gain insights into the inner workings of the visual system. Long before scientists were studying the properties of neurons, artists had devised a series of techniques to deceive the brain into thinking that a flat canvas was three-dimensional or that a series of brushstrokes was indeed a still life.

Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event. When we experience a visual illusion, we may see something that is not there or fail to see something that is there. Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. By studying these failings, we can learn about the computational methods used by the brain to construct visual experience.

Brightness, color, shading, eye movement and other factors can have powerful effects on what we “see.” In this series of images, we showcase several basic categories of visual illusions and what they can teach us about perception in the brain.