Perceiving objects

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Perceiving objects

• Chapter 5
• CS4005 Lecture tomorrow

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Perceiving objects

• Inverse projection problem a particular image
  on the retina can be caused by many different
  objects
• The image on the retina is ambiguous e.g.
  bistable figures
• What we see is not solely dependent on the
  retinal image
• Can perceive objects that are partially hidden or
  blurred - loss of some detail figure 5.5
• occlusion

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viewpoints

• Viewpoint invariance our ability to recognise
  objects from different viewpoints e.g. face
  recognition - figure 5.7
• Our ability to detect changes in lightness and
  darkness in a scene as being due to object colour
  or light illumination. knowledge of shadows
• How do we do this?
• Structuralist approach to object recognition
  perceptions are created by combining elements
  called sensations

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Gestalt psychology

• 1911
• The Gestalt psychologists Max Wertheimer, Kurt
  Koffka and Ivo Kohler
• Illusory contours contours perceived that are
  not actually present (lab work)
• Bistable figures switch back and fourth between
  different images as they are viewed
• Bistable figures and illusory contours are
  difficult to explain in terms of just our
  sensations

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Gestalt psychology

• The Gestalt psychologists rejected the idea that
  our perception can be explained just in terms of
  our sensations and proposed the idea that the
  whole differs from the sum of its parts.
• e.g. apparent motion
• They determined a set of principles to explain
  our perceptual organisation
• Perceptual organisation how small elements are
  grouped to form larger objects e.g. Fig 5.12

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The Gestalt laws of perceptual organisation

• A series of rules that specify how we organise
  small parts into wholes
• Law of pragnanz / of good figure / of simplicity
  every stimulus pattern is seen in such a way
  that the resulting structure is as simple as
  possible 5 circles
• Law of similarity - similar things appear to be
  grouped together

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Gestalt laws of perceptual organisation

• Auditory - Grouping according to similarity in
  pitch, timbre (e.g. Wessel demonstration) etc
• Law of good continuation points that, when
  connected, result in straight or smoothly curving
  lines are seen as belonging together, and the
  lines tend to be seen in such a way as to follow
  the smoothest path.
• Law of proximity or nearness things that are
  near to each other appear to be grouped together

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Gestalt laws of perceptual organisation

• Law of common fate things that are moving in
  the same direction appear to be grouped together
  (to perceive objects in motion e.g. bike, car)
• Law of familiarity things are more likely to
  form groups if the groups appear familiar or
  meaningful demonstration faces in a landscape
• Other gestalt grouping principles proposed by
  Palmer and Rock common region and uniform
  connectedness.

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Gestalt laws of perceptual organisation

• Principle of common region elements that are
  within the same region of space appear to be
  grouped together (a)
• Principle of uniform connectedness a connected
  region of visual properties, such as lightness,
  colour, texture, or motion are perceived as a
  single unit (c)
• Principle of synchrony visual events that occur
  at the same time will be perceived as belonging
  together synchrony in changing events e.g.
  lights flicking on and off

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Gestalt heuristics

• Laws v.s. heuristics
• Gestalt heuristics rules of thumb that provide
  a best-guess solution to a problem
• An incorrect perception just one object behind
  the tree? Continuation, similarity, colour,
  contour

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Perceptual segregation

• How are objects separated?
• Principles of grouping help determine how
  portions of a stimulus become grouped to form
  larger objects
• Also necessary to explain how / why objects are
  perceived as separated from the scene in which
  they appear problem of perceptual segregation
• Figure-group segregation figure v.s. background

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Perceptual segregation

• What are the properties that cause us to perceive
  one area as figure and the other as ground?
• Studied by considering patterns like Rubins
  face-vase figure a reversible figure-ground
• Some properties of figure and ground
• Figure is more memorable that the ground maybe
  the object of focus / attention
• Figure seen in front of the ground

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Figure / background

• Ground viewed as unformed extends behind the
  figure
• The contour separating the figure from the ground
  belongs to the figure border ownership

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Global superiority effect

• Global precedence effect
• the finding in various experiments that the
  properties of the whole object take precedence
  over the properties of parts of the object
• Evidence that the visual system first processes
  large objects and then divides them into smaller
  parts
• Identifying the small (local) letters takes
  longer than identifying the larger (global)
  letters