Chapter 8: Perceiving Depth and Size

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Chapter 8 Perceiving Depth and Size
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• Figure 8.1 (a) The house is farther away than the
  tree, but (b) the images of points F on the house
  and N on the tree both fall on the
  two-dimensional surface of the retina, so (c)
  these two points, considered by themselves, do
  not tell us the distances of the house and the
  tree.

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Cue Approach to Depth Perception

• Oculomotor - cues based on sensing the position
  of the eyes and muscle tension
• Convergence - inward movement of the eyes when we
  focus on nearby objects
• Accommodation - change in the shape of the lens
  when we focus on objects at different distances

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• Figure 8.2 (a) Convergence of the eyes occurs
  when a person looks at something that is very
  close. (b) The eyes look straight ahead when the
  person observes something that is far away.

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Cue Approach to Depth Perception - continued

• Monocular - cues that come from one eye
• Pictorial cues - sources of depth information
  that come from 2-D images, such as pictures
• Occlusion - when one object partially covers
  another
• Relative height - objects that are higher in the
  field of vision are more distant

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Pictorial Cues

• Relative size - when objects are equal size, the
  closer one will take up more of your visual field
• Perspective convergence - parallel lines appear
  to come together in the distance
• Familiar size - distance information based on our
  knowledge of object size

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• Figure 8.3 A scene in Tucson, Arizona containing
  a number of depth cues occlusion (the cactus
  occludes the hill, which occludes the mountain)
  perspective convergence (the sides of the road
  converge in the distance) relative size (the far
  motorcycle is smaller than the near one) and
  relative height (the far motorcycle is higher in
  the field of view the far cloud is lower).

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Pictorial Cues - continued

• Atmospheric perspective - distance objects are
  fuzzy and have a blue tint
• Texture gradient - equally spaced elements are
  more closely packed as distance increases
• Shadows - indicate where objects are located

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• Figure 8.5 A scene along the coast of California
  that illustrates atmospheric perspective.

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• Figure 8.6 A texture gradient in Death Valley,
  California.

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• Figure 8.7 (a) Occlusion indicates that the
  tapered glass is in front of the round glass and
  vase. (b) Overlap now indicates that the vase is
  in front of the tapered glass, but there is
  something strange about this picture. (c) The
  cast shadow under the vase provides additional
  information about its position in space, which
  helps clear up the confusion.

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Motion-Produced Cues

• Motion parallax - close objects in direction of
  movement glide rapidly past but objects in the
  distance appear to move slowly
• Deletion and accretion - objects are covered or
  uncovered as we move relative to them
• Also called occlusion-in-motion

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• Figure 8.8 One eye, moving from left to right,
  showing how the images of a nearby tree and a
  faraway house change their position on the retina
  because of this movement. The image of the tree
  moves farther on the retina than the image of the
  house.

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• Figure 8.9 Starting with the view in the center
  (a), moving to the left causes deletion
  (covering) of the rear object (b). Moving to the
  right causes accretion (uncovering) of the rear
  object (c).

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• Table 8.1 Range of effectiveness of different
  depth cues

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Binocular Depth Information

• Binocular disparity - difference in images
  between the two eyes
• Difference can be described by examining
  corresponding points on the retina that connect
  to same places in the cortex

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• Figure 8.16 The two images of a stereoscopic
  photograph. The difference between the two
  images, such as the distances between the front
  cactus and the window in the two views, creates
  retinal disparity. This creates a perception of
  depth when (a) the left image is viewed by the
  left eye and (b) the right image is viewed by the
  right eye.

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Binocular Depth Information - continued

• Stereopsis - depth information provided by
  binocular disparity
• Stereoscope uses two pictures from slightly
  different viewpoints
• 3-D movies use the same principle and viewers
  wear glasses to see the effect
• Random-dot stereogram has two identical patterns
  with one shifted to the right

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• Figure 8.19 Top a random-dot stereogram.
  Bottom the principle for constructing the
  stereogram. See text for an explanation.

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Physiology of Depth Perception

• Experiment by Tsutsui et al.
• Monkeys matched texture gradients that were 2-D
  pictures and 3-D stereograms
• Recordings from a neuron in the parietal lobe
  showed
• Cell responded to pictorial cues
• Cell also responded to binocular disparity

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• Figure 8.20 Top gradient stimuli. Bottom
  response of neurons in the parietal cortex to
  each gradient. This neuron fires to the pattern
  in (c), which the monkey perceives as slanting to
  the left. (From Tsutsui et al., 2002, 2005.)

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Physiology of Depth Perception - continued

• Neurons have been found that respond best to
  binocular disparity
• Called binocular depth cells or disparity
  selective cells
• These cells respond best to a specific degree of
  disparity between images on the right and left
  retinas

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• Figure 8.21 Disparity tuning curve for a
  disparity-sensitive neuron. This curve indicates
  the neural response that occurs when stimuli
  presented the left and right eyes create
  different amounts of disparity. (From Uka
  DeAngelis, 2003.)

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• Figure 8.22 Where images of Frieda and Lee fall
  on the retina. See text for explanation.

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Connecting Binocular Disparity and Depth
Perception

• Experiment by Blake and Hirsch
• Cats were reared by alternating vision between
  two eyes
• Results showed that they
• Had few binocular neurons
• Were unable to use binocular disparity to
  perceive depth

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Connecting Binocular Disparity and Depth
Perception - continued

• Experiment by DeAngelis et al.
• Monkey trained to indicate depth from disparate
  images
• Disparity-selective neurons were activated by
  this process
• Experimenter used microstimulation to activate
  different disparity-selective neurons
• Monkey shifted judgment to the artificially
  stimulated disparity

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Size Perception

• Distance and size perception are interrelated
• Experiment by Holway and Boring
• Observer was at the intersection of two hallways
• A luminous test circle was in the right hallway
  placed from 10 to 120 feet away
• A luminous comparison circle was in the left
  hallway at 10 feet away

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• Figure 8.24 Setup of Holway and Borings (1941)
  experiment. The observer changes the diameter of
  the comparison circle to match his or her
  perception of the size of the text circle. Each
  of the test circles has a visual angle of 1
  degree. This diagram is not drawn to scale. The
  actual distance of the test circle was 100 feet.

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Experiment by Holway and Boring

• On each trial the observer was to adjust the
  diameter of the test circle to match the
  comparison
• Test stimuli all had same visual angle (angle of
  object relative to observers eye)
• Visual angle depends on both the size of the
  object and the distance from the observer

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• Figure 8.25 (a) The visual angle depends on the
  size of the stimulus (the woman in this example)
  and its distance from the observer. (b) When the
  woman moves closer to the observer, the visual
  angle and the size of the image on the retina
  increases. This example shows how halving the
  distance between the stimulus and observer
  doubles the size of the image on the retina.

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• Figure 8.26 The thumb method of determining the
  visual angle of an object. When the thumb is at
  arms length, whatever it covers has a visual
  angle of about 2 degrees. The womans thumb
  covers half the width of her iPod, so we can
  determine that the visual angle of the iPods
  total width is about 4 degrees.

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Experiment by Holway and Boring - continued

• Part 1 of the experiment provided observers with
  depth cues
• Judgments of size were based on physical size
• Part 2 of the experiment provided no depth
  information
• Judgments of size were based on size of the
  retinal images

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• Figure 8.28 Results of Holway and Borings
  experiment. The dashed line marked physical size
  is the result that would be expected if the
  observers adjusted the diameter of the comparison
  circle to match the actual diameter of each test
  circle. The line marked visual angle is the
  result that would be expected if the observers
  adjusted the diameter of the comparison circle to
  match the visual angle of each test circle.
  (From Determinants of Apparent Visual Size with
  Distance Variant, by A. H. Holway and E. G.
  Boring, 1941, American Journal of Psychology, 54,
  21-34, figure 2. University of Illinois Press.)

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• Figure 8.29 The moons disk almost exactly covers
  the sun during an eclipse because the sun and the
  moon have the same visual angles.

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Size Constancy

• Perception of an objects size remains relatively
  constant
• This effect remains even if the size of the
  object on the retina changes
• Size-distance scaling equation
• S K (R X D)
• Changes in distance and retinal size balance each
  other

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Size-Distance Scaling

• Emmerts law
• Retinal size of an afterimage remains constant
• Perceived size will change depending on distance
  of projection
• This follows the size-distance scaling equation

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• Figure 8.31 The principle behind the observation
  that the size of an afterimage increases as the
  afterimage is viewed against more distant
  surfaces.

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• Figure 8.33 Two cylinders resting on a texture
  gradient. The fact that the bases of both
  cylinders cover the same number of units on the
  gradient indicates that the bases of the two
  cylinders are the same size.

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Visual Illusions

• Nonveridical perception occurs during visual
  illusions
• Müller-Lyer illusion
• Straight lines with inward fins appear shorter
  than straight lines with outward fins
• Lines are actually the same length

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• Figure 8.34 The Müller-Lyer illusion. Both lines
  are actually the same length.

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Müller-Lyer Illusion

• Why does this illusion occur?
• Misapplied size-constancy scaling
• Size constancy scaling that works in 3-D is
  misapplied for 2-D objects
• Observers unconsciously perceive the fins as
  belonging to outside and inside corners
• Outside corners would be closer and inside would
  be further away

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Müller-Lyer Illusion - continued

• Since the retinal images are the same, the lines
  must be different sizes
• Problems with this explanation
• The dumbbell version shows the same perception
  even though there are no corners
• The illusion also occurs for some 3-D displays

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• Figure 8.35 According to Gregory (1973), the
  Müller-Lyer line on the left corresponds to an
  outside corner, and the line on the right to an
  inside corner. Note that the two vertical lines
  are the same length (measure them!).

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• Figure 8.36 The dumbbell version of the
  Müller-Lyer illusion. As in the original
  Müller-Lyer illusion, the two lines are actually
  the same length.

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• Figure 8.37 A three-dimensional Müller-Lyer
  illusion. Although the distances x and y are the
  same, distance y appears larger, just as in the
  two-dimensional Müller-Lyer illusion.

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Müller-Lyer Illusion - continued

• Another possible explanation
• Conflicting cues theory - our perception of line
  length depends on
• The actual length of the vertical lines
• The overall length of the figure
• The conflicting cues are integrated into a
  compromise perception of length

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• Figure 8.38 An alternate version of the
  Müller-Lyer illusion. We perceive that the
  distance between the dots in (a) is less than the
  distance in (b), even though the distances are
  the same. (From Day, 1989.)

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Ponzo Illusion

• Horizontal rectangular objects are placed over
  railroad tracks in a picture
• Far rectangle appears larger than closer
  rectangle but both are the same size
• One possible explanation is misapplied
  size-constancy scaling

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• Figure 8.39 The Ponzo (or railroad track)
  illusion. The two horizontal rectangles are the
  same length on the page (measure them), but the
  far one appears larger.

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The Ames Room

• Two people of equal size appear very different in
  size in this room
• The room is constructed so that
• Shape looks like normal room when viewed with one
  eye
• Actual shape has left corner twice as far away as
  right corner

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The Ames Room
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• Figure 8.41 The Ames room, showing its true
  shape. The woman on the left is actually almost
  twice as far away from the observer as the woman
  on the right however, when the room is viewed
  through the peephole, this difference in distance
  is not seen. In order for the room to look
  normal when viewed through the peephole, it is
  necessary to enlarge the left side of the room.

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The Ames Room - continued

• Why does the illusion occur?
• One possible explanation
• Observer thinks the room is normal
• Women would be at same distance
• One has smaller visual angle (R)
• Due to the perceived distance (D) being the same
• Her perceived size (S) is smaller

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The Ames Room - continued

• Another possible explanation
• Perception of size depends on relative size
• One woman fills the distance between the top and
  bottom of the room
• Other woman only fills part of the distance
• Thus, first woman appears taller

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Moon Illusion

• Moon appears larger on horizon than when it is
  higher in the sky
• One possible explanation
• Apparent-distance theory - horizon moon is
  surrounded by depth cues while moon higher in the
  sky has none
• Horizon is perceived as further away than the sky
  - called flattened heavens

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Moon Illusion - continued

• Since the moon in both cases has the same visual
  angle, it must appear larger at the horizon
• Another possible explanation
• Angular size-contrast theory - moon appears
  smaller when surrounded by larger objects
• Thus, the large expanse of the sky makes it
  appear smaller
• Actual explanation may be a combination of a
  number of cues

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• Figure 8.42 An artists conception of the moon
  illusion showing the moon on the horizon and high
  in the sky simultaneously.

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• Figure 8.43 When observers are asked to consider
  that the sky is a surface and are asked to
  compare the distance to the horizon (H) and the
  distance to the top of the sky on a clear
  moonless night, they usually say that the horizon
  appears farther away. This results in the
  flattened heavens shown above.