ILLUSIONSILLUSIONS

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What is an illusion?What is an illusion? Illusion of lengthIllusion of length Illusion of shapeIllusion of shape Illusion of sizeIllusion of size Illusion of touch Illusion of touch Illusion of temperatureIllusion of temperature

What is an illusion?What is an illusion?

Illusion, a mistake in the perception of a Illusion, a mistake in the perception of a sensory experience. An illusion occurs sensory experience. An illusion occurs when what the brain perceives differs when what the brain perceives differs substantially from the actual qualities of an substantially from the actual qualities of an object or stimulus. Illusions may occur in object or stimulus. Illusions may occur in any of the human senses, including vision, any of the human senses, including vision, hearing, touch, smell, and taste. hearing, touch, smell, and taste.

However, errors in perception are only However, errors in perception are only considered illusions if they are considered illusions if they are experienced by a large number of people. experienced by a large number of people. For example, if you are the only person For example, if you are the only person who misreads a word, that is not an who misreads a word, that is not an illusion. But if a large number of people illusion. But if a large number of people misread the word in exactly the same way, misread the word in exactly the same way, then it may be considered an illusion.then it may be considered an illusion.

Illusion of lengthIllusion of length

The first illusion is probably the most The first illusion is probably the most famous and most studied illusion in famous and most studied illusion in psychology—the psychology—the Müller-Lyer illusion,Müller-Lyer illusion, created by German psychiatrist Franz created by German psychiatrist Franz Müller-Lyer in 1889.Müller-Lyer in 1889.

The second illusion is called the The second illusion is called the Ponzo Ponzo illusion,illusion, named after Italian psychologist named after Italian psychologist Mario Ponzo. Mario Ponzo.

Explanation to Explanation to Müller-Lyer illusionMüller-Lyer illusion:: Because we take distance into account Because we take distance into account when judging size, if two objects project when judging size, if two objects project the same image size on the retinas of our the same image size on the retinas of our eyes, we interpret the object that seems eyes, we interpret the object that seems farther away as larger.farther away as larger.

The explanation for The explanation for second second illusion also illusion also involves depth perception. You interpret involves depth perception. You interpret the oblique lines as indicating depth, which the oblique lines as indicating depth, which leads you to perceive the upper horizontal leads you to perceive the upper horizontal line as farther away than the bottom line. line as farther away than the bottom line. Although the images formed on your Although the images formed on your retinas by the two horizontal lines are retinas by the two horizontal lines are actually equal in length, you perceive the actually equal in length, you perceive the upper line as longer because you think upper line as longer because you think that it is farther away.that it is farther away.

Illusion of shapeIllusion of shape

Look at the illustration entitled “Zöllner Look at the illustration entitled “Zöllner illusion.” In this example, a perfect square illusion.” In this example, a perfect square appears to be trapezoidal—that is, wider appears to be trapezoidal—that is, wider at the top than at the bottom—because of at the top than at the bottom—because of the background against which it is the background against which it is perceived. Why? Once again, the illusion perceived. Why? Once again, the illusion arises because the oblique lines act as a arises because the oblique lines act as a cue to depth.cue to depth.

Illusion of sizeIllusion of size

This unusual room is called the This unusual room is called the

Ames roomAmes room after American after American ophthalmologist Adelbert Ames, who ophthalmologist Adelbert Ames, who first constructed such a room in the first constructed such a room in the 1940s.1940s.

The room itself is constructed in The room itself is constructed in such a way that it provides such a way that it provides misleading cues about distance. misleading cues about distance. The left corner of the room is The left corner of the room is actually farther away from the actually farther away from the viewer than the right corner of the viewer than the right corner of the room. In addition, the right side of room. In addition, the right side of the room is elevated so that the the room is elevated so that the feet of both people appear at feet of both people appear at roughly the same relative height roughly the same relative height on the visual field. on the visual field.

Illusion of touchIllusion of touchHold your right hand in front of you Hold your right hand in front of you

with all the fingers next to one with all the fingers next to one another. With your eyes closed, run a another. With your eyes closed, run a pencil along the groove where the pencil along the groove where the index and middle fingers meet. Note index and middle fingers meet. Note that you will perceive a single touch of that you will perceive a single touch of the pencil. Now, cross your index the pencil. Now, cross your index finger over your middle finger. Once finger over your middle finger. Once again, close your eyes and run the again, close your eyes and run the pencil between the two fingers. Now pencil between the two fingers. Now you should experience two separate you should experience two separate touch perceptions. (by Aristotle)touch perceptions. (by Aristotle)

Presumably, this illusion arises Presumably, this illusion arises because the touch receptors of the because the touch receptors of the hand allow us to link the input from hand allow us to link the input from the right side of the index finger and the right side of the index finger and the left side of the middle finger. the left side of the middle finger. However, when the fingers are However, when the fingers are crossed, we cannot link the input from crossed, we cannot link the input from the right side of the middle finger with the right side of the middle finger with the left side of the index finger. Thus, the left side of the index finger. Thus, we perceive two separate touches.we perceive two separate touches.

Illusion of temperatureIllusion of temperature

A person puts one hand in a bowl of A person puts one hand in a bowl of cold water and the other hand in a cold water and the other hand in a bowl of hot water, and allows both bowl of hot water, and allows both hands to adjust until the sensation of hands to adjust until the sensation of temperature disappears. temperature disappears.

If the person then plunges both hands If the person then plunges both hands into lukewarm water, the hand that into lukewarm water, the hand that had adapted to the hot water feels had adapted to the hot water feels cold in a lukewarm water, whereas cold in a lukewarm water, whereas the hand that had adapted to the cold the hand that had adapted to the cold water feels warm in the same water. water feels warm in the same water. This shows that the skin’s perception This shows that the skin’s perception to temperature is relative, but not to temperature is relative, but not absolute. (by John Locke)absolute. (by John Locke)

ConConclusionclusionTThere still are disagreements on what here still are disagreements on what

exactly causes different illusionsexactly causes different illusions..TThis field still needs researches. his field still needs researches. Explanation of illusions lead to Explanation of illusions lead to

understanding perception.understanding perception.

ReferencesReferences

Contributed By:Contributed By:Hugh J. FoleyHugh J. Foley

Microsoft ® Encarta ® Encyclopedia Microsoft ® Encarta ® Encyclopedia 2004.2004.

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