It may seem to have little or no pattern. However, hearing it two or three times

will help you to perceive the pattern. To the extent that you, or Tina, can hear

any pattern at all on the first presentation, it is probably due to the Gestalt laws.

The sharpening of perception on repeated presentations can be attributed to

learning.

One way to explain this sharpening of perception is to suggest that patterns of

stimulation set off chain reactions in neurons located, let us say, in the association

areas of the brain’s cortex. Each time a given stimulus is presented, the same set of

neurons fire. The research of the Canadian psychologist Donald O. Hebb suggests

that repeated firings form a cell assembly, a stable group of neurons that are used

over and over by the brain to create a representation of the external pattern. A pattern

can, of course, be quite complex. If this is so, a given cell assembly may represent

only a portion of a pattern. Hebb called a set of cell assemblies grouped together to form a larger pattern a phase sequence.

The existence of cell assemblies helps account for a memory of patterns and

perceptual objects. When you hear a melody or recognize something you have

seen before, it is quite possibly because an established cell assembly is firing.

Learning also plays a role in perception because we are conscious beings who

attach labels to perceptual objects. This brings us to the cognitive hypothesis in

perception, the hypothesis that we not only perceive, but know what we are perceiving. If you see a friend and think, “There’s Erin,” or hear a song and think,

“That’s ‘God Bless America’ by Irving Berlin,” then you have increased the acuity

of your perceptual world. Cognitive learning, learning in which consciousness

plays an important role, is an important aspect of the perceptual process.

Illusions: What Do They Teach Us about Perception?

An illusion is a false perception, a perception that does not fit an objective

description of a stimulus situation. An illusion is usually associated with a particular

sense. Consequently, there are optical illusions, auditory illusions, and so

forth. Illusions tend to be remarkably stable. They affect most normal observers in

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 hallucination. A delusion is 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 delusion.

A hallucination is 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. Illusions teach us that perceptions are, to some extent, created by the brain and nervous system, that we are not passive observers of our world. Let’s return to figureground perception. We perceive the relationship between a figure and its associated ground as being a fact about the world itself. But is it? The vase-faces illusion can be perceived in two different ways. 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 alternations

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 vasefaces

drawing is said to be ambiguous, meaning that it can be perceived in more

than one way. The process of attention, characterized by a tendency to focus on

some stimuli and ignore others, determines that one organization will be temporarily

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.