Perception of Depth and Distance

114
FIGURE
Chapter 3 Sensation and Perception
3.20
Gestalt Principles of Perceptual
Grouping
We tend to perceive Part A as two groups
of two circles plus one single circle, rather
than as, say, five circles. In Part B, we see
two columns of Xs and two columns of Os,
not four rows of XOXO. We see the X in Part
C as being made out of two continuous
lines, not a combination of the odd forms
shown. In Part D, we fill gaps so as to perceive a hollow cube. In Part E, we tend to
pair up dots in the same oval even though
they are far apart. Part F shows that connected objects are grouped together.
=
+
=
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Proximity
Similarity
Continuity
(A)
(B)
(C)
Closure
Common Region
Connectedness
(D)
(E)
(F)
4. Closure. We tend to mentally “fill in” missing parts of incomplete objects, as
in Figure 3.20(D). The gaps are easy to see, but the tendency to link disconnected parts can be so strong that you may perceive faint connections that are
not actually there (Meng, Remus, & Tong, 2005).
5. Texture. When basic features of stimuli have the same texture (such as the angle
of several elements), we tend to group those stimuli together. So we group standing trees together and perceive them as separate from their fallen neighbors.
6. Simplicity. We tend to group features of a stimulus in a way that provides the
simplest interpretation of the world. You can see the simplicity principle in
action in Figure 3.20(D), where it is simpler to see a single cube than an assortment of separate, unrelated arrows and Ys.
7. Common fate. Sets of objects that move in the same direction at the same
speed are perceived together. So a flock of birds, though separated in space, will
be perceived as a group. Choreographers and marching-band directors use the
principle of common fate when they arrange for groups or subgroups of dancers
or musicians to move in unison, causing the audience to perceive waves of
motion or a single large moving object.
Stephen Palmer (1999) has identified three additional grouping principles:
8. Synchrony. Stimuli that occur at the same time are likely to be perceived as
coming from the same source. For example, if you see a car up ahead stop violently at the same instant you hear a crash, you will probably perceive these
visual and auditory stimuli as part of the same event.
9. Common region. Stimuli located within some boundary tend to be grouped
together. The boundary can be created by an enclosing perimeter, as in Figure
3.20(E), a region of color, or other factors.
10. Connectedness. Stimuli that are connected by other elements tend to be
grouped together. In Figure 3.20(F), the circles connected by dotted lines seem
to go together even though they are farther apart than some pairs of unconnected circles. Here, the principle of connectedness appears more important
than the principle of proximity.
Perception of Depth and Distance
depth perception Perception of distance, allowing us to experience the
world in three dimensions.
We are able to experience the world in three-dimensional depth even though the visual
information we receive from it is projected onto two-dimensional retinas. This is possible because of depth perception, our ability to perceive distance. Depth perception,
Organizing the Perceptual World
FIGURE
115
3.21
Stimulus Cues for Depth
Perception
See if you can identify the cues
of relative size, interposition,
linear perspective, height in
the visual field, textural gradient, and
shadows that combine to create a sense
of three-dimensional depth in this photograph. Notice, too, that sidewalk artist
Kurt Wenner has used some of these same
cues to create a dramatic illusion of depth
in his drawing. (You can see more of
Wenner’s amazing work at http://www.
kurtwenner.com/street/.)
doing
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in turn, is made possible by stimulus cues provided by the environment and also by the
properties of our visual system (Anderson, 2004).
Stimulus Cues To some extent, people perceive depth through the same cues that
artists use to create the impression of depth and distance on a two-dimensional canvas. Figure 3.21 demonstrates several of these cues:
■ One of the most important depth cues is interposition: Closer objects block the view
of things farther away. This cue is illustrated in Figure 3.21 by the couple walking
away from the camera. Because their bodies block out part of the buildings, we perceive them as being closer to us than the buildings are.
■ You can see the principle of relative size operating in Figure 3.21 by measuring the
images of that same couple and compare it to the size of the man in the foreground.
If two objects are assumed to be about the same size, the object producing a larger
image on the retina is perceived as closer than the one producing a smaller image.
■ Another cue comes from height in the visual field: On the ground, more-distant
objects are usually higher in the visual field than those nearby. Because the building
in the center of Figure 3.21 is higher than the people in the restaurant, the building
appears to be farther away from you. This is one reason why objects higher in the
visual field are more likely to be interpreted as the background for objects that are
lower in a scene (Vecera, Vogel, & Woodman, 2002).
■ The tiny figures near the center of Figure 3.21 are seen as very far away because they
are near a point where the buildings on each edge of the plaza, like all parallel lines
116
FIGURE
Chapter 3 Sensation and Perception
3.22
Light, Shadow, and Depth
Perception
The shadows cast by these
protruding rivets and deep
dents make it easy to see them
in three dimensions. But if you turn the
book upside down, the rivets now look
like dents, and the dents look like bumps.
This reversal in depth perception occurs
partly because people normally assume
that illumination comes from above and
interpret the pattern of light and shadow
accordingly (Adams, Graf, & Ernst, 2004).
With the picture upside down, light coming from the top would produce the observed pattern of shadows only if the
circles were dents, not rivets.
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that recede into the distance, appear to converge toward a single point. This apparent convergence provides a cue called linear perspective. The closer together two converging lines are, the greater the perceived distance.
■ Notice that the street in Figure 3.21 fades into a hazy background. Increased dis-
tance usually produces less clarity, and this reduced clarity is interpreted as a cue for
greater distance. (Hazy, distant objects also tend to take on a bluish tone, which is
why art students are taught to add a little blue when mixing paint for deep background features.)
■ Light and shadow also contribute to the perception of three dimensions (Kingdom,
2003; Ramachandran, 1988). The buildings in Figure 3.21 are seen as three-dimensional,
not flat, because of the shadows on some of their surfaces. Figure 3.22 shows a more
dramatic example.
■ An additional stimulus-based depth cue comes from continuous changes across the
visual field, called gradients. For example, a textural gradient is a graduated change in
the texture, or “grain,” of the visual field, as you can see in the plaza and the street in
Figure 3.21. Texture appears finer and less detailed as distance increases. So, as the texture of a surface changes across the retinal image, you perceive a change in distance.
Some depth cues result from
the way human eyes are built and positioned. Recall that to bring an image into focus
on the retina, the lens of the eye changes shape, or accommodates. Information about
the muscle activity involved is relayed to the brain, and this accommodation cue helps
create the perception of distance.
Two other depth cues are produced by the relative location of our two eyes. The first
is convergence. Each eye is located at a different place on the skull, so the eyes must
converge, or rotate inward, to project the same image on each retina. The closer the
object, the more the eyes must converge. Eye muscles send information about this convergence to the brain, which processes it as a distance cue. You can experilearn ence this feedback from your eye muscles by holding up a finger at arm’s
by
doing length and then try to keep it in focus as you move it toward your nose.
Second, because they are in slightly different locations, each eye sees the world from
a slightly different angle. The difference between these different retinal images is called
binocular disparity. The difference, or disparity, between images gets smaller for
objects that are far away and larger for objects that are nearby. The brain not only
combines the two images of an object but also takes into account how much they differ. This information helps to generate the impression of a single object that has depth,
Cues Based on Properties of the Visual System
2
convergence
A depth cue resulting
when the eyes rotate to project the image of an object on each retina.
binocular disparity A depth cue based
on the difference between the retinal
images received by each eye.