Visual Perception Is a Creative Process

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Visual Perception Is a Creative Process

• Instructor Dr. S. Gharibzadeh
• Presented By J. Razjouyan

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• Visual perception has often been compared to the
  operation of a camera.
• Like a camera, retina.
• three-dimensional perception of the world from
  the two-dimensional
• cognitive function of the visual system
• different visual conditions

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• the visual system transforms transient light
  patterns on the retina into a coherent and stable
  interpretation of a three-dimensional world.
• recognize in a melody is not simply the sequence
  of particular notes but their interrelationship.
• played in different keys will still be
  recognized as the same melody because the
  relationship of the notes remains the same.
• Likewise, different images under a variety of
  visual conditions, because the relationships
  between the components of the image are
  maintained by the brain.

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Your perception rapidly alternates between the
two figures. The perceptual distinction between
figure (or object) and ground is similar to the
communication engineer's distinction between
signal and noise. As we focus on one signal,
other information is relegated to background
noise.
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An outline drawing, typical of children's
drawings, has clearly recognizable objects
because edges are powerful cues in the perceptual
organization of the visual field.
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Illusions

• misreading of visual information by the brain,
  also illustrate how the brain applies certain
  assumptions about the visual world to the sensory
  information it receives.
• the brain uses shape as an indicator of size

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Illusions

• demonstrate certain organizational mechanisms of
  visual perception
• selection,
• distortion,
• filling in of omissions.

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Filling-in
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• spatial relationships of objects
• we judge the size of an object by comparing it to
  its immediate surroundings.
• In this comparison we also rely on our
  familiarity with objects in the visual field

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assumption of occlusion

• The integration of distinctive objects into a
  coherent visual scene is aided by another central
  fact of vision closer structures cover those
  that are more distant.

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Perception on inferencesshadow ?convex / concave

• You can reverse the depth of these objects by
  imagining a shift in the light source from the
  top of the figure to the bottom.

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• In this array, once you see one column as convex
  the other column will appear concave. It is
  almost impossible to see both rows as
  simultaneously convex or concave
• The assumption of a single light source may have
  evolved because our natural environment has only
  one source of light, the sun, and we assume that
  the source of light is always above.

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• Gestalt theorists,
• What are the basic components of this
  perception?
• How does the brain produce this perception?
• framework

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Visual Information Is Processed in Multiple
Cortical Areas
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• various unrelated attributes
• motion,
• depth,
• form,
• Color
• are all coordinated in a single percept.

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• hierarchical neural system
• multiple visual areas
• interacting neural pathways.
• Distributed processing

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• photoreceptors
• bipolar cells
• retinal ganglion cells,
• the output cells of the retina.
• The axons of ganglion cells of the retina form
  the optic nerve,
• lateral geniculate nucleus
• in the thalamus.
• primary visual cortex
• (Brodmann's area 17 or V1, also called the
  striate cortex )

Projection definition
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32 representations of the retina in the
extrastriate areas
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• Over 50 of the neocortex of the macaque monkey
  is devoted to processing visual information,
• while only 11 is somatosensory cortex and
• 3 is auditory cortex.

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Different Cortical Areas Make Different
Contributions to the Processing of Motion, Depth,
Form, and Color
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• How separable is processing of motion from that
  of form, and either of these from processing of
  color?

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Frames of Reference
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• A major task for the brain is to construct three
  successive frames of reference for visual
  perception and the control of movement
• a retinotopic frame of reference,
• a head-centered frame of reference,
• and a body-centered frame of reference.

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We refer to this map as a retinotopic map or a
retinotopic frame of reference.

• Visual information leaving the retina is
  organized into a two-dimensional map of the
  visual field.

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• Each time the eye moves the retinotopic frame of
  reference moves as well.
• Anything that is anchored to the frame of
  reference, such as the afterimage produced by a
  flash of light, moves with it.

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• same visual field with respect to the head. In
  this frame of reference
• anything in the visual field that moves with the
  head
• remains stable.
• The brain constructs this head-centered frame of
  reference by combining
• the retinotopic frame of reference
• with added information about
• the eye position.

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• Likewise, a body-centered frame of reference can
  be constructed by combining
• information about eye movement
• and head movement with
• information about posture.
• Thus one frame of reference is built upon
  another.

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• clinical observations
• parietal cortex is specialized for spatial
  representation,
• whereas the temporal cortex is specialized for
  object recognition.

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tests of monkeys with lesions in the posterior
parietal or inferior temporal cortex

• Ablation of the posterior parietal cortex altered
  the monkey's ability to locate objects visually,
  including the ability to guide hand movements to
  reach them, but did not affect the ability of the
  monkey to identify objects. (orientation)
• In contrast, lesions of the inferior temporal
  cortex impaired the monkey's ability to identify
  objects when the discriminations required use of
  color, orientation, pattern, or shape but did not
  affect the monkey's ability to locate objects in
  space. (identification)

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agnosias (loss of knowledge)
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Parallel Pathways Convey Information From the
Retina to Parietal and Temporal Cortical Areas