Outline

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Outline

• Human Visual Information Processing cont.
• Models of some visual processing stages

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Visual Pathway
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How to understand the visual perception

• Neurophysiology
• Recording of cell responses
• Functional MRI
• Psychophysics
• Determination of the relationship between the
  magnitude of a sensation and the magnitude of the
  stimulus that gave rise to the that perceptual
  sensation

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Retinal Ganglion Cell Responses
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Retinal Ganglion Receptive Fields

• Has a circular center-surround organization
• Two major classes
• On-center
• Off-center
• How do they respond to a small spot of light?

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Simple Cells in the Visual Cortex
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Simple Cells

• rectangular shaped receptive fields
• segregated ON and OFF zones
• respond to a bright or dark bar
• represent a restricted region in the visual field
  
• respond best to a specific orientation
• non-optimally oriented stimuli will be
  ineffective in stimulating the neuron

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Complex Cells

• larger receptive field than simple cells
• orientation tuned
• ON and OFF zones are mixed in the receptive field
  
• respond well to a moving bar
• direction selective

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Hyper-complex Cells

• receptive field is selective for the length of
  the stimulus
• similar to complex cell receptive fields
  (orientation and direction selective)
• selective for features of shape such as length
  and width of the bar of light.

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Visual Perception

• Modern view is that visual transformation is a
  creative process
• Vision transforms light stimuli on the retina
  into mental constructs of a stable 3D world
• Visual perception is a 3D percept of the world
  that is invariant to a wide range of changes in
  illumination, size, shape, and brightness of the
  image

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Adaptation

• Adaptation
• Prior exposure affects the perception of
  brightness

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Webs Law

• The difference threshold is not constant
• The difference threshold changes as a function of
  the magnitude of the standard stimulus

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Contrast sensitivity function
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Single Channel or Multiple Channels
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Neural Spatial Frequency Channels

• Neural receptive fields are tuned to the spatial
  frequency of the stimulus
• There seems to be a range of neural spatial
  frequency channels, each tuned to a different
  spatial frequency
• A spatial frequency channel can be adapted

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Virtual Contours
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Reconstruction of Visual Perception
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Reconstruction of Visual Perception
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Exercise 1
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Answers to Exercise 1
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Answers to Exercise 1 cont.
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Answers to Exercise 1 cont.

• Paolina Borgheseby Antonio Canova (1757-1822) -
  Galleria Borghese, Rome