Visual Illusions
"Whilst part of what we perceive comes through our senses from the object before us, another part (and it may be the larger part) always comes out of our own mind." -- William James
Most of us take vision for granted. We seem to do it so effortlessly; however, perceiving images, objects, colour, and motion is a very complicated process.
Take a moment to observe the world around you. For example, if you tilt your head, the world doesn't tilt. If you shut one eye, you don't immediately lose depth perception. Look at what happens to colour under varying types of illumination. Move around an object: The shape you see changes, yet the object remains stable. The illusions we have been looking at tell us that even though you may intellectually know that you are being fooled, it does not stop the effect from continuing to trick you. This indicates a split between your perception of something and your conception of it. In many cases your higher order cognitive abilities cannot influence your lower order perceptions.
Only in the last one hundred years, and especially in the last twenty years, have scientists started to make some progress in understanding vision and perception. Illusions can be a wonderful window into this process. And they are fun too! Because they combine both the element of joy with the element of surprise.
The late, great physicist Richard Feynman wrote, "It's quite wonderful that we can 'see,' or figure it out so easily. Someone who's standing at my left can see somebody who's standing at my right - that is, the light can be going this way across, or that way across, or this way up, or that way down; it's a complete network. Some quantity is shaking about, in a combination of motions so elaborate and complicated the net result is to produce an influence which makes me see you, completely undisturbed by the fact that at the same time there are influences that represent the guy on my left side seeing the guy on my right side. The light's there anyway....it bounces off this, and it bounces off that - all this is going on, and yet we can sort it out with this instrument, our eye."
This is not the end of this wonderful process. Light waves enter your eye and then enter photoreceptive cells on your retina. The image that forms on your retina is flat, yet you perceive a world of shape, colour, depth, and motion.
How does our visual system recover three-dimensional information? This is an important question. Our retinal images, whether from a two-dimensional image or from the three-dimensional world, are flat representations on a curved surface. Yet, for the most part, we perceive an accurate world of depth, surfaces and objects.
A closely related problem is that any one aspect of a visual scene is spatially ambiguous. There is an innate ambiguity in the retinal input (many to many mappings between objects and retinal images). In other words, for any given retinal image, there are an infinite variety of possible three-dimensional structures that can give rise to it. Our visual system, however, usually settles for the correct interpretation. When a mistake is made, an illusion occurs.
It is impossible to recover accurate three-dimensional information from a visually ambiguous two-dimensional representation which means that some very powerful constraints must be imposed on our interpretations of two-dimensional images.
These constraints must also account for many illusions. In fact, illusions are a powerful and fun tool for revealing constraints that mediate vision and perception. In some cases, illusions take place because the constraints for interpreting an image are ambiguous. Your visual system can interpret the scene in more than one way. Even though the image on your retina remains constant, you never see an odd mixture of the two perceptions - it is always one or the other, although they may perceptually flip back and forth. Normally, this does not happen in the real world, as your visual system has evolved many different ways to resolve ambiguity. Visual perception is essentially an ambiguity-solving process. This process is called "inverse optics".
The early visual process behaves intelligently, but mostly in a bottom-up fashion (separated from cognitive processes). The visual system is also highly adaptive, e.g., visual adaptation is not merely fatigue. It should be understood that both evolution and learning contribute to visual capabilities.
Artists have also been trying to understand how we perceive, and much of our understanding of vision comes from learning how artists manipulate images into meaningful and realistic scenes. Artists have always created illusions. That's their business.
Artists and scientists over the years have experimented with these rules to produce illusions either by reducing the number of visual cues for interpreting images or by deliberately setting up situations where the rules come into conflict.
Adapted from http://www.illusionworks.com/