Illusions are experiences of perception in which one perceives stimuli incorrectly, not completely, or in distorted way. Some are caused by external factors (unrelated to the receiver's sensory organs and brain), as in the case of the “Doppler effect”, when one hears, say, the siren of an ambulance change its pitch as it passes by when, in reality, the pitch stays the same. The ears and brain of the receiver function normally, but this function is impeded by external natural factors like distance, interference of elements in the air, wind and its direction, etc. Illusions of this kind are called stimulus-distortion illusions. Other illusions are more receiver-related, and are referred to as perceiver-distortion illusions. These are caused by the way in which the human brain functions and, in particular, its tendency to group similar objects together as well as to “complete” unfinished objects. The latter function is what makes watching a movie possible, since it “blocks” perception of the intervals between projected stills and makes them look like a “moving” picture.

Researchers have designed various optical perceiver-distortion illusions. Some of the best known are the “figure and ground” illusion, occurring when looking at a white illustration on a solid black background; the Müller-Lyer illusion, where two equally long straight lines feature inward and outward arrow-shaped ends which make them look unequal; the Zöllner illusion, in which long parallel lines look as if they tend to intersect by means of other, shorter lines which do intersect them, and the Ponzo illusion, where two equal circles are placed inside an acute angle, resulting in the circle located closer to the angle's top giving the impression of being larger.