Depth Perception

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Depth Perception
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Depth Perception

• The image we receive on our retinas is only 2-D,
  however, we are able translate this in such a way
  that we perceive depth.
• We do this by using one of three categories of
  cues.
• 1) Monocular cues
• 2) Kinetic cues
• 3) Binocular cues

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Monocular Cues

• Require the use of only one eye.
• 1) Pictorial cues (static cues) We can use
  these same cues from stationary inspection of a
  scene (e.g. looking at a picture).
• Size The closer an object, the larger the
  retinal image.

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Monocular Cues

• We use the familiar size cue. If we are familiar
  with the size of an object, we can judge distance
  based on the size of the retinal image.
• E.g., We can judge how far away a car is based on
  the size of the retinal image.

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Monocular cues (contd)

• We also use the cue of relative size.
• This is when we look at two or more similar or
  identical objects and judge their distances based
  on the relative size of the retinal images (Fig
  11.1).

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Monocular cues (contd)

• Interposition When one object occludes another,
  we assume the occluding object is closer.
• Perhaps the most primitive of all pictorial cues.
• This cue arises from our tendency to choose
  simple objects over complicated forms (see Figs.
  11.2 and 11.3)

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Monocular cues (contd)

• Lighting and Shadow Objects that have depth
  cast shadows that give them the impression of
  being 3-D.
• Flat 2-D images do not cast shadows, so the
  visual system associates shading with 3-D objects
  only.
• Greater depth is indicated by deeper shadows (Fig
  11.4).

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Monocular cues (contd)

• Clarity and Elevation Dust and moisture in the
  atmosphere scatter light. Thus, objects that are
  further away are less clear.
• Close objects are more clear.
• As you look from nearer to further, objects that
  are further away are generally higher in the
  visual field (Fig 8.1).

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Monocular cues (contd)

• Linear Perspective Parallel lines tend to
  converge of in the distance (Fig. 11.7).

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Monocular cues (contd)

• Texture Most surfaces have a visible texture.
  The density of the this texture will vary with
  distance.
• Textures are finer in the distance.
• Equally spaced objects tend to appear closer as
  distance increases (Fig 11.9).
• This cue appears to depend on the magnocellular
  pathway.

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Monocular cues (contd)

• Livingstone and Hubel (1987) presented this
  figure to subjects in color
• Thus only the parvocellular system could detect
  the lines.
• There was no impression of depth.

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Monocular cues (contd)

• 2) Accommodation The lens must accommodate as
  you view objects at different distances.
• Controlled by ciliary muscles. Perhaps the brain
  could use activity in these muscles to determine
  depth.
• Humans probably make little use of this cue.

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Kinetic Cues

• Kinetic Cues When we move in space, objects
  images change positions on the retina.
• The relative movement of these objects at
  different distances from the observer is known as
  motion parallax.

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Kinetic Cues (contd)

• Objects closer than the spot youre fixating on
  move quickly in the opposite direction.
• Objects that are more distant than the object
  youre fixating on appear to move slowly in the
  same direction.

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Kinetic Cues (contd)

• There is also a kinetic cue based on the movement
  of the object.
• This is called the kinetic depth effect (Wallach
  OConnell, 1953).
• A rotating rod was projected onto a 2-D screen.

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Kinetic Cues (contd)

• Since it was projected onto a 2-D screen, it
  should have been seen as a line in which the
  length was constantly changing.
• Instead, it induced induced the sensation of
  depth due to its motion.

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Binocular Depth Cues

• 1) Convergence The eyes make disjunctive
  movements when following objects that move
  towards you or away from you.
• When an object moves toward you, the eyes
  converge in order to fixate the image on both
  foveae.

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Binocular Cues (contd)

• The angle at which the eyes converge is
  proportionate to the distance of the object.
• When an object is moved away from you, your eyes
  diverge.
• Evidence does suggest that we use convergence in
  order to perceive depth.

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Binocular Cues (contd)

• If accommodation and convergence are in
  disagreement, humans will rely on convergence in
  order to judge depth.

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Binocular cues (contd)

• 2) Binocular disparity The two eyes receive
  slightly different views of the world because
  they are in different positions in the head.
• Corresponding parts of the two retinas do not
  always receive the exact same visual image (see
  Fig. 11.16).
• This disparity produces the sensation of true
  depth which is known as stereopsis.

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Binocular cues (contd)

• The amount and direction of binocular disparity
  provides information about which of two or more
  objects is closer and how far apart they may be.
• There are two types of disparity (Fig 11.17).
• Uncrossed disparity it occurs for objects that
  are further away than the fixation point.

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Binocular cues (contd)

• Crossed disparity it occurs for objects that are
  closer than the fixation point.
• Evidence that retinal disparity produces depth
  comes from stereoscopes.
• Pictures are taken of the same visual scene from
  a slightly different perspective.
• Observer perceives depth.

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Binocular cues (contd)

• This sensation of depth is not due to contour or
  monocular cues.
• Julesz (1964) had subjects view two random dot
  stereogram dichoptically (Fig11.19).
• These consisted of black and white dots and thus
  no familiar contours.

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Binocular Cues (contd)

• The stereograms were mirror image except a
  central portion was displaced.
• When viewed dichoptically, this caused disparity,
  and thus, depth.

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Horopters

• When you fixate an object, other objects at the
  same distance appear at corresponding points of
  the retinas.
• This forms a curve known as a horopter (Fig
  11.18).
• The horopter is surrounded by Panums Fusion
  area.

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Horopters (contd)

• Objects that are not on the horopter but within
  the fusion area form images on disparate regions
  of the retinas.
• Thus, they are seen at a different depth than the
  object that is fixated.
• Objects outside this fusion area produce diplopia
  (i.e., double-vision).

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Physiology of Depth

• Binocular depth perception is likely mediated in
  the visual cortex as this is where binocular
  cells are located.
• To perceive depth, we require binocular cortical
  cells whose receptive fields are on disparate
  retinal locations.
• These cells are disparity selective.

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Physiology of Depth (contd)

• Such cells have been found in cats and primates
  (Hubel Weisel, 1970).
• Weisel and Hubel (1970) found these binocular
  cortical cells in V2 and V3 of the visual cortex.
• These cells rarely respond to monocular
  stimulation and respond only to certain
  disparities.

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Physiology of Depth (contd)

• These cells are part of the magnocellular
  pathway.
• Blake and Hirsch (1975) raised cats that received
  no binocular stimulation.
• This prevented the formation of binocular
  cortical cells.
• These cats possessed inferior depth perception.

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Physiology of Depth (contd)

• Blake and Cormack (1979) studied humans who were
  stereoblind (i.e, did not possess stereopsis).
• They found that they possessed only monocular
  cells.
• Thus, these binocular cortical cells are critical
  to stereopsis.

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Stereoblindness

• What leads to stereoblindness?
• Lack of binocular cortical cells.
• In infancy, the eyes compete for connections with
  cortical cells.
• This leads to the formation of binocular cortical
  cells.

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Stereoblindness

• If one eye is deprived, it can not compete.
• Cataract, eye turn, a weak eye.
• Thus, all cells in the visual cortex are
  monocular.
• Leads to stereoblindness.