PERCEPTION

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PERCEPTION

• Main Questions
• 1. Atomism vs. holism
• dynamics of perception
• 2. Constructivism vs. direct perception

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Constructivism
Top-down processing

• Stimulation from environment scarce and
  ambiguous. Perception relies on knowledge.
• A percept arises from the interaction of stimuli
  and knowledge (expectations) -- ALWAYS
• Main theorists and researchers Boring, Bruner,
  Neisser (70), Rock

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Constructivism explains

• The influence of experience on perception
• Familiarity ? perception (e.g., Bruner i Postman,
  1949)
• Using context in situations of impoverished
  stimulation
• Illusions

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Critique of constructivism

• Everyday perception is rather accurate
• The problem of the origin of knowledge and
  expectations
• Critique of research conducted in situations of
  insufficient stimulation (short presentations,
  limited visibility, passive observer)

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Direct perception

• James Gibson (1966, 1979), Eleanor Gibson
• Other names gibsonism, ecological psychology,
  bottom-up processing
• Ask not whats in your head but what your head
  is inside of.

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• HOW? -gt WHAT?
• Goal to account for everyday functioning in the
  environment
• Information is rich enough to directly specify
  possible reactions
• Processing and knowledge influence are
  unnecessary
• Information about affordances (perception and
  action are inseparable)
• Information is contained in optic array,
  received by an active, moving observer
  structured light.
• Tuning to the world effect of organisms
  evolution in a specific environment (tuning not
  learning).

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Advantages

• Accounts for accuracy of perception
• Emphasis on
• What is really picked up from the environment
• The importance of innate constraints of
  perception
• Dynamic character of perception (movement,
  activity of observer)
• The importance of studying perception in natural
  conditions (ecological validity of research)

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Disadvantages

• - Difficult to explain
• Perception errors
• Perception in situations of limited stimulation
  (context and knowledge influences)
• Re-cognition and multi-cognition the
  possibility of taking multiple views on an object
• - Affordances of artifacts?

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Synthesis Neisser 1976

• Perceptual cycle relative importance of ??
  depends on a situation

Information from environment
modifies
samples
guides
Knowledge
Exploration
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The debate is still alive

• Constructivists Irvin Rock Indirect Perception
  1997
• Perception is intelligent
• Perception is based on prior perception.
• Eleanor Gibson, ecological psychology (Michael
  Turvey, Claudia Carello)

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Theory of TWO visual systems (Norman, 2001)

• Earlier theories
• Schneiders system where? system what?
  (indication vs identification)
• Cortical visual system ablation no pattern
  discrimination
• Undercutting the tectum disrupted orientation in
  space
• Systems ambient (subcortical) focal (cortical)
• Neisser 1994 separate systems direct perception
  and recognition/representation
• Norman gibsonian dorsal system and
  constructivist ventral system

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Evidence

• Neurophysiological
• Neuropsychological (brain damage)
• dissociations double dissociations
• Psychological experiments

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Neurophysiological Evidence

• Ungerleider Mishkin 2 separate pathways in
  visual cortex. From the occipital lobe
• dorsal stream to the posterior parietal cortex,
• ventral stream to the inferior temporal cortex,
• Similar to Schneiders what and where but
  both cortical
• Signals from visual cortex
• dorsal movement analysis an space signals
  coming from peripheral areas of retina, signals
  from subcortical structures
• ventral analysis of shape and color from areas
  representing central parts of the retina

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• Processing of similar information (shape,
  distance, color) but for different reasons
• dorsal visual control of movement, egocentric
• ventral object recognition, allocentric
• Confirmation from PET research.

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Neuropsychological evidence(Milner and
Goodale95)

• Patients with damage in their parietal lobe
• optic ataxia indication and reaching impairment
  but no trouble in discrimination and
  identification of visual stimuli

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• Damage to inferior temporal lobe, patient DF
  visual form agnosia
• Is not able to recognize faces and objects,
• Incapable of making siple discriminations e.g.
  Between a triangle and a circle,
• Cannot copy pictures nor recognize what she has
  drawn (from memory),
• Capable of carrying out visuomotor activities
• Inserting a card into a slot
• Picking up disks of various widths (information
  about size is available to her but she cannot
  judge which one is bigger)
• Using tools is ok, describing not

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Psychological experiments

• Comparing judgmental responses (evaluative) to
  motor responses (reaching, grasping) to the same
  stimuli in healthy subjects.
• Eg. Reaching for vs. Evaluating the size of a
  disk in the Titchener-Ebbinghaus illusion

• Dorsal system not prone to visual illusions?
• Induced movement illusion reported induced
  eye-movements followed the real movement
  direction!
• Judgmental estimates of distance vs. motor
  responses to distance (e.g. blindfolded walking),
  angle
• Summary motor responses are more accurate.
  Errors do not correlate with verbal evaluation
  errors.

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Comparison of the dorsal and ventral systems
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Two parallel systems corresponding to the two
theories of perception

• Concepts and methods used by gibsonists
  correspond to functions of the dorsal system
• Concepts and methods used by constructivists
  correspond to functions of the ventral system
• Two systems having different functions but
  synergistic, cooperating