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Chapter 8: Perceiving Depth and Size

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Chapter 8: Perceiving Depth and Size ... Cue Approach to Depth Perception ... This creates a perception of depth when (a) the left image is viewed by the left ... – PowerPoint PPT presentation

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Title: Chapter 8: Perceiving Depth and Size


1
Chapter 8 Perceiving Depth and Size
2
  • 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.

3
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

4
  • 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.

5
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

6
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

7
  • 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).

8
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

9
  • Figure 8.5 A scene along the coast of California
    that illustrates atmospheric perspective.

10
  • Figure 8.6 A texture gradient in Death Valley,
    California.

11
  • 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.

12
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

13
  • 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.

14
  • 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).

15
  • Table 8.1 Range of effectiveness of different
    depth cues

16
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

17
  • 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.

18
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

19
  • Figure 8.19 Top a random-dot stereogram.
    Bottom the principle for constructing the
    stereogram. See text for an explanation.

20
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

21
  • 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.)

22
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

23
  • 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.)

24
  • Figure 8.22 Where images of Frieda and Lee fall
    on the retina. See text for explanation.

25
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

26
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

27
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

28
  • 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.

29
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

30
  • 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.

31
  • 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.

32
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

33
  • 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.)

34
  • 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.

35
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

36
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

37
  • Figure 8.31 The principle behind the observation
    that the size of an afterimage increases as the
    afterimage is viewed against more distant
    surfaces.

38
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40
  • 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.

41
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

42
  • Figure 8.34 The Müller-Lyer illusion. Both lines
    are actually the same length.

43
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

44
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

45
  • 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!).

46
  • 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.

47
  • 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.

48
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

49
  • 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.)

50
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

51
  • 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.

52
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

53
The Ames Room
Mac OS 8-9
Mac OS X
Windows
54
  • 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.

55
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

56
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

57
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

58
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

59
  • Figure 8.42 An artists conception of the moon
    illusion showing the moon on the horizon and high
    in the sky simultaneously.

60
  • 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.
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