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Perceptual Constancy

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Perceptual Constancy
117
Organizing the Perceptual World
as well as height and width, and that is located at a particular distance. Three-dimensional
movies and some virtual reality systems use binocular disparity cues to create the
appearance of depth in a two-dimensional stimulus. They show each eye an image of
a scene as viewed from a slightly different angle.
In short, many cues—some present in the environment and in retinal images, others arising from the structure of the visual system—combine to give us a powerful and
accurate sense of depth and distance.
Perception of Motion
A CASE OF DEPTH MISPERCEPTION
The runner in this photo is actually farther away than the man on the pitcher’s
mound. But because he is lower, not
higher, in the visual field—and because
his leg can be seen as in front of, not behind, the pitcher’s leg—the runner appears smaller than normal rather than
farther away.
Sometimes the most important property of an object is its motion—how fast it is going
and where it is heading. Many of the cues about motion come from optical flow, or the
changes in retinal images across the entire visual field. As in the case of depth perception,
you automatically translate this two-dimensional information into a three-dimensional
experience. One particularly meaningful pattern of optical flow is known as looming,
the rapid expansion in the size of an image so that it fills the retina. When an image
looms, there is an automatic tendency to perceive it as an approaching object. If the
expansion is as fast to the right as to the left, and as fast above as below, this information signals that the object is directly approaching the eyes. In other words: Duck!
We are lucky that movement of the retinal image is not the only factor contributing to motion perception. If it were, everything in sight would appear to move every
time you moved your eyes and head (Ölveczky, Baccus, & Meister, 2003). This does not
happen, because as noted earlier, the brain receives and processes information about
the motion of the eyes and head (Wexler, 2005). If you look around the room
learn right now, tables, chairs, and other stationary objects will not appear to
by
doing move, because your brain determines that all the movement of images on
your retinas is due to your eye and head movements (Goltz et al., 2003). But now close
one eye, and wiggle your open eyeball by gently pushing your lower eyelid. Because
your brain receives no signals that your eye is being moved by its own muscles, everything in the room will appear to move.
When your body is moving, as in a car, the flow of visual information across the
retina combines with information from the vestibular and touch senses to give you the
experience of being in motion. So if the car accelerates, you feel pressure from the back
of the seat and feel your head tilting backward. If visual flow is perceived without
appropriate sensations from other parts of the body, particularly the vestibular senses,
motion sickness may result. This explains why you might feel nauseous while in a
motion simulator or playing certain video games, especially those with virtual reality
technology. The images suggest that you are moving through space when there is no
real motion.
Other illusions of motion are more enjoyable. The most important of these occurs
when still images appear, one at a time, in rapid succession, as they do on films, videos,
and DVDs. Because each image differs slightly from the preceding one, the brain sees
the objects in each image at one location for only a fraction of a second before they
disappear and immediately reappear in a slightly different location. The entertaining
result is the illusion of stroboscopic motion; when objects disappear and then quickly
reappear nearby, the brain assumes that they have moved smoothly from one location
to another. The same illusion is at work when it appears that flashing lights on a theater or casino sign are moving around the sign.
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Perceptual Constancy
looming
A motion cue whereby rapid
expansion in the size of an image fills
the available space on the retina.
stroboscopic motion An illusion in
which lights or images flashed in rapid
succession are perceived as moving.
Suppose that one sunny day you are watching someone walking toward you along a treelined path. The visual sensations produced by this person are actually very strange. For
one thing, the size of the image on your retinas keeps getting larger as the person gets
closer. To see this for yourself, hold out a hand at arm’s length and look at
learn someone far away. The retinal image of that person will be so small that you
by
doing can cover it with your hand. If you do the same thing when the person is three
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118
doing
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learn
by
Chapter 3 Sensation and Perception
BINOCULAR DISPARITY
AND DISTANCE There is a
smaller difference, or disparity,
between each eye’s view of an object
when the object is far away than when it
is close by. The amount of this binocular
disparity helps us estimate the object’s
distance from us. To see for yourself how
binocular disparity changes with distance, hold a pencil vertically about six
inches in front of your nose; then close
one eye and notice where the pencil is in
relation to the background. Now open
that eye, close the other one, and notice
how much the pencil “shifts.” These are
the two different views your eyes have
of the pencil. Repeat this procedure while
holding the pencil at arm’s length. Notice
that there is now less disparity or “shift,”
because there is less difference in the angles from which your two eyes see the
pencil.
feet away, the retinal image will be too large to be covered by your hand, yet you perceive
the person as being closer now, not bigger. Similarly, as you watch the person pass from
bright sunshine through the shadows of trees, your retinas receive images that shift back
and forth from dark to light. Still, you perceive the person’s coloring as staying the same.
These examples illustrate perceptual constancy, the perception that objects keep
their size, shape, color, and other properties despite changes in their retinal image.
Without this aspect of perception, you would experience the world as a place in which
solid objects continuously changed their properties.
Size Constancy Why does an object’s perceived size stay more or less constant,
regardless of changes in the size of its retinal image? One reason is that the brain perceives a change in the distance of an object and automatically adjusts the perception
of size. Specifically, the perceived size of an object is equal to the size of the retinal image
multiplied by the perceived distance (Holway & Boring, 1941). As an object moves
closer, the size of its retinal image increases, but the perceived distance decreases at the
same rate. So the perceived size remains constant. If a balloon is inflated in front of
your eyes, perceived distance remains constant, and the perceived size (correctly)
increases as the size of the retinal image increases.
The principles behind shape constancy are closely related to
those of size constancy. To see shape constancy at work, remember what page you are
close this book, and tilt it toward and away from you several times. The
learn on,
book will continue to look rectangular, even though the shape of its retinal
by
doing image changes dramatically as you move it. Your brain automatically combines information about retinal images and distance as movement occurs. In this case,
the distance information has to do with the difference in distance between the near and
far edges of the book.
Shape Constancy
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Even with dramatic changes in the amount of light striking an object, the object’s perceived brightness remains relatively constant. To see this
for yourself, place a piece of charcoal in sunlight and a piece of white paper
learn in nearby shade. The charcoal will look very dark and the paper very bright,
by
doing yet a light meter would tell you that much more light energy is reflected from
the sun-bathed coal than from the shaded paper. The reason is partly that the charcoal is
Brightness Constancy
perceptual constancy The perception
of objects as retaining the same size,
shape, color, and other properties despite changes in their retinal image.
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