Thursday , December 13 2018


Neverending staircase in sepia tone

An illusion is an inaccurate perception of a stimulus. The term is also broadly used to refer to inaccurate beliefs or perceptions. In scientific usage, however, an illusion is a sensory distortion.


Illusions provide powerful clues about how the brain processes information. Scientifically, they can pose a problem for empirical research as they demonstrate the ways in which even direct observation can be misleading. Most people can be tricked by optical illusions, and scientists can use information about this visual phenomenon to better understand perception and brain organization. Some conditions that affect the brain may also cause illusions. For example, people who experience migraine headaches frequently report seeing auras, which consist of movement or colors along the outer edges of a person’s view.


Illusions can occur with any of the five senses. Examples include:

  • Optical illusions, which may be seen when an image is constructed in such a way that it relays misleading information to the brain. For example, two people of different heights standing on a slanted floor covered in check marks may appear to be standing on a flat floor and thus appear to be the same size.
  • Auditory illusions, which occur when a person hears sounds that are not actually being made or sounds that are distortions of the actual tones. One well-known example is the Shepard tone, which seems to be constantly rising or falling in pitch but is actually doing neither.
  • Tactile illusions, which cause the brain to perceive touch stimuli that is not actually present, or that is not present in the way the brain perceives it. Phantom limb syndrome, or the experience of feeling an amputated limb, is an example of a tactile illusion.
  • Smell and taste illusions, which are not as common as other types of illusions. However, certain people may perceive smells differently than others do, especially when given conflicting information about the stimuli producing the smell. Similar phenomena can occur with taste.


Illusions are different from hallucinations in that hallucinations occur without an external stimuli. Like hallucinations, though, illusions are not necessarily a sign of a psychiatric condition, and anyone might experience them. They can occur for many reasons, such as the effect of light on an object, insufficient sensory information about an object, or errors in an individual’s processing of sensory details. The refraction of light can cause rainbows and mirages, two illusions that are dependent on the atmosphere.

Certain illusions, known as pseudohallucinations, can be signs of a psychiatric disturbance. One may experience a pseudohallucination under conditions of anxiety or fear or when he or she projects their feelings onto external objects or people. People in intensive psychiatric care have been reported to see people around them as monsters or devils, for example. Illusions can also be characteristic of certain mental health conditions, such as schizophrenia.


Synesthesia is a particular type of illusory phenomenon where individuals experience certain sounds as colors. A musician might see green when he or she hears a particular piece of music, for example. Some writers have also reported hearing musical tones when they see a particular word or image. In certain rare and extreme cases, people with synesthesia may become unable to differentiate between seeing and hearing.


  1. American Psychological Association. APA concise dictionary of psychology. Washington, DC: American Psychological Association, 2009. Print.
  2. Colman, A. M. (2006). Oxford dictionary of psychology. New York, NY: Oxford University Press.
  3. Schizophrenia. (2009, January 1). Retrieved from
  4. West, L. (2015, February 5). Tactile illusions. Retrieved from