the same way. For example, for almost all of us the Moon is perceived to be larger
when low and near the horizon than when it is high and overhead.
It is important to distinguish the concept of an illusion from a delusion and a hal-
lucination. A delusionis a false belief. If Ray, a schizophrenic mental patient,
believes that he has an eye with X-ray vision on the back of his head, this is a delu-
sion. A hallucinationis a perception created by the individual. It has no relationship
to reality at all. If Ray sees and hears an invisible companion that nobody else can see
or hear, this is a hallucination. Illusions are thought to be normal and experienced by
most of us. Delusions and hallucinations are thought to be abnormal and experienced
in an idiosyncratic fashion. (There is more about abnormal behavior in chapter 14.)
(a) An illusion is a.
(b) A delusion is a.
(c) A hallucination is a created by the individual.
Answers: (a) false perception; (b) false belief; (c) perception.
Illusions teach us that perceptions are, to some extent, created by the brain and nerv-
ous system, that we are not passive observers of our world. Let’s return to figure-
ground perception. We perceive the relationship between a figure and its associated
ground as being a fact about the world itself. But is it? Thevase-faces illusioncan
be perceived in two different ways. (See the illustration on page 64.) It can be seen as
a vase. Or it can be seen as two profiles facing each other. When seen as a vase, this
becomes figure and tends to stand forward a little in perception. The faces disappear
and become absorbed into a receding ground. When seen as two faces, these become
figure, and both tend to stand forward a little in perception. The vase disappears and
becomes absorbed into a receding ground. These two different perceptual alterna-
tions will take place for most observers on a predictable basis. Also, it is impossible to
simultaneously perceive both organizations. All of this suggests that figure and ground
are organizing tendencies linked to perception, not facts about the external world.
How can the vase-faces illusion be explained? Here is one approach. The vase-
faces drawing is said to be ambiguous,meaning that it can be perceived in more
than one way. The process ofattention,characterized by a tendency to focus on
some stimuli and ignore others, determines that one organization will be tem-
porarily favored over another. Let us say that the first organization favored is the
vase. The region of the brain being stimulated by the vase organization becomes
satiated (“overfilled”) with the vase organization. It spontaneously rejects it for a
second organization, one that is briefly refreshing. The satiation hypothesis
suggests that the brain tends to reject excessive stimulation of one kind and tends
to seek novel stimulation of another kind. Ambiguity, attention, and satiation are
factors that all work together to produce the fluctuations in perception that take
place when one experiences the vase-faces illusion.
Perception: Why Do Things Look the Way They Do? 63