move your head back to a normal reading distance. Although the book stimu-
lated a much larger part of your retina when it was up close than when it was
far away, didn’t you perceive the book’s size to remain the same? Now set the
book upright and try tilting your head clockwise. When you do this, the image
of the book rotates counterclockwise on your retina, but didn’t you still per-
ceive the book to be upright?
In general, you see the world asinvariant, constant,andstabledespite changes
in the stimulation of your sensory receptors. Psychologists refer to this phe-
nomenon asperceptual constancy. Roughly speaking, it means that you perceive
the properties of the distal stimuli, which are usually constant, rather than the
properties of proximal stimuli, which change every time you move your eyes or
head. For survival, it is critical that you perceive constant and stable properties
of objects in the world despite the enormous variations in the properties of the
light patterns that stimulate your eyes. The critical task of perception is to dis-
coverinvariantproperties of your environment despite thevariationsin your
retinal impressions of them. We will see how this works for size, shape, and
orientation.
Size and Shape Constancy What determines your perception of the size of an
object? In part, you perceive an object’s actual size on the basis of the size of its
retinal image. However, the demonstration with your book shows that the size
of the retinal image depends on both the actual size of the book and itsdistance
from the eye. As you now know, information about distance is available from
a variety of depth cues. Your visual system combines that information with
retinal information about image size to yield a perception of an object size that
usually corresponds to the actual size of the distal stimulus.Size constancy
refers to your ability to perceive the true size of an object despite variations in
the size of its retinal image.
If the size of an object is perceived by taking distance cues into account, then
you should be fooled about size whenever you are fooled about distance. One
suchillusionoccursintheAmesroomshowninfigure7.31.Incomparisonto
his 4-foot daughter, Tanya Zimbardo, your 6-foot-tall author looks quite short
in the left corner of this room, but he looks enormous in the right corner. The
reason for this illusion is that you perceive the room to be rectangular, with the
two back corners equally distant from you. Thus you perceive Tanya’s actual
size as being consistent with the size of the images on your retina in both cases.
In fact, Tanya is not at the same distance, because the Ames room creates a
clever illusion. It appears to be a rectangular room, but it is actually made from
nonrectangular surfaces at odd angles in depth and height, as you can see in
the drawings that accompany the photos. Any person on the right will make a
larger retinal image, because he or she is twice as close to the observer.
Another way that the perceptual system can infer objective size is by using
prior knowledge about the characteristic size of similarly shaped objects. For
instance, once you recognize the shape of a house, a tree, or a dog, you have a
pretty good idea of how big each is, even without knowing its distance from
you. Universal Studios in Hollywood uses your expectations about the normal
sizes of doors to make its actors in westerns look bigger or smaller to you. The
doorsononesideofthestreetonawesternsetaremadetobesmallerthanthe
172 Philip G. Zimbardo and Richard J. Gerrig