202 Chapter 6 Sensation and Perception
you hold a Frisbee directly in front of your face,
its image on the retina will be round. When you
set the Frisbee on a table, its image becomes el-
liptical, yet you continue to see it as round.3
Location constancy. We perceive stationary ob-
jects as remaining in the same place even
though the retinal image moves about as we move
our eyes, heads, and body. As you drive along the
highway, telephone poles and trees fly by on your
retina. But you know that these objects do not
move on their own, and you also know that your
body is moving, so you perceive the poles and
trees as staying put.4
Brightness constancy. We see objects as having
a relatively constant brightness even though
the amount of light they reflect changes as the
overall level of illumination changes. Snow re-
mains white even on a cloudy day and a black
car remains black even on a sunny day. We are
not fooled because the brain registers the to-
tal illumination in the scene and automatically
adjusts for it.5
Color constancy. Outdoor light is “bluer” than
indoor light, and objects outdoors therefore
reflect more “blue” light than those indoors.
Conversely, indoor light from incandescent lamps
is rich in long wavelengths and is therefore “yel-
lower.” Yet an apple looks red whether you look
at it in your kitchen or outside on the patio;
why? Part of the explanation involves sensory
adaptation: Outdoors, we quickly adapt to short-
wavelength (bluish) light, and indoors, we adapt
to long-wavelength light. As a result, our visual
responses are similar in the two situations. Also,
when computing the color of an object, the brain
takes into account all the wavelengths in the visual
field immediately around it. If an apple is bathed in
bluish light, so, usually, is everything else around it.
The increase in blue light reflected by the apple is
canceled in the visual cortex by the increase in blue
light reflected by the apple’s surroundings, and so
the apple continues to look red. Color constancy is
further aided by our knowledge of the world. We
know that apples are usually red and bananas are
usually yellow, and the brain uses that knowledge
to recalibrate the colors in those objects when the
lighting changes (Mitterer & de Ruiter, 2008).Visual illusions: When Seeing is Mislead-
ing. Perceptual constancies allow us to make
sense of the world. Occasionally, however, we can
be fooled, and the result is a perceptual illusion. For
psychologists, illusions are valuable because they
are systematic errors that provide us with hints
about the perceptual strategies of the mind.Visual Constancies: When Seeing is Be-
lieving. Your perceptual world would be a con-
fusing place without still another perceptual skill.
Lighting conditions, viewing angles, and the dis-
tances of stationary objects are all continually
changing as we move about, yet we rarely confuse
these changes with changes in the objects them-
selves. This ability to perceive objects as stable
or unchanging, even though the sensory patterns
they produce are constantly shifting, is called
perceptual constancy. The best-studied constancies
are visual and include the following:1
Size constancy. We see an object as having a con-
stant size even when its retinal image becomes
smaller or larger. A friend approaching on the street
does not seem to be growing; a car pulling away
from the curb does not seem to be shrinking. Size
constancy depends in part on familiarity with ob-
jects; you know that people and cars do not change
size from moment to moment. It also depends on
the apparent distance of an object. An object that
is close produces a larger retinal image than the
same object farther away, and the brain takes this
into account. When you move your hand toward
your face, your brain registers the fact that the hand
is getting closer, and you correctly perceive its un-
changing size despite the growing size of its retinal
image. There is, then, an intimate relationship be-
tween perceived size and perceived distance.2
Shape constancy. We continue to perceive an
object as having a constant shape even though
the shape of the retinal image produced by the
object changes when our point of view changes. Ifperceptual constancy
The accurate perception
of objects as stable or un-
changed despite changes
in the sensory patterns
they produce.
© 1999 Dan Piraro. Reprinted with special permission of King Features Syndicate.