What are Illusions?

Mostly described as “misinterpretations of a sensory stimulus”, illusions are thought to occur when stimuli from a perceived object are combined with mental images to produce a false perception.

Illusions are altered perceptions in which a real external object is combined with imagery to produce a false internal percept.

Thus, illusions are misperceptions of real external stimuli, e.g. in a dark room, a dressing gown hanging on a bedroom wall is perceived as a person.

Illusions are associated with inattention when external sensory stimuli are meager or when attention is impaired due to delirium Opens in new window.

Illusions are also associated with prevailing affect, thus shadows may appear like human figures to a frightened individual.

Illusions almost always disappear when sensory stimuli increase or when attention improves.

Both lowered attention and heightened affect will predispose to experiencing illusions.

Illusions often occur in healthy people and are usually associated with inattention or strong emotion. The following are common types:

  • Affect illusions, which occur at times of heightened emotion (e.g. while walking through a dangerous area late at night a person may see a tree blowing in the wind as an attacker lunging at them).
  • Completion illusions rely on our brain’s tendency to ‘fill-in’ presumed missing parts of an object to produce a meaningful percept and are the basis for many types of optical illusion. Completion illusions and affect illusions (discussed earlier) resolve on closer attention.
  • Pareidolic illusions are meaningful percepts produced when experiencing a poorly defined stimulus (e.g. seeing faces in a fire or in clouds).

Illusions lack diagnostic significance and are different from hallucination Opens in new window by the presence of a stimulus.

    Adapted from:
  1. Oxford Handbook of Psychiatry By David Semple, Roger Smyth
  2. Crash Course Psychiatry - E-Book By Katie FM Marwick, Steven Birrell
  3. Essentials of Psychiatry By Dr Sandeep K Goyal