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Perceptual constancy
is a phenomenon
in which physical objects are perceived as unvarying and consistent despite changes
in their appearance or in the physical environment. Perceptual constancy leads us to
view objects as having an unvarying size, shape, color, and brightness, even if the
image on our retina varies. For example, despite the varying images on the retina as
an airplane approaches, fl ies overhead, and disappears, we do not perceive the air-
plane as changing shape (Garrigan & Kellman, 2008; Redding, 2002; Wickelgren, 2004).
In some cases, though, our application of perceptual constancy can mislead us.
One good example of this involves the rising moon. When the moon fi rst appears at
night, close to the horizon, it seems to be huge—much larger than when it is high
perceptual constancy
The phenom-
enon in which physical objects are
perceived as unvarying and consistent
despite changes in their appearance or
in the physical environment .
When the moon is near the horizon, we
do not see it by itself, and perceptual
constancy leads us to take into account
a misleading sense of distance.
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Module
13
Perceptual Organization: Constructing Our View of the World
131
in the sky later in the evening. You may have thought that the apparent change in
the size of the moon was caused by the moon’s being physically closer to the earth
when it fi rst appears. In fact, though, this is not the case at all: the actual image of
the moon on our retina is the same, whether it is low or high in the sky.
There are several explanations for the moon illusion. One suggests that the moon
appears to be larger when it is close to the horizon primarily because of perceptual
constancy. When the moon is near the horizon, the perceptual cues of intervening
terrain and objects such as trees on the horizon produce a misleading sense of dis-
tance, leading us to misperceive the moon as relatively large.
In contrast, when the moon is high in the sky, we see it by itself, and we don’t
try to compensate for its distance from us. In this case, then, perceptual constancy
leads us to perceive it as relatively small. To experience perceptual constancy, try
looking at the moon when it is relatively low on the horizon through a paper-towel
tube; the moon suddenly will appear to “shrink” back to normal size (Coren, 1992;
Imamura & Nakamizo, 2006; Kaufman, Johnson, & Liu, 2008; Ross & Plug, 2002).
Perceptual constancy is not the only explanation for the moon illusion, and it
remains a puzzle to psychologists. It may be that several different perceptual pro-
cesses are involved in the illusion (Gregory, 2008; Kim, 2008).
Motion Perception:
As the World Turns
When a batter tries to hit a pitched ball, the most important factor is the motion of
the ball. How is a batter able to judge the speed and location of a target that is mov-
ing at some 90 miles per hour?
The answer rests in part on several cues that provide us with relevant informa-
tion about the perception of motion. For one thing, the movement of an object across
the retina is typically perceived relative to some stable, unmoving background. More-
over, if the stimulus is heading toward us, the image on the retina expands in size,
fi lling more and more of the visual fi eld. In such cases, we assume that the stimulus
is approaching—not that it is an expanding stimulus viewed at a constant distance.
It is not, however, just the movement of images across the retina that brings
about the perception of motion. If it were, we would perceive the world as moving
every time we moved our heads. Instead, one of the critical things we learn about
perception is to factor information about our own head and eye movements along
with information about changes in the retinal image.
Sometimes we perceive motion when it doesn’t occur. Have you ever been on a
stationary train that feels as if it is moving, because a train on an adjacent track
begins to slowly move past? Or have you been in an IMAX movie theater, in which
you feel as if you were falling as a huge image of plane moves across the screen? In
both cases, the experience of motion is convincing. Apparent movement is the percep-
tion that a stationary object is moving. It occurs when different areas of the retina
are quickly stimulated, leading us to interpret motion (Ekroll & Scherzer, 2009;
Lindemann & Bekkering, 2009).
Perceptual Illusions:
The Deceptions of Perceptions
If you look carefully at the Parthenon, one of the most famous buildings of ancient
Greece, still standing at the top of an Athens hill, you’ll see that it was built with a
bulge on one side. If it didn’t have that bulge—and quite a few other architectural
“tricks” like it, such as columns that incline inward—it would look as if it were
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