Beyond Bottom-Up Processing • 53the background (blue arrows). In addition, other signals, which are
associated with a person’s knowledge and expectations, are being
transmitted down from higher levels in the brain (dashed arrow).
Signals such as this, that travel down from higher centers to infl u-
ence incoming signals, are called feedback signals (Di Lollo, 2010).
From the physiological point of view, therefore, perception of
an object is based on signals representing the object plus signals
representing other aspects of the environment and feedback signals
representing prior knowledge or expectations (Figure 3.7). Looking
at perception in this way, we can draw an analogy between percep-
tion and baking a loaf of bread. The basic ingredients for bread
are fl our and water, plus extra ingredients such as poppy seeds or
salt, depending on the recipe. But if you just mix these ingredients
together, the bread doesn’t rise. A little yeast is also necessary to
make the bread rise. Add the yeast to these other ingredients, bake,
and you get a loaf of bread. (Without the yeast, unleavened bread
such as matzo, fl atbread, or communion wafers results.)
Just as creating a loaf of bread requires the basic ingredients plus
yeast, perception depends on information provided by stimulation of
receptors plus additional information such as information about the
environment and a person’s prior knowledge. This information is
carried in the additional physiological signals we have described, but
we can also use perceptual examples to demonstrate how the per-
ceptual system takes additional information into account. We will
do this by describing two different kinds of perceptions: perception
of the size of an object and perception of the intensity of an odor.PERCEIVING SIZE:
TAKING DISTANCE INTO ACCOUNT
Imagine that you are walking down some railroad tracks when you suddenly come
upon the scene in ● Figure 3.8. The small creature near you seems harmless, but you’re
a little worried about the larger one! You perceive the two creatures to be very different
in size, yet they both cover the same distance across your fi eld of view and therefore
have the same-sized image on your retina (● Figure 3.9). (Check this out by measuring
them!) This means that something in addition to the size of the creature’s image on the
retina determines your perception of its size.
What other information is available? Perhaps
the most obvious is that the creatures are at dif-
ferent distances. A large amount of research has
shown that if two objects are perceived to be at dif-
ferent distances but cast the same-sized image on
the retina, the perceptual system takes the distance
of the farther object into account, so it is perceived
as its true, larger size. This makes sense, because in
our everyday experience a distant object can result
in the same-sized image on the retina as a much
smaller object that is closer (see ● Figure 3.10),
so the way the perceptual system takes depth into
account helps us more accurately perceive the size
of the faraway object.
In addition to depth, the perceptual system
could also be taking into account the size of the
object relative to other objects in the environment.
Returning to our creatures on the railroad tracks,
we can see that the near creature fi ts within the
two tracks with space to spare, while the far one● (^) FIGURE 3.6 “Multiple personalities of a blob.” What
we expect to see in diff erent contexts infl uences our
interpretation of the identity of the “blob” inside the circles.
(Source: Adapted from A. Oliva & A. Torralba, “The Role of Context in
Object Recognition,” Trends in Cognitive Sciences, 11 , Figure 2, 520–527.
Copyright © 2007, with permission from Elsevier. Photographs courtesy
of Antonio Torralba.)
(a)(b)
(c) (d)
blob
● (^) FIGURE 3.7 Perception is determined by three sources of information:
(1) information originating from stimulation of the receptors (bottom-
up = green arrows); (2) additional information such as the context in which
an object appears (blue arrows); (3) knowledge or expectations of the
perceiver (top-down = dashed arrow). The dashed red arrow represents
feedback signals.
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