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Perceptual Illusions

Page history last edited by Pooja Patel 11 years, 5 months ago


One of the most popular illusions in psychology is Ebbinghaus' Tichtener Circles: Which center circle is bigger, the one on the right or the one on the left?


     The dictionary definition of a perceptual illusion is, "the perception of something objectively existing in such a way as to cause misinterpretation of its actual nature." While this is true, it is not the only definition of an illusion. An illusion is not always a misinterpretation, but rather it convinces us that the real life version of an object is untrue or false. One important factor to remember is that no two different interpretations of an object of picture can be given at one moment. Besides sensory illusions, there are three types of optical illusions we study: literal, physiological, and cognitive.

      We have sensory receptors in our brain that detect light, sound, temperature, etc., and most of these receptors are on the surface of our bodies (i.e. eyes, ears, mouth, etc.). There are, however, internal receptors such as those that register pain. Regardless of the type, each receptor is responsible for detecting its own form of energy which is used to transmit signals to the brain. When the brain receives these signals, most of the time it interprets the signals correctly, but when it doesn't, that is what we call a sensory illusion.

      A literal illusion is an image created by smaller images that are in no way related to the larger, overall image created. This is the simplest type of optical illusion which is why it is often the least studied in psychology. Here is an example of a literal illusion: The smaller images (sailboats) make up the underlying, larger image (the bridge).



     A physiological illusion can occur after prolonged visual stimulation. An afterimage, for example, is a physiological illusion. According to theorists, a viewer's perception may be changed as a result of a physiological imbalance. This imbalance is usually caused by over-active or over-stimulated nerve paths caused by competition between the light and dark receptors in the retina. This competition is essentially what leads to the physiological imbalance. Here is an example of a physiological illusion: Stare at the four black dots in the middle of the image below for thirty seconds, and then look away to a blank (preferably white) wall, and an after-image should appear. What do you see?



     A cognitive illusion can be defined as the viewer's knowledge and assumptions about the world, or unconscious inferences. There are three categories of cognitive illusions: ambiguous illusions, distorting illusions, and paradox, or fiction, illusions. Ambiguous illusions are images or objects that allow for the viewer to have two valid interpretations of what the object represents or what it actually is. The viewer is usually able to mentally visualize one interpretation right away and eventually the second, after some time. However, both interpretations cannot be seen at the same time because that would interfere with the full perception of either one, and the brain simply does not allow that. Perhaps the best example of an ambiguous illusion is the Necker Cube, which is a line-frame drawing of a cube with all of it's edges parallel. When the two lines meet, it is hard to tell whether the corner is coming out or going into the page, which is what makes it an ambiguous figure. Distorting illusions are images or objects that are distorted in their geometrical make (i.e. size, length, position, or curvature). One famous example of a distortion illusion is the Muller-Lyer Illusion, where two separate lines with arrows at either end of each line appear to be of different length because on one line, the arrows point out, while on the other line, the arrows point in. Lastly, a paradox or fiction illusion is an image or object that is simply impossible in 3-D or "real life", however  they look very real and very possible in 2-D representations. One of the best examples of a paradox illusion is the impossible staircase, or the Penrose stairs. As we look at the 2-D picture of the stairs, it seems like a normal set of stairs, however it becomes evident that we are either ascending or descending, but getting nowhere.


                                               Necker Cube                                 Müller-Lyer                                 Penrose Stairs




What is an Illusion? 

This link is an article that discusses the overall concept of an illusion, written by Dr. Block of Hofstra University.


Perceptual Illusions and Brain Models

This is a paper written by Dr. Richard Gregory that describes visual illusions and what role our senses play in creating visual illusions.


Types of Optical Illusions

This article describes in detail the three main types of illusions: physiological, literal, and cognitive.


The Neuroscience of Illusions

This is an article from the magazine Scientific America that describes how tricking the eye can reveal how the brain functions.


Not Just Your Imagination

This article from Science Daily describes how tricking the eye may actually lead to the brain actually perceiving a "moving" illusion as actually moving.


Well-known/Common Illusions

This is an article that details several types of illusions as well as famous illusions used as teaching tools in most psychology classes.


The Importance of Perceptual Illusion Research

This is an article from National Geographic that highlights why it is important to study illusions because they can have various effects on the functioning of the brain.


Is That Even Possible!

This is a link to several optical illusions, some common and some new.


Illusions Galore!

This is a link to more illusions used at the University of Massachusetts.


Perceptual Constancy vs. Perceptual Illusion

This is a link to an article that discusses why our retina projects an image differently than what it actually is. Why do illusions occur?




*This page was developed by Pooja Patel

**To my knowledge, all images, links, and text used on this page are public. If there are any copyright issues, please email me at: Pooja.V.Patel@live.mercer.edu

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