TopDown Processing

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Chapter 3 Sensation and Perception
of letters or judgments of shape. Color and motion are other sensory features that
appear to be analyzed separately, in different parts of the brain, before full perceptual
recognition takes place (Beatty, 1995; Cowey, 1994; Treisman, 1999).
The brain also apparently analyzes patterns of light and darkness in the visual scene.
Analyzing these patterns may help us to perceive textural gradients, which in turn help
us to judge depth and recognize the general shape of blurry images.
Top-Down Processing
Bottom-up feature analysis can explain why you recognize the letters in a sign for
Barney’s Diner. But why is it that you can recognize the sign more easily if it appears
where you were told to expect it rather than a block earlier? And why can you
recognize it even if a few letters are missing from the sign? Top-down processing
seems to be at work in those cases. In top-down processing, people rely on their
knowledge in making inferences, or “educated guesses,” that help them recognize
objects, words, or melodies, especially when sensory information is vague or
ambiguous (DeWitt & Samuel, 1990; Rock, 1983). For example, once you knew that
there was a dog in Figure 3.25, it became much easier for you to perceive one. Similarly, police officers find it easy to recognize familiar people on blurry security camera videos, but it is much more difficult for them to recognize strangers (Burton
et al., 1999).
Top-down processing illustrates that our experiences create schemas, mental representations of what we know and expect about the world. Schemas can bias our perception toward one recognition or another by creating a perceptual set—that is, a
readiness to perceive a stimulus in a certain way. A tragic example occurred several
years ago in London when police shot a man who they were told was carrying a sawedoff shotgun. The object in his hand was actually a table leg, but it was dark, so when
he raised his arm, they assumed he was about to fire at them.
Expectancy can be shaped by the context in which a stimulus occurs. For example,
we know of a woman who saw a masked man in the darkened hallway of a house she
was visiting. Her first perception was that the man who lived there was playing a joke;
in fact, she had confronted a burglar. Context has biasing effects for sounds as well as
sights. When shots are heard on a downtown street, they are often perceived as firecrackers or a car backfiring. The same sounds heard at a shooting range would immediately be interpreted as gunfire.
Motivation is another aspect of top-down processing that can affect perception. A
hungry person, for example, might initially mistake a sign for “Burger’s Body Shop” as
FIGURE
3.26
Another Version of Figure 3.25
Now that you can identify a
dog in this figure, it should be
by
much easier to recognize when
you look back at the original version.
doing
2
learn
schemas Mental representations of
what we know and expect about the
world.