Human Visual Processing

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Human Visual Processing

• Sanketh Shetty
• Image Analysis Group
• NC State University

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Overview

• Structure of the system
• Retina
• Lateral Geniculate Nucleus
• Superior Colliculus
• Primary Visual Cortex
• Higher Visual Areas (later)

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Visual Field
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Three Layers

• Receptors ( Rods and Cones)
• Bipolar Cells
• Horizontal and Amacrine Cells
• Retinal Ganglion Cells

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The Retina
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Receptors

• Rods
• More numerous
• Concentrated on Periphery
• More sensitive to Light (dark vision)
• Long and slender
• Only one type of pigment

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• Cones
• Concentrated in the fovea (0.5 mm in di)
• Daylight vision
• Also responsible for color vision (3 types of
  pigments) (Wald at Harvard first proposed the
  bleaching theory)
• Short and tapered
• Receptors are normally active and when light
  falls on them they are hyperpolarized.

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The Retina
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Second layer

• Bipolars
• Horizontal
• Amacrine Cells

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Bipolars

• Involved in both Direct and indirect paths
• Center Surround Structure that is reflected onto
  the Retinal Ganglion Cells.
• Activity in terms of transmitter release
• Surround gt Horizontal Cells

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The Retina
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Retinal Ganglion Cells

• First look at receptive fields
• 1251 reduction
• Direct and indirect paths from receptors

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Parvo and Magno Ganglion Cells

• Center Surround
• Small receptive fields
• Low contrast sensitivity
• High Spatial acuity
• Low Speed

• Center Surround and Center only
• Large RF
• High Contrast Sensitivities
• Low Spatial Acuity
• High Speed (SC)

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Receptive Fields of Ganglion Cells

• Fire irregular but steady pulses in dark or
  diffuse light.
• 2 types of Cells
• On Center
• Off Center
• Center Surround Structure

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Center Surround Structure

• Smallest in the fovea and largest in the
  periphery.

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A Look at the Structure
Topographic
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Lateral Geniculate Nucleus

• Each receives inputs from Ganglion Cells
• Receives inputs from Higher Cortical Areas
  (dynamic receptive fields)
• Intra-region connections also exist

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Layers of the LGN

• Alternating R-L
• Dorsal Parvo (ganglion cells) top 4 layers
• Ventral Magno (ganglion cells) bottom 2 layers
• Same Center Surround Receptive Field.

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Superior Colliculus

• Magno cells input
• Feedback from higher cortical areas
• Quick to respond
• Sensitive to luminance changes in time and space
• Junction box for interaction between senses
• Projects directly to V5 (MT and FEF)
• Constantly updates new information (does not feed
  to V1)

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Primary Visual Cortex

• Architecture
• Three types of Cells
• Simple
• Complex
• End Stopped

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Architecture of the Visual Cortex

• Area 17
• Area 17 gt Area 18 (V2)
• Area 18 projects to
• Medial Temporal
• V3
• V4

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Area 17 (Striate Cortex)

• The richest connections run vertically
• Very little horizontal or diagonal spreading
• The analysis in V1 is local
• Not the seat of perception
• Architecture is complex (No details for now)

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Overview
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Simple Cells

• No longer Center Surround
• Spatial Summation
• Respond best to an optimally oriented stationary
  slit that lies within the receptive field
• Smallest in the foveal region
• Sensitive to spatial frequency also. (De Valois
  and De Valois)
• How are they constructed?

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One possibility
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Complex Cells

• 75 of cortical cells
• Orientation Specific
• Respond only in a receptive field
• Not responsive to stationary slits or spots
  shining on them.
• Motion Sensitive
• Sometimes sensitive to only one direction of
  motion (20 of cells in the striate)

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Directional Sensitivity (Barlow-Levicks Model)
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More on how we see

• 75 of cells in the V1 are tuned to Motion
• How do we see stationary scenes?
• Saccades
• Micro-saccades
• Stationary Scenes disappear (if we stabilize the
  image on the retina)

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End Stopping

• Spatial Summation (only over fixed space)
• Possibly constructed from Simple cells. Complex
  Cell architecture is also possible
• Optimal response
• Curve with appropriate orientation within
• Appropriately oriented line of fixed length

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Patterns in V1

• Ocular Dominance Columns
• Orientation Columns

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Orientation Columns

• Vertically same orientation preference (except
  layer 4Cß)
• Horizontally the preference changes by 10 degrees
  every 0.05mm

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Visual Processing

• 2mm x 2mm area of cortex
• Complete processing of one area of VF
• Layer 3 (layer 4 has 4x4)
• In each module (hyper column) corresponding to
  each area in the VF
• Orientation
• Eye dominance

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(No Transcript)
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Orientation Columns

• 400 degrees per mm

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Magnification

• Fovea
• 1 degree of VF maps to 6mm on cortex
• Periphery
• 1 degree of VF maps to 0.15mm on cortex

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Disproportionate Representation
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Looking Ahead

• 2 Parts of the Visual Stream
• Ventral Stream (V1gtIT)
• Objects, Shapes, Color, Texture
• Dorsal Stream(V1gtMTgtPC)
• Visual Information, Position, Movement, Action

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The Where and What Streams
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Visual Hierarchy

• Dorsal and Ventral Streams arent independent
• The visual information flows 2 ways
• It needs to emphasize
• What it is looking at (to the Dorsal)
• Where it is (to the Ventral)

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More about the Visual Streams

• Speed
• Time
• Feedback, horizontal and feed forward connections
• Dynamic Receptive Fields

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What we need to look into

• The Ventral Path (V1 and IT)
• Significance of Feedback to our algorithm

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Resources

• Eye, Brain and Vision by Dr. David Hubel
• The Thinking Eye, The Seeing Brain by James Enns
• We need to look at Topographica/LISSOM (from
  University of Texas at Austin)

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The End

• Images courtesy Dr.Hubels Homepage