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The vase-faces illusion.

Returning to the Moon illusion, why does the Moon appear larger on the

horizon than when it’s overhead? The illusion is a variation of the Ponzo illusion,

an illusion associated with linear perspective. Parallel lines, like those associated

with railroad tracks or the sides of a roadway, appear to converge as they

approach the horizon. At the horizon itself they meet, and this is called the vanishing

point. If in a drawing two objects of the same size are simultaneously placed so that the first object is far from the horizon and the second one is near the horizon, the second object will be perceived as being larger than the first one. This is because, in a drawing, the retinal size of both objects is the same. However, the second object seems to be larger than it is in terms of comparisons we automatically make with other objects near the horizon.

Note that in everyday perception the Ponzo illusion does not occur. This is

because the retinal size of an object near the horizon is smaller than that of an

object closer to you. When the size of an image projected on the retina shrinks

with distance, the apparent size of the object remains the same. This is a perceptual

phenomenon called size constancy. For example, an approaching friend first

seen when twenty feet away and then when closer to you appears to be the same

size. However, in the case of the Moon illusion, the size of the Moon’s image pro-

jected on your retina is about the same size when it is near the horizon and when

It’s “far” from it (when it’s overhead). As the Moon orbits our planet, its actual

distance from the Earth doesn’t change significantly. Consequently, the conditions

of the Ponzo illusion are met.

What we learn from illusions is that the world appears to us the way it does

not only because it actually is the way it is. We also interpret sensory information,

transforming it into a constructed perceptual, or psychological, world. And it is

our perception of the world that determines much of our behavior.

Depth Perception: Living in a Three-dimensional World

One of the fascinating questions of perception is this one:Why do we perceive a

world of rounded shapes, of near and far things, of depth instead of a flat world

with one surface? A second, related question is: How is this accomplished?

A given eye’s retina is basically a surface, not a cube. (Although the eye itself

Is a three-dimensional “ball,” the surface of the retina is not.) Think of the information

on the surface of the retina as having some similarity to an oil painting made on a flat canvas. Note that it is possible to perceive depth in a landscape painting made on a flat canvas.

Depth perception is made possible by various cues, signals or stimuli that provide

an observer with information. Depth perception is made possible by cues arising from binocular vision and monocular vision.

Binocular vision is vision with two eyes. The principal cue for depth perception

associated with binocular vision is retinal disparity. The pupils of the eyes are about three inches apart. This gives the right eye a somewhat different view of a scene than the one obtained with the left eye. Notice that although you sense two images, you only perceive one. (This is another example of the difference between sensation and perception.) This is sometimes called the zipper function of the brain, the capacity of the visual portion of the cortex to integrate two images into a meaningful whole. The whole image, in part because of retinal disparity, appears to be three-dimensional.

Monocular vision is vision with one eye. If a person is deprived of binocular