Illusions

Illusions are perceptions that misrepresent physical stimuli. They occur when sensory input is real, but the brain interprets it in ways that differ from objective reality. Unlike hallucinations, which involve perceiving something that is not there, illusions distort real sensations and often do so in predictable, explainable patterns.

Visual illusions are the best known. Classic examples include the Müller-Lyer illusion, where line segments with arrow-like tails appear different in length; the Ponzo illusion, where perspective cues affect size judgments; and the Ebbinghaus circle illusion, where surrounding circles alter the perceived size of a central circle. Illusions arise from interactions between early sensory processing and higher-level interpretation, and they can be influenced by attention, lighting, context, and cultural experience. They illustrate that perception is constructive rather than a direct readout of the world.

Illusions are not limited to vision. Auditory illusions, such as the missing fundamental and the use of Shepard tones, create sounds that can seem to sustain or rise in pitch despite the actual frequencies. Tactile and motion illusions also occur, revealing how the nervous system integrates time, space, and prior expectations to shape perception. Studying these effects helps map neural pathways and understand how context shapes experience.

Illusions have applications in psychology, art, design, and technology. They highlight the distinction between sensation and perception, show how expectations influence judgment, and provide practical insights for fields such as user interface design, virtual reality, and clinical assessment.