Week 3: Physiology of HigherOrder Visual Processing

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Week 3 Physiology of Higher-Order Visual
Processing
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Review Important concepts from Week 2

• Neurons and Neural Signaling
• Light, Light Reception and Image Formation
• The Retina and Transduction
• Dark Adaptation and the Absolute Luminance
  Threshold

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Week 3 Outline

• Sensitivity vs. Acuity
• Receptive Fields
• Visual Pathways

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The Retina Sensitivity vs. Acuity

• Different areas of the retina are specialized for
  high sensitivity and high acuity
• Receptor distribution
• Retinal Periphery is specialized for sensitivity
  the ability to see dimly illuminated objects as
  measured by the luminance threshold
• Fovea is specialized for acuity ability to
  distinguish spatial detail as measured by visual
  angle
• Recognition acuity (Snellen letters)
• Vernier acuity (offset lines)
• Resolution acuity (gratings, Landolt rings,
  Figures)
• Dynamic acuity (detecting moving targets)
• Visual acuity across the retina

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Consequences of Retinal Specialization

• Eye placement (frontal vs. lateral) and visual
  field size
• Eye movements
• Saccades
• Saccadic suppression or omission (eye-blinks too)
• Pursuit and vergence
• Vestibular Ocular Reflex (VOR)
• Optokinetic Nystagmous

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Visual Receptive Fields

• Receptive field of visual neuron
• the area of the visual field (defined in retinal
  coordinates) that, when stimulated with light,
  produces a change in the neurons firing rate
• Excitation increase in firing rate above
  spontaneous firing rate
• Inhibition decrease in firing rate below
  spontaneous firing rate
• Receptive fields can be tuned to specific
  properties of a light stimulus (e.g.,
  orientation, movement, color, shape, which eye is
  stimulated, etc.)

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Retinal Circuits for Sensitivity and Acuity

• Neural wiring for high sensitivity sensitive
  receptors (rods) connected to ganglion cells
  through circuits with convergence (spatial
  summation)
• Peripheral retina
• Peripheral ganglion cells have large receptive
  fields and low luminance thresholds
• Neural Wiring for high acuity smaller receptors
  (cones), which can be more densely packed,
  connected to ganglion cells through linear
  circuits (11 ratio between receptors and
  ganglion cells)
• Fovea
• Foveal ganglion cells have small receptive fields
  and high luminance thresholds

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Spatial Properties of Ganglion Cell Receptive
Fields

• Center-surround organization
• On-off and off-on cells
• Lateral inhibition
• Hartline, Wagner, Ratcliff (1956) the limulus
• Contrast enhancement
• Psychological phenomena resulting from contrast
  enhancement
• Hermann Grid
• Mach Bands

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Temporal Properties of Ganglion Cell Receptive
Fields

• P-cells vs. M-cells
• P-cells exhibit slow-sustained response
• M-cells exhibit fast-transient response
• W-cells have more specialized responses

on
on
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Visual Pathways from Eye to Brain

• Optic Chiasm
• Left visual field projects to right hemisphere of
  brain, right visual field projects to left
  hemisphere of brain
• Tectal Pathway to superior colliculus
• Eye movements, visual and postural reflexes
• Pretectal pathway to the thalamus and lateral
  geniculate nucleus (LGN)
• Retinotopic map
• Parvocellular division
• Magnocellular division

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Primary Visual Cortex

• Also known as V1, striate
• Receptive Fields in primary visual cortex (V1)
• Hubel Wiesel (1959, 1962)
• Simple Cells (orientation selective)
• Complex Cells (Motion direction selective)
• Hypercomplex (end-stopped, motion sensitive)
• Functional Architecture of V1
• 1 mm square hypercolumns of neurons
• selective for orientation and ocular dominance
• Retinotopically organized receptive fields
• Cortical magnification emphasizes representation
  of fovea relative to peripheral retina

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Extrastriate Visual Cortex

• Two Functionally-specialized pathways
• Dorsal pathway to parietal lobe where or how
• Ventral pathway to temporal lobe what
• Examples of extrastriate areas
• V4 color coding
• Infero-temporal (IT) Cortex respond to specific
  shapes
• Medial-temporal (MT) Cortex (V5) cells respond
  to motion, binocular disparity

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Links to More Information

• LGN
• http//www.ks.uiuc.edu/zeller/morphogenesis.html
  
• Visual Pathways
• http//www.lea-test.fi/en/vistests/pathways/pathwa
  ys.html
• http//www.biols.susx.ac.uk/home/George_Mather/Lin
  ked20Pages/Physiol/Cortex.htmlanchor973576

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End of Week 3

• Be sure to do the homework and write a journal
  entry!
• For the next Lesson read Chapter 7 of your text
  book
• Whats coming up in Week 4?
• Color Vision