Illusion

An illusion is a distortion of a sensory perception, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people. Illusions may occur with more of the human senses than vision, but visual illusions, optical illusions, are the most well known and understood. The emphasis on visual illusions occurs because vision often dominates the other senses. For example, individuals watching a ventriloquist will perceive the voice is coming from the dummy since they are able to see the dummy mouth the words. Some illusions are based on general assumptions the brain makes during perception. These assumptions are made using organizational principles, like Gestalt, an individual's ability of depth perception and motion perception, and perceptual constancy. Other illusions occur because of biological sensory structures within the human body or conditions outside of the body within one’s physical environment.



Perceptual constancies
Perceptual constancies are sources of many illusions. Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light reflecting from it. An illusion of color difference can be created, however, when the luminosity of the area surrounding an unfamiliar object is changed. The color of the object will appear darker against a black field which reflects less light compared to a white field even though the object itself did not change in color. Like color, the brain has the ability to understand familiar objects as having a consistent shape or size. For example a door is perceived as rectangle regardless as to how the image may change on the retina as the door is opened and closed. Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The Shepard illusion of the changing table is an example of an illusion based on distortions in shape constancy.

Biological approach
Vision
The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, where in the receptive field of the retina light and dark receptors compete with one another to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands. Once a receptor is active it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion the grey spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surround.
Other senses
Illusions can occur with the other senses including that of taste, smell and touch. It was discovered that even if some portion of the taste receptor on the tongue became damaged that illusory taste could be produced by tactile stimulation.
Examples of Touch illusions include Phantom limb, the Thermal grill illusion, and the tactile illusion which occurs when the middle finger is crossed over the pointer finger and the fingers are ran along the bridge of the nose to the tip with one finger on each side of the nose . In this illusion two “noses” are felt at the tip. Interestingly, with Touch illusions similar brain sights are activated during illusory stimulation as actual stimulation.
Disorders
Some illusions occur as result of an illness or a disorder. While these types of illusions are not shared with everyone they are typical of each condition. For example migraine suffers often report Fortification illusions….


Physical approach
Mirages are optical distortions through the atmosphere that may be photographed. While the perceived reality (such as water in the desert) is illusory, the visual image (of a reflective surface) is real.
Rainbows
Antisolar rays
Reflection
Refraction
Illusion in art and magic
Stage magic is a popular form of entertainment based on illusion. Magicians use tricks to give their audiences the impression that seemingly impossible events have occurred. See magic (illusion).
In fantasy works, actual magic may work by affecting the senses or producing an image, rather than producing a real change; this magic is frequently called illusion to distinguish it from more substantive forms of magic.
Mimes are known for a repertoire of illusions that are created by physical means. The mime artist creates an illusion of acting upon or being acted upon an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls", "climbing stairs", "leaning", "descending ladders", "pulling and pushing". Amongst mimes, these illusions are sometimes referred to as pantomime

In psychiatry and philosophy the term illusion refers to a specific form of sensory distortion. Unlike a hallucination, which is a sensory experience in the absence of a stimulus, an illusion describes a misinterpretation of a true sensation so it is perceived in a distorted manner. For example, hearing voices regardless of the environment would be a hallucination, whereas hearing voices in the sound of running water (or other auditory source) would be an illusion.

Some well-known illusion

Gravity Hill
A gravity hill, also known as a magnetic hill (and sometimes a mystery hill or a gravity road), is a place where the layout of the surrounding land produces the optical illusion that a very slight downhill slope appears to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill. There are hundreds of known gravity hill locations around the world.

Café wall illusion

The café wall illusion
The café wall illusion is an optical illusion, first described by Doctor Richard Gregory. He observed this curious effect in the tiles of the wall of a café at the bottom of St Michael's Hill, Bristol.
This optical illusion makes the parallel straight horizontal lines appear to be bent.
To construct the illusion, alternating light and dark "bricks" are laid in staggered rows. It is essential for the illusion that each "brick" is surrounded by a layer of "mortar" (the grey in the image). This should ideally be of a colour in between the dark and light colour of the "bricks"

Some Exercises

#Concentrate on center point and move your head forward and backward!!!!! What do you observe?????

#The gray strip in the center is the same color throughout!!!!!