Types of Illusions

Optical Illusions

An optical or visual illusion is a kind of illusion in which the images perceived through the sense of sight tend to be misleading or deceptive, causing errors in perception. An optical illusion is based on the process through which the brain creates a visual world in one's mind using either or both these two sources: previous memory stored in it and the current presentation of the object in the environment.

In order for perception to occur, the brain tries to organize sensory information related to the object as gathered by the eye. This leads to the formation of a percept. If there are any gaps once the percept is created, the brain attempts to fill in such gaps. However, the percept may not represent or interpret the real, physical measurement of the stimulus. Thus, an optical illusion emerges. Here is a list of some amazing optical illusions:

1. Blivet – an undecipherable figure
2. Bezold effect – a color seems different due to its adjacent colors
3. Ebbinghaus illusion - an illusion related to relative size perception
4. Hermann Grid Illusion – ghost-like grey blobs appear in the middle of the black squares on a white background
5. Necker cube – an impossible cube with edges that are apparently solid beams

Auditory Illusions

While optical illusions deceive the eyes through visual images, auditory illusions mislead the ears through sounds. These sounds are usually those that are not really present in the physical stimulus, but is heard by the ears and perceived as a sound related to the stimulus in the environment. There are also auditory illusions that come from "impossible sounds", such as hearing a missing fundamental frequency, provided that there are other portions of the harmonic series, and different psychoacoustic tricks of lossy audio compression.

Tactile Illusion

While optical and auditory illusions are common manifestations of several psychological disorders such as schizophrenia and psychosis, tactile illusion is experienced by patients who have undergone amputation. The phantom limb is a tactile illusion wherein the patient still 'feels' pain on the leg, arm, or digit that has already been removed.