Visual Illusions

Synonyms

Optical Illusions; Mirages; Hallucinations

Definition

Illusion is the difference between the perception of a stimulus and the actual stimulus. Illusions are either adaptations or byproducts of differential selection on information processing mechanisms.

Introduction

An illusion is the difference between an observer’s perception of a stimulus and the objectively measurable qualities of that stimulus in the real world. Classic examples of illusions include the moon illusion, where the moon looks larger when it is at the horizon than when it is high in the sky (see Fig. 1). Other classic illusions include the Ponzo illusion, where objects located near the convergence of distant, seemingly parallel lines look larger than the same objects located at the near, diverging end of those lines (see Fig. 1). We typically describe phenomena as illusory only when they occur across individuals in a systematic fashion. There is often no clear or meaningful way to distinguish such phenomena...