PERCEPTION

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PERCEPTION

• HP502 Lecture 12 (week 13)
• Dr Meredith McKague.

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Perception

• The organisation and interpretation of sensations
  into meaningful units.
• For example Speech perception to understand
  speech we need to organise speech sounds
  (phonemes) into meaningful units (words).
• Phonemes delivered at up to 40 per second.
• Additionally, the sound must then be interpreted
  (recognised) as speech.

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Perception.

• Four aspects of perceptual organisation are
• Form perception
• Depth or distance perception
• Motion perception
• Perceptual constancy

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Form Perception

• The organisation of sensations into meaningful
  shapes and patterns - object recognition.
• The study of form perception was greatly
  influenced by the Gestalt psychologists.
• Max Werteimer is considered the founder of
  Gestalt Psychology.

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Form Perception Gestalt principles.

• Gestalt psychologists studied form perception at
  the beginning of the 20th C.
• Gestalt German for whole or form.
• The whole (percept) is greater than the sum of
  its parts (sensations).

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Form Perception Gestalt principles.

• Gestalt psychologists proposed a set of
  principles that are followed automatically and
  unconsciously by the brain to organise sensory
  experience.
• Figure-ground perception
• Similarity
• Proximity
• Good continuation
• Simplicity
• Closure

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Form Perception Gestalt principles.

• Figure-ground perception.
• The brain distinguishes between the object of
  attention (the figure) and the background scene
  (ground).
• For example, try imagining the negative space
  around the text you are reading on the screen as
  being the figure instead of the ground.
• Do you see an old woman or a young woman in the
  picture opposite.
• The black line in the centre of the image can
  either be seen as the old womans mouth or the
  young womans necklace, but not both at the same
  time.

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Figure-ground reversal

• Ambiguous Figures
• Ambiguous figures demonstrate our ability to
  shift between figure and ground.
• It is the shift between figure and ground that
  provides the basis for the two interpretations of
  these figures.
• Ambiguous figures demonstrate how the same
  perceptual input can lead to very different
  representations.
• The mind is actively involved in interpreting the
  input.

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Figure-ground reversal
www.world-mysteries.com/illusions/volti01.jpg
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Figure-ground reversal

• Look at the red dot. Is it located in the upper
  right front or the upper right rear?
• To help you see the different options,the front
  wall of the cube is colored gray.

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Figure-ground reversal
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Form Perception Gestalt principles.

• Proximity
• Elements close together tend to organise into
  units
• Similarity
• Objects that look alike tend to be grouped
  together
• Good continuation
• We see lines as being continuous if they do not
  bend sharply
• Closure
• We prefer to see regular shapes, inferring
  occlusion to do so.

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Gestalt Principles
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Gestalt Principles Similiarity

• The matrices show that the similarity in colour
  is a factor that influences the way we group
  these elements.

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Gestalt principles Good continuation

• In these examples, we tend to group the dots as
  forming two lines
• In the top and middle figures we tend to see the
  A and C segments grouped as a single "line" and
  the B segment grouped as a line rather than the
  A, B, and C segments each as a line.
• In the bottom figure se see the A and b figures
  grouped together and the C as a separate line.

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Gestalt principles Context

• These examples provide evidence for the effect of
  past experience and context on the grouping of
  elements.
• Notice that in the top figure the middle lines
  are grouped into single whole, but in the next
  figure they are grouped into two elements.
• And, finally the image below shows only the
  centre element which is, of course, the same in
  both examples

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Form Perception Gestalt principles.

• The gestalists were suggesting that we need to
  consider the context in which an element occurs
  to understand how it contributes to the whole, or
  gestalt.
• The mind rarely combines things or organizes
  things independent of the context in which they
  appear.
• Consider the two configurations shown on the
  following slide

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Form Perception Gestalt principles.

• In both configurations a larger rectangle is
  placed between two smaller rectangles.
• But note that the "large rectangle" in the top
  figure is exactly the same size as the two
  "smaller rectangles" in the lower figure.
• Here the "context" is influencing the way in
  which the rectangles are encoded.

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Form Perception Gestalt principles.

• The fact that there are two possibilities poses a
  problem for a "passive behaviourist mind.
• The environment does not decide which of these
  alternatives is seen by the viewer.
• The "stimulus, from a behaviourist point of
  view, becomes ambiguous.
• Ambiguous figures present exactly the same
  problem and this is one of the reasons that they
  have been studied so extensively in perception.

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Object recognition Combining features.

• Different areas of the visual cortex process
  separate elements of a scene, e.g., colour,
  texture, line orientation etc.
• Object recognition requires the combining of
  these features into a whole unit, which can then
  be matched against a stored template in the brain
  (recognition memory).
• This process occurs along the what visual
  pathway.

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Object recognition Combining features.

• Biederman developed a theory of object
  recognition called recognition-by-components.
• Perception and recognition of objects in our
  environment occurs by breaking them down into
  component parts and then matching the components
  and their arrangement to stored representations
  in memory.

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Object recognition Combining features.

• Biederman (1987) proposed an alphabet of 36
  elementary geometrial forms called geons
  (geometric icons).
• Geons composed of features such as lines.
• Objects constructed from geons in the same way
  that words are constructed from letters.
• Feature approach to object perception
• Basic idea Analysis precedes synthesis
• 1. Analysis stage feature units
• 2. Synthesis stage(s) object units

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Recognition by Components
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Recognition by Components
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Recognition by Components

• Biederman et al. (1987, 1993, 2001)
• Geons are robust to degraded perceptual input.

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Object recognition Combining features.

• The combination of primitive visual sensations
  into geons allows rapid identification of
  objects.
• This explains how we recognise objects when parts
  of them are occluded or missing.
• Gestalt principles apply to geons.
• Beidermans theory predicts that failures of
  object recognition should occur when the line
  where two geons intersect is missing.

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Recognition by Components
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Perceptual illusions.

• Perceptual illusions occur when normal perceptual
  processes produce misinterpretations.
• Impossible figures provide conflicting cues about
  3D organisation.
• As soon as the brain organises the image one way
  it is invalidated by the alternative.

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Perceptual Illusions
The IKEA instructions from HELL.
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Depth or Distance Perception

• The organisation of perception in three
  dimensions
• Two kinds of visual information provide important
  information about depth and distance
• Monocular cues visual input from one eye
• Binocular cues visual input integrated from the
  two eyes

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Monocular Depth Perception Cues

• Interposition
• One object blocks another
• Linear perspective
• Lines converge
• Texture gradient
• Distant objects finer
• Shading
• 3D objects cast shadows

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Monocular Depth Perception Cues

• Aerial Perspective
• Far objects are fuzzy
• Familiar size
• Familiar objects that appear small are inferred
  to be distant
• Relative size
• The smaller of 2 objects is seen as further away
  

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Monocular Depth Perception Cues

• Motion Parallax.
• Created by the apparent difference in the speed
  with which objects near and far away seem to move
  across the field of vision.
• The relative motion of nearby and distant objects
  is obvious when looking out of the window of a
  moving car or train.
• Nearby trees or posts seem to speed by, whereas
  distant objects barely seem to move.

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Binocular Depth Perception Cues

• Because each eye is in a different location
  objects produce separate images on each retina.
• This creates retinal disparity (diminishes with
  distance).
• Convergence when you look at something close
  you get the sensation of your eye balls
  converging (moving towards each other), when an
  object is further away you sense divergence.
• Note, convergence is actually a kinesthetic
  sensation.

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Motion Perception

• Visual system is wired to detect motion
• Rods in retina are sensitive to motion
• Parafoveal motion detector cells in the retina
  enable us to turn our eyes quickly towards
  something moving towards us.
• Motion detector cells have large visual fields.
• Neurons in visual cortex respond to motion.
• Information then travels along the where
  pathway to the medial temporal lobes (MT).
• Cells in MT tuned to detect movement in different
  directions.

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Motion Perception

• Two systems for processing movement
• A eye is stationary as object moves across the
  visual field.
• B eye moves to maintain object at same place on
  the retina muscle signals are used to infer
  movement.

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Motion Perception
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Illusions of motion
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Illusions of motion.

• The "Enigma", by the French painter Isia Leviant,
  is a painting which exhibits a strong sensation
  of motion around the blue bands. 

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Illusions of motion.

• On the next slide is the rotating snake by
  Akiyoshi Kitaoka.  
• The rapidly decreasing size of the shapes inside
  the circles results in an illusion of motion.
• This causes your brain to make the circles spin
  when in fact they are motionless.
• This tells us that ordinary perception may
  recognize a sudden decrease in size in a circular
  form as an indication of motion.
• www.cogsci.rpi.edu/cogworks/cogpsy/illusions.html

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Perceptual Constancy

• The perception of objects remains relatively
  stable despite changes in the stimulation of
  sensory receptors.
• For example still recognise a song on the radio
  despite turning the volume down.
• Still recognise someone walking away from you as
  the same size despite shrinking image on the
  retina.
• Cultural differences Rainforest dwellers
  confused by ants turning into Buffalo when
  taken out onto a savana.

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Perceptual constancy

• Size constancy.
• Helmholtz (1909) described how the brain adjusts
  for distance when assessing the size of objects.
• This process of adjustment occurs by unconscious
  inference because we have no conscious awareness
  of the processes involved.
• Perception is intelligent.
• However, unconscious inferences can lead to
  visual illusions.

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Perceptual constancy Muller-Lyer illusion.
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Perceptual constancy Ponzo illusion.

• According to Gregory, we interpret the Ponzo
  illusion as a 3-dimensional figure, with the
  upper horizontal assumed to be farther away than
  the lower. 
• Because of size constancy, we assume that of two
  objects casting the same size retinal image, the
  more distant one must be larger.
• So that's what we perceive.

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Top-down and Bottom-up Processing

• Recognition of objects, animals, faces, etc is
  influenced both by bottom-up sensory input and
  top-down knowledge of previous experience.
• Top-down processing is influenced by the
  observers expectations and stored knowledge.
• The brain forms a hypothesis about an incoming
  pattern of sensory stimulation.
• Again, perception is intelligent.

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Top-down and Bottom-up Processing
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Top-down and Bottom-up Processing
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Bottom-up and top-down processing in letter
recognition.

• The interactive activation model (McClelland and
  Rumelhart (1981).
• Word-superiority effect easier to recognise a
  letter in the context of a word than it is to
  recognise it in isolation.
• Perception of letters is influenced by top down
  and bottom up processes.

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AN A is an A Shared features.
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The importance of context (and knowledge).
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Lecture summary.

• Form perception
• Gestalt principles especially figure-ground and
  ambiguous figures.
• Object recognition Biedermans theory of
  recognition-by-components.
• Depth or distance perception.
• Monocular (esp. motion parallax).
• Binocular cues (esp. retinal disparity and
  convergence).
• Motion perception
• Two systems for processing movement.
• Perceptual constancy.
• Define.
• Unconscious inference.
• Explanation of Ponzo illusion in terms of size
  constancy.
• Top-down and bottom-up processing
• Define.
• Example of the interactive activation model of
  letter recognition.

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Final words.

• THANKYOU for a great semester and GOOD LUCK for
  the future.
• Cheers, Meredith.