central visual processes. anthony j greene2 central visual pathways i.primary visual cortex...
TRANSCRIPT
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Central Visual Processes
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Anthony J Greene 2
Central Visual Pathways
I. Primary Visual CortexReceptive Field
• Columns• Hypercolumns
II. Spatial Frequency
III. Nerve or Cortical Damage
IV. Higher Visual Areas
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Occipital Lobe
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Occipital Lobe: Calcarine Sulcus -- V1 -- Striate Cortex
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Cells in V1
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Single-Cell Recording (Hubel & Weisel, 1962)
• Attempted to discover what sorts of information cells in (cat) V1 respond to
• Accidentally discovered orientation specific cells organized into columns and hypercolumns
V1
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Cells In V1
Cells in V1 receive messages from certain ganglion cells such that they respond to stimuli of a certain orientation from a small portion of the retina - Orientation Specific
~ 200 Million Cells in V1
Inputs from Ganglion Cells
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Cells In V1
• One V1 cell receives inputs from many ganglion cells
• One ganglion cell may send inputs to numerous V1 cells
• Stimuli from every possible orientation, and from every position in the visual field are detected by different cells in V1
• Simple Cells detect only orientation -- Complex Cells detect orientation and motion
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How to Make a Complex Cell
• Orientation specific inputs from ganglion cells is similar to simple cells
• However, the receptive field is much larger and is designed to respond maximally when inputs from sub-fields are sequential
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Cells in V1
Occular Dominance
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Columns in V1
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Organization of Cells in V1• Columns are sections of cortex which all respond to
the same orientation from approximately the same region of cortex
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Organization of Cells in V1• Hypercolumns are groups of columns, from both
eyes, which are influenced by the same minute portion of the visual field
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Organization of Cells in V1• What sort of information are these cells detecting? • Is the information from any single cell in V1
informative?
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Response Properties of Cells in V1The extent to which columns will respond to stimuli with no interactions from other columns
Orientation Column Position on Occular Dominance Slab
Cel
lula
r A
ctiv
ity
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Lateral InhibitionThe column with the strongest response to a given stimuli will suppress the respondse of neighboring columns
+
-
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Response Properties of Cells in V1The extent to which columns will respond to stimuli with lateral inhibition from other columns
Orientation Column Position on Occular Dominance Slab
Cel
lula
r A
ctiv
ity
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Processing at V1 Is Edge Detection
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Edge Detection
• While lateral inhibition normally improves the accuracy of edge detection, in this case it creates the “Deli Wall Illusion”
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Understanding Acuity: Spatial Frequency Analysis
• Measuring visual acuity:
– Eye doctors use distance (e.g., 20/20)
– Vision scientists use visual angle
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Understanding Acuity: Spatial Frequency
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Understanding Acuity: Spatial Frequency
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Describing Processes in V1: Spatial Frequency Analysis
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Describing Processes in V1: Spatial Frequency Analysis (cont.)
• Orientation
• Frequency
• Contrast
Orientation
Decreasing Contrast
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Spatial Frequency Analysis (cont.)
• Fourier - French mathematician, came up with theory that one can create any complex wave through a summation of Sinusoids (or sub-parts, sub-waves)
• Fourier Analysis divides all orientation specific cells in V1 according to the width of their receptive fields or Spatial Frequency
• 1) Low
• 2) Medium
• 3) High
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Spatial Frequency Analysis (cont.)• Neurons can then be viewed as Spatial Filters which
separately analyze differing levels of detail or scale
• Any scene can then be decomposed into images with varying spatial frequencies - low frequency images are blurry and only the most prominent features are represented - high frequency images exaggerate the fine details
• Construing form vision in terms of an emergent property of these different scales of receptors is referred to as the Multichannel Model
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Spatial frequency Analysis (cont.)
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Spatial frequency Analysis (cont.)
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Spatial frequency Analysis (cont.)
• Once divided by width, cells can further be grouped according to their orientation specificity
• This allows a vastly simplified organization of neural activity - 3 major variables - Spatial Frequency, Orientation & Contrast
• Additionally, Fourier analysis helps explain how individual cells may contribute information to the aggregate
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Spatial frequency Analysis (cont.)
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Spatial frequency Analysis (cont.)
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Spatial Frequency Analysis (cont.)
• 1f gives the fundamental waveform
• 2f ... xf : are called harmonics - increasing details
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Spatial Frequency Analysis (cont.)
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Spatial Frequency Illusions
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Spatial Frequency Illusions
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Spatial Frequency Illusions
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Spatial Frequency Illusions
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Color at V1
• Among cells selective for orientation are patches of cells selective for color (and not orientation), which are known as Blobs.
• Other cell (orientation specific cells) regions are known as interblobs.
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Organization of V2
• Thin Stripes receive information from Blobs and pass it to V4
• Thick Stripes recieve information from complex cells and send it to V5 and V3
• Interstripes recieve information from simple cells and send it to V3 and V4
• Information at V2 is 3-D
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Nerve or Cortical Damage1) Retina / Optic Nerve2) Optic Chiasm3) Optic Tract4) V1/V2
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Nerve or Cortical DamageRetina/Optic Nerve: Monocular blindness
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Nerve or Cortical DamageOptic Chiasm: Nasal field blindness
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Nerve or Cortical DamageOptic Chiasm: Bitemporal field blindness
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Nerve or Cortical Damage
Optic Tract/LGN/Radiations: Homonymous Blindness
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Nerve or Cortical DamageV1: Quadrantic blindness
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Nerve or Cortical Damage
V1/V2:
• Scotoma
• Complete blindness
• case of Blindsight
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Higher Visual Areas
• V3: Form & Dynamic Form
• V4: Color
• V5: Motion
• IT: What System: Object Recognition– Lingual Gyrus of IT: Face Recognition
• PP: Where System: Object Location and Navigation
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Simplified Functional Visual Anatomy
Type Lobe Area Function
Primary Occipital V1 (17) Initial Processing
Secondary Occipital V2 (18) 3-D Form
Secondary Occipital V3 (19) Dynamic Form
Secondary Occipital V4 (19) Color & Form
Secondary Occipital V5/MT (37) Motion
Tertiary Temporal IT (20, 21, 22) "What"
Tertiary Parietal PP (7) "Where"
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Simplified Flow Diagram of the Visual System
LGN
Parvo
Magno
Thalamus Occipital Lobe
Temporal Lobe
Parietal Lobe
V1 V2
V3
V4
V5 or MT IT
Optic Nerve
PP
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Summary
Color Form Motion
Eye / LGN P P M
V1 Blobs Simple Complex
V2 Thin Stripes Interstripes Thick Stripes
V4Form &Color
V3Dynamic
Form
V5Motion
} } }
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