Spatial Vision

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Spatial Vision
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Visual Acuity Oh Say, Can You See?

• The King said, I havent sent the two
  Messengers, either. Theyre both gone to the
  town. Just look along the road, and tell me if
  you can see either of them.
• I see nobody on the road, said Alice.
• I only wish I had such eyes, the King remarked
  in a fretful tone.
• To be able to see Nobody! And at that distance,
  too!
• Lewis Carroll, Through the Looking Glass

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Spatial Vision

• Visual Acuity Oh Say, Can You See?
• Retinal Ganglion Cells and Stripes
• The Lateral Geniculate Nucleus
• Striate Cortex
• Receptive Fields in the Striate Cortex
• Columns and Hypercolumns
• Selective Adaptation The Psychologists
  Electrode
• The Girl Who Almost Couldnt See Stripes

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Visual Acuity Oh Say, Can You See? (contd)

• What is the path of image processing from the
  eyeball to the brain?

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Visual Acuity Oh Say, Can You See? (contd)

• Contrast The difference in illumination between
  a figure and its background

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Visual Acuity Oh Say, Can You See? (contd)

• Acuity The smallest spatial detail that can be
  resolved

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Visual Acuity Oh Say, Can You See? (contd)

• Measuring visual acuity
• Eye doctors use distance (e.g., 20/20)
• Vision scientists use the smallest visual angle
  of a cycle of grating

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Visual Acuity Oh Say, Can You See? (contd)

• The visual system samples the grating discretely

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Sine Wave Gratings
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Visual Acuity Oh Say, Can You See? (contd)

• Herman Snellen invented method for designating
  visual acuity in 1862

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Visual Acuity Oh Say, Can You See? (contd)

• Spatial Frequency The number of cycles of a
  grating per unit of visual angle (usually
  specified in degrees)

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Visual Acuity Oh Say, Can You See? (contd)

• Cycles per degree The number of dark and bright
  bars per degree of visual angle

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Visual Acuity Oh Say, Can You See? (contd)

• Why sine gratings?
• Patterns of stripes with fuzzy boundaries are
  quite common
• The edge of any object produces a single stripe,
  often blurred by a shadow, in the retinal image
• The visual system appears to break down images
  into vast number of components, each is a sine
  wave grating with particular spatial frequency

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Retinal Ganglion Cells and Stripes

• Retinal cells like spots of light

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Response of ON-center Retinal Ganglion Cell
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The Lateral Geniculate Nucleus

• Two lateral geniculate nuclei (LGNs) Axons of
  retinal ganglion cells synapse there

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The Lateral Geniculate Nucleus (contd)

• Two types of layers in LGN Magnocellular vs.
  Parvocellular

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The Lateral Geniculate Nucleus (contd)

• Ipsilateral Referring to the same side of the
  body (or brain)
• Contralateral Referring to the opposite side of
  the body (or brain)

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Striate Cortex

• Striate cortex
• Also known as primary visual cortex or V1
• Major transformation of visual information takes
  place in striate cortex
• It has about 200 million cells!

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Striate Cortex (contd)

• Two important features of striate cortex
• Topographical mapping
• Dramatic scaling of information from different
  parts of visual field

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Mapping of Objects in Space onto the Striate
Cortex
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Striate Cortex (contd)

• Visual acuity declines in an orderly fashion with
  eccentricity

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Receptive Fields in Striate Cortex

• Selective Responsiveness Orientation tuning
  tendency of neurons in striate cortex to respond
  optimally to certain orientations, and less to
  others

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Orientation Tuning Function of a Cortical Cell
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Receptive Fields in Striate Cortex (contd)

• How are the circular receptive fields in the LGN
  transformed into the elongated receptive fields
  in striate cortex?
• Hubel and Wiesel Very simple scheme to
  accomplish this transformation

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Receptive Fields in Striate Cortex (contd)
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Receptive Fields in Striate Cortex (contd)

• Many cortical cells respond especially well to
• Moving lines
• Bars
• Edges
• Gratings
• Direction

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Receptive Fields in Striate Cortex (contd)

• Each LGN cell responds to one eye or the other,
  but never to both, but
• Each striate cortex cell can respond to input
  from both eyes

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Receptive Fields in Striate Cortex (contd)

• Simple cells vs. complex cells

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Receptive Fields in Striate Cortex (contd)

• End Stopping Process by which cells in the
  cortex first increase their firing rate as the
  bar length increases to fill up its receptive
  field, and then decrease their firing rate as the
  bar is lengthened further

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Response of Simple vs. Complex Cells
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Stimulus Extending Beyond the Receptive Field
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Columns and Hypercolumns

• Column A vertical arrangement of neurons
• Hubel and Wiesel Found systematic, progressive
  change in preferred orientation all orientations
  were encountered in a distance of about 0.5 mm
• Hypercolumn A 1-mm block of striate cortex
  containing all the machinery necessary to look
  after everything the striate cortex is
  responsible for, in a certain small part of the
  visual world (Hubel, 1982)

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Orientation and Ocular Dominance Columns in the
Cortex
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Columns and Hypercolumns (contd)

• Regular array of CO blobs in systematic
  columnar arrangement (discovered by using
  cytochrome oxidase staining technique)

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Selective Adaptation The Psychologists Electrode

• Method of Adaptation The diminishing response of
  a sense organ to a sustained stimulus

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Selective Adaptation (Part 1)
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Selective Adaptation (Part 2)
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Stimuli for Demonstrating Selective Adaptation
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Selective Adaptation The Psychologists
Electrode (contd)

• Tilt aftereffect Perceptual illusion of tilt,
  provided by adapting to a pattern of a given
  orientation
• Supports the idea that the human visual system
  contains individual neurons selective for
  different orientations

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Selective Adaptation The Psychologists
Electrode (contd)

• Selective Adaptation Evidence that human visual
  system contains neurons selective for spatial
  frequency

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Adaptation that Is Specific to Spatial Frequency
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Selective Adaptation The Psychologists
Electrode (contd)

• Adaptation experiments provide strong evidence
  that orientation and spatial frequency are coded
  by neurons somewhere in the human visual system
• Cats, Monkeys Striate cortex, not in retina or
  LGN
• Humans operate the same way as cats and monkeys
  with respect to selective adaptation

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Selective Adaptation The Psychologists
Electrode (contd)
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Selective Adaptation The Psychologists
Electrode (contd)

• Spatial frequency channels
• Why would the visual system use spatial frequency
  filters to analyze images?
• Different spatial frequencies emphasize different
  types of information

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Frequency Components of an Image
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High-Spatial-Frequency Mask
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The Girl Who Almost Couldnt See Stripes

• Story of Jane Abnormal early visual experience
  resulting in possibly permanent consequences

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The Girl Who Almost Couldnt See Stripes (contd)

• Monocular from deprivation can cause massive
  changes in cortical physiology, resulting in
  devastating and permanent loss of spatial vision

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The Girl Who Almost Couldnt See Stripes (contd)

• Cataracts strabismus can lead to serious
  problems, but early detection and care can
  prevent such problems