Chapter Summary
Distance Perception
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Of the monocular depth cues, seven pictorial cues do not require movement.
a. Occlusion means that we judge a partly covered object to be farther away than the object that covers it; it is a primary
source of distance information.
b. Linear perspective, by which parallel lines appear to meet in the distance, is an important pictorial cue.
c. Size (either familiar or relative) is a depth cue that is related to linear perspective.
d. Texture gradient, or the increase in surface density at greater distances, is also related to linear perspective. Gibson
made use of texture gradients in his approach.
e. Atmospheric perspective means that distant objects often look blurry and blue.
f. Shading conveys depth information, because the lighting is not uniform across a surface and because objects far from a
light source are more shadowy.
g. Height cues tell us that objects near the horizon are farther away from the observer.
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Artists can make use of the pictorial depth cues to create a very realistic representation of a three-dimensional world on a
two-dimensional surface.
Two monocular depth cues involve movement—either of the observer or of objects around the observer.
a. Motion parallax means that as you move your head sideways, objects at different distances seem to move in different
directions.
b. Because of the kinetic depth effect, the two-dimensional projection of an object seems to have depth when the object
rotates.
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In addition to the pictorial cues, movies can make use of motion cues to depth.
In binocular disparity, the two eyes present two slightly different points of view; this is an important source of
information about the distance of nearby objects. Stereopsis is the sense of depth that emerges from binocular disparity
cues.
Binocular disparity can be represented in stereograms, random-dot stereograms, and autostereograms.
Two less useful depth cues come from eye muscles. Accommodation of the lens may provide some weak depth
information. Convergence is a binocular cue in which the eyes move together to look at a nearby object. Convergence
might sometimes be a helpful source of depth information, at least for objects within an arm’s reach.
Gibson’s direct theory of depth perception argues that the visual stimulus is rich with information; texture gradients and
motion perspective are particularly important.
The empiricist position on depth perception states that we perceive distance by associating various cues for distance with
kinesthetic information; the visual stimulus itself is inadequate. Modern variations of empiricism, including constructivist
theory, emphasize the importance of our expectations and problem-solving abilities in determining what we perceive.
The computational approach blends aspects of constructivist theory and Gibson’s direct perception theory. Researchers in
this interdisciplinary field have developed mathematical models that can be used by computers to perceive depth and
distance.
Through the various depth cues, people are able to represent the three-dimensional world in which we live. This spatial
representation allows them to navigate the world successfully.
Using visual angle information about the trajectory of a moving object, people can track the object and catch it.
People with unilateral neglect or directional deficits illustrate the fragility of our space perception mechanisms. Damage to
our brains (e.g., parietal lobe) may cause a portion of space to disappear.
Many areas of the brain process spatial information. The M pathway carries motion information, and the P pathway
carries information for binocular disparity. The primary visual cortex (V1) has cells that process disparity information, but
MT (V5) seems to integrate various types of depth information.
The hippocampus is important for memory, including spatial memory. Place cells respond to particular locations.
Size Perception
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The egocentric distance of the object is important in the accurate perception of an object’s size. Several other factors also
influence the perceived size of an object, including its shape and the surrounding context.
Emmert’s law illustrates the crucial relationship between size perception and distance. Although the law was formulated
on the basis of afterimages, it is useful for explaining how size perception generally depends on distance perception.
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The classic experiment by Holway and Boring points out the importance of distance information for the accurate
perception of size. In the absence of distance information, people judge size based on visual angle.
Size constancy arises when the distal size of an object remains constant in spite of proximal changes. Several theories
have been proposed to explain size constancy, including the size-distance invariance hypothesis, Gibson’s direct
perception explanation, and the two-stage transformation theory.
Illusions of Distance and Size
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The line-length and distance illusions include the Müller-Lyer illusion (and its many variants), the Sander parallelogram,
the horizontal-vertical illusion, and the Ponzo illusion.
A popular psychological explanation for some illusions is the misapplied constancy explanation, in which cues in the
illusion are interpreted as cues for maintaining size constancy. Research has demonstrated that experience with depth
cues enhances these illusions, a finding that supports the misapplied constancy explanation. However, this explanation
cannot explain all the illusions.
The incorrect comparison explanation proposes that illusions occur when observers base their judgments on the incorrect
parts of the figures. Thus, when judging the horizontal lines in the Ponzo or Müller-Lyer illusions, people’s judgments are
influenced by the inducing elements in the illusions.
More physiological explanations, such as eye-movement patterns, may play a role in some illusory effects. Ultimately,
brain research is likely to address many issues regarding the perception of illusions.
Two important area illusions are the Ames room and the moon illusion, in which the horizon moon looks as much as 50%
bigger than the zenith moon.
A number of factors likely contribute to the moon illusion. A major factor is contextual, with the horizon moon appearing
to be farther away because of the objects between the observer and the moon. However, estimating the size of the moon
is an extremely complex judgment. Thus, only with a more complete theory of size perception will researchers be able to
explain the moon illusion.