In most cases, perception occurs automatically and unconsciously. You don't have to focus your attention on interpreting your sensory experience of the world — it simply happens with minimal mental effort on your part. Because this process is largely automatic, errors or mistakes in perception can occur. Optical illusions take place when you perceive optical information in a way that is misleading, false, or not in tune with reality.
There are two major types of optical illusions. Physiological illusions are often the result of overstimulation of the visual system. For example, if you stare for a long period of time at a brightly colored object, you might continue to see an “afterimage” of the object even after you look away. Cognitive illusions occur when you make unconscious cognitive assumptions about how things “ought” to be, sometimes making incorrect inferences about how things are in reality. For example, artist M. C. Escher created a famous lithograph print involving a staircase that appeared to both ascend and descend at the same time. The distance, depth, and edge cues used to perceive the direction of the stairs are incompatible, creating a perceptual contradiction.
Why do optical illusions occur? In many cases, the same rules of perceptual organization that allow you to determine size, distance, and location contribute to these errors. For example, the figure-ground principle allows you to determine which objects are closer or further away, but it also creates the well-known “vases or faces” illusion discussed earlier, in which the image can be perceived as either two faces or as a central vase.
Your expectations also play a role in the experience of optical illusions. Researchers refer to these expectations and prior assumptions as a perceptual set. Of course, what you expect to see in certain situations depends a great deal on factors such as culture and past experience. For example, if you just accidentally drove through a red stoplight, you would probably be more likely to perceive flashing blue lights in your rearview mirror as a police cruiser rather than a neon sign from a local used-car dealership.