Visual Illusions

Introduction To Illusions

Whether you know it or not, visual illusions are occurring around you all the time. Visual illusions are often present in nature and appear when you least expect it. For instance, have you ever placed a stick in water and wondered why the part in the water appeared to be bent? Have you ever remembered a time when you watched a car's tires spinning and they appeared to spin in the opposite direction than the car was moving? These are just a few examples of illusions that occur in the world around us. But why do they happen?

Ancient Greek philosophers asked just that question. By about 500BC, the Greeks had already formed two opposing theories to explain the phenomenon of the visual illusion:

1. Sensory inputs are variable and inaccurate. The mind is used to correct these inaccuracies and;

2. The senses are inherently accurate and it is the mind that is limited (Coren, 1978).

Aristotle contemplated both of these viewpoints and decided that both were correct in their own way. He argued that some perceptual qualities were immediately and accurately perceived by our senses (Coren, 1978). He proposed that "each sense has one kind of object which it discerns and never errs when reporting what [it] is" (Coren, 1978). However, he also argued that qualities such as movement, number, figure qualities and magnitude were not exclusive properties of any one "sense" but common to all (Coren, 1978). Aristotle felt that these qualities could only be properly perceived through the mind and not through the senses. Remarkably, Aristotle's view of illusions survived until the nineteenth century, at which time scientists became interested in the study of illusions. This interest in illusions has continued to the present day.

Recently, perceptual theories have been used to explain illusions. Cue theory developed from studying perceptual illusions. This theory viewed perceptions as the product of incoming sensory information and top-down processing from the brain (Allard, 2001). Soon scientists such as Gibson (1950) noticed that cue theory could not describe how people reacted to certain real-life situations. Gibson created direct perception theory to explain these real-life situations. Direct perception stated that all information comes to us from our senses and does not need processing from the higher centers of the brain (Allard, 2001). The contradiction between the two theories caused scientists to look outside the field of behaviour sciences to neuroanatomy (Allard, 2001). Studies in neuroanatomy found evidence for two visual processing systems: the dorsal and ventral processing streams. The dorsal stream seemed responsible for the unconscious control of on-going movement (Allard, 2001). This stream seemed to best correlate to the direct perception theory. The ventral stream was responsible for the conscious processing involved in the identification of objects (Allard, 2001). This stream seemed to correspond to cue theory and was the stream most easily deceived by visual illusions. The current debate regarding these two streams questions whether the dorsal stream can also be fooled by visual illusions.

Illusions have often been considered to be negative phenomenon. The word illusion, itself, comes from the Latin root word, "illudere" which means "to mock" (Coren, 1978). Richard Gregory in his book "Eye and Brain" (1997) feels that people are too harsh in judging sensory and perceptual systems. He describes the intricate workings of re-constructing the visual world from the image presented to us on the retina:

We are so familiar with seeing, that it takes a leap of imagination to realise that there are problems to be solved. But consider it. We are given tiny distorted upside-down images in the eyes, and we see separate solid objects in surrounding space. From the patterns of stimulation on the retinas we perceive the world of objects, and this is nothing short of a miracle (Gregory, 1997).

Gregory (1997) also explains that the act of perceiving is a dynamic process. Perception is the brain's search for the best interpretation of the data that is being presented. Sometimes the "perceptual hypothesis" is incorrect and an illusion results (Gregory, 1997).

The next time you look at a visual illusion do not think of it as a phenomenon that points to the inadequacies of our visual and perceptual systems. Think of an illusion as a phenomenon that allows us to become consciously aware of the intricate process that is always unfolding behind the scenes. The visual world that we perceive is always being created by an active mind continuously searching for patterns and interpretations out of the sensory data it is receiving.

Cue Theory

Direct Perception

Examples

Click To View Full Size Illusion

Links

Written By:
Christine Cheung
Janice Douglas
Damien Marion
Matt Preston
Heather Williams


Assisted By:
J. Dunkley

References

University of Waterloo Homepage