Page 170 - Foundations of Cognitive Psychology : Core Readings

174 Philip G. Zimbardo and Richard J. Gerrig

Figure 7.32
Shape constancy. As a coin is rotated, its image becomes an ellipse that grows narrower and nar-
rower until it becomes a thin rectangle, an ellipsis again, and then a circle. At each orientation,
however, it is still perceived as a circular coin.

larger, and though he was as courageous as any Pygmy, he moved over
and sat close to me and muttered that it was witchcraft.... Finally, when
he realized that they were real buffalo he was no longer afraid, but what
puzzled him still was why they had been so small, and whether they really
had been small and had so suddenly grown larger, or whether it had been
some kind of trickery. (Turnbull, 1961, p. 305)
In this unfamiliar perceptual environment, Kenge first tried to fit his novel
perceptions into a familiar context, by assuming the tiny, distant specks he saw
were insects. With no previous experience seeing buffalo at a distance, he had
no basis for size constancy, and as the fast-moving car approached them and
Kenge’s retinal images got larger and larger, he had the frightening illusion
that the animals were changing in size. We can assume that, over time, Kenge
would have come to see them as Turnbull did. The knowledge he acquired
would allow him to arrive at an appropriate perceptual interpretation for his
sensory experience.
Shape constancy is closely related to size constancy. You perceive an object’s
actual shape correctly even when the object is slanted away from you, making
the shape of the retinal image substantially different from that of the object
itself. For instance, a rectangle tipped away projects a trapezoidal image onto
your retina; a circle tipped away from you projects an elliptical image (see fig-
ure 7.32). Yet you usually perceive the shapes accurately as a circle and a rect-
angle slanted away in space. When there is good depth information available,
your visual system can determine an object’s true shape simply by taking into
account your distance from its different parts.
Orientation Constancy When you tilted your head to the side in viewing your
book, the world did not seem to tilt; only your own head did. Orientation con-
stancy is your ability to recognize the true orientation of the figure in the real
world, even though its orientation in the retinal image is changed. Orientation
constancy relies on output from the vestibular system in your inner ear—which
makes available information about the way in which your head is tilted. By
combining the output of the vestibular system with retinal orientation, your
visual system is usually able to give you an accurate perception of the orienta-
tion of an object in the environment.
In familiar environments, prior knowledge provides additional information
about objective orientation. However, you may not be good at recognizing
complex and unfamiliar figures when they are seen in unusual orientations.