Our perceptual interpretations of visual information are usually correct. For example, on the basis of changes in color or texture, we can conclude that a dog is on the rug. On the basis of a continuous increase in size, we conclude that a train is coming toward us. Sometimes, though, our interpretations or inferences are wrong. The result is a visual illusion, a mismatch between reality and our perception of it. More than two hundred different types of illusions have been discovered.

One of the most famous visual illusions is the Müller-Lyer illusion. Although line b looks longer than line a, the two horizontal lines are exactly the same length.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072937629/41654/0530.jpg','popWin', 'width=690,height=550,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (11.0K)</a>

A similar illusion is the horizontal-vertical illusion, in which a vertical line looks longer than a horizontal line even though the two are equal.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072937629/41654/0531.jpg','popWin', 'width=690,height=550,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (9.0K)</a>

In the Ponzo illusion, the top horizontal line looks much longer than the bottom horizontal line.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072937629/41654/0532.jpg','popWin', 'width=690,height=550,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (19.0K)</a>

Why do these illusions trick us? One reason is that we mistakenly use certain cues for maintaining size constancy. For example, in the Ponzo illusion the upper line spans more lines (even though those lines are closer together at the top). The Müller-Lyer illusion, though, is not so easily explained. We might make judgments about the lines by comparing incorrect parts of the figures. For example, when the wings are a different color than the horizontal lines in the Müller-Lyer illusion, the illusion is greatly reduced (Coren & Girus, 1972). Cultural experiences also influence an individual's perception of the Müller-Lyer illusion.

Another well-known illusion is the moon illusion. The moon is 2,000 miles in diameter and 289,000 miles away. Because both the moon's size and its distance from us are beyond our own experience, we have difficulty judging just how far away it really is. When the moon is high in the sky, directly above us, little information is present to help us judge its distance. But when the moon is on the horizon, we can judge its distance in relation to familiar objects, such as trees and buildings, which makes it appear farther away. The result is that we estimate the size of the moon as much larger when it is on the horizon than when it is overhead.

The devil's tuning fork is another fascinating illusion. Look at the diagram for about 30 seconds, and then try to draw the tuning fork without looking back at the diagram. You will undoubtedly find this to be a difficult, if not impossible, task. Why? Look carefully at the figure again. You'll see that the figure's depth cues are ambiguous.

<a onClick="window.open('/olcweb/cgi/pluginpop.cgi?it=jpg:: ::/sites/dl/free/0072937629/41654/0534.jpg','popWin', 'width=690,height=550,resizable,scrollbars');" href="#"><img valign="absmiddle" height="16" width="16" border="0" src="/olcweb/styles/shared/linkicons/image.gif"> (14.0K)</a>

1		A discrepancy or incongruency between reality and the perception of it is a .
2		What is a visual (or optical) illusion? Describe the perceptual experiences you have at http://dragon.uml.edu/psych/illusion.html in terms of shape perception, depth perception, motion perception, and perceptual constancy.