PERCEPTION

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PERCEPTION
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Perception of Form

• Gestalt school of psychology (1930s) concerned
  with figureground separation
• FG separation follows laws of pragnanz
• Proximity
• Closure
• Similarity
• Continuity

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The Constancies

• Tendency to see shape, size, brightness and
  colour as constant across changes in proximal
  stimulus
• Shape constancy shape of object remains
  constant, even though outline on the retina
  depends on the angle from which it is seen
• Size constancy Object is perceived to be a
  constant size, regardless of how close or far
  away it is
• explained by the sizedistance invariance
  hypothesis (Kilpatrick Ittleson, 1953)

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The Constancies

• Brightness constancy same perceived brightness
  across different light intensities
• Colour constancy same perceived colour across
  different coloured illumination
• Constancies provide evidence that
• perception of the world is not dependent only on
  the relevant aspect of the stimulus
• It depends on other aspects of the stimulus, such
  as its perceived distance, its context and
  knowledge of the world

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Depth Perception

• Three kinds of cues to depth perception
• Binocular cues
• Retinal disparity
• Binocular convergence
• Monocular cues
• Interposition
• Texture gradient
• Linear perspective
• Motion cues
• Motion parallax
• Optical flow pattern

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Pattern Recognition

• Two-part process
• Perceptual processes, which analyse the input and
  produce a percept
• Recognition processes
• Pattern-recognition theories
• Template matching deals with second part
• Feature theory deals with first part

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Pattern Recognition

• Template matching (Selfridge Neisser, 1960)
• Store templates of various patterns, e.g. we have
  templates for each letter of the alphabet and
  each number (19) in memory
• The percept of the input (letter or number) is
  compared with the store of templates and
  recognition occurs when a match is found
• Problems requires vast store of templates
  difficulty in dealing with dirty stimuli in
  different orientations

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Pattern Recognition

• Feature theory (Neisser, 1967). Each pattern
  specified as list of distinctive features
• This list compared with feature lists stored in
  memory (one for each pattern)
• When input list matches a memory list the pattern
  is recognised
• Hubel Wiesel (1966) found cells (feature
  detectors) responding to lines, corners, etc
• Problem is that feature lists dont specify how
  the features are put together

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Object Recognition

• Patterns are two-dimensional objects are
  three-dimensional
• Notion of features needs modification to cope
  with the three-dimensional world
• Biedermans (1987) recognition-by-components
  theory
• Geons are three-dimensional building blocks, such
  as bricks, cylinders, wedges, cones and their
  curved axis counterparts

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Object Recognition

• Geons combine to form objects
• Structural descriptions of combinations of geons
  are matched to similar descriptions in memory
• When a match is found the object is recognised
• Problem is many objects in the natural world are
  not specifiable in terms of geons, e.g. a bush,
  the sea and a beach

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Context

• The context in which an object is seen can affect
  its perception
• Palmer (1975) recognition of the parts of a face
  (nose, ears, eyes, etc) was better when presented
  as parts of a face than when presented in
  isolation. More detail was needed when presented
  out of context

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Direct vs. Constructive Perception

• Direct perception (bottom-up processing), e.g.
  Gibson (1950, 1964)
• Perceptual processes are driven entirely by the
  stimulus or input
• Enough information in the stimulus for us to
  perceive the complexities of the world
• No higher cognitive processes, inference or
  knowledge of the world necessary

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Direct vs. Constructive Perception

• Gibson on depth perception
• All information needed to perceive depth is
  present in the stimulus
• Linear perspective and texture gradient are
  invariant properties of depth
• Pattern of optical flow is also an invariant
  property of depth (remains the same when an
  observer is moving towards a point)
• Existence of invariants is evidence that
  perception is direct

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Direct vs. Constructive Perception

• Constructive perception (top-down processing)
• The stimulus provides only the basic working
  material
• Perceptual processes heavily influenced by
  inference and knowledge of world
• Perception involves forming hypotheses about the
  percept using the sensory data as well as
  knowledge and inference

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Direct vs. Constructive Perception

• Linear perspective and texture gradient used to
  infer depth and distance
• Context effects are consistent with top-down
  processing
• Constancies and illusions suggest that perception
  is based on more than the properties of the
  stimulus and that factors such as inference,
  experience and knowledge of the world play an
  important part

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Speech Perception

• Invariance of speech perception
• Perception of speech remains consistent across a
  very large range of physical properties of the
  speech signal
• Pitch, volume, accents, speed
• Indicates that it is not the absolute values of
  the input that matters, but the relationship
  between the speech sounds of a particular speaker

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Speech Perception

• Segmentation problem
• When listening to speech we hear discrete
  words, i.e. words and gaps of silence
• But not so when listening to a foreign language
  we hear a continuous stream
• Also impossible to find gaps in the right places
  in the physical speech signal
• So segmentation is an illusion
• Implies top-down processing

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Speech Perception

• Warren Warren (1970) presented these spoken
  sentences to listeners
• It was found that the eel was on the axle
• It was found that the eel was on the shoe
• It was found that the eel was on the orange
• It was found that the eel was on the table
• The signifies a missing sound.
• Found that participants heard the eel as wheel,
  heel, peel or meal, depending on the sentence,
  i.e. constructive perception