Download Ch03 Lecture Part I

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Visual Acuity
• Acuity: The smallest spatial detail that can be resolved
Spatial Vision
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Visual Acuity (cont’d)
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Discrete Sampling of Gratings
• 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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Snellen Letters
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• Herman Snellen invented method for designating
visual acuity in 1862
Contrast Sensitivity and Spatial Frequency
• Spatial Frequency: The number of cycles of a
grating per unit of visual angle (usually specified in
degrees)
• Contrast: The difference in illumination between a
figure and its background (difference between darker
and lighter regions)
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Visual Acuity: Oh Say, Can You See? (cont’d)
• Contrast Sensitivity Function
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Retinal Ganglion Cells and Stripes
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Response of ON-center Retinal Ganglion Cell
Pathways from the Eye
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The Lateral Geniculate Nucleus
• Retinal cells: like spots
of light (of the right size)
• Also like gratings (of the
right size)
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• Two lateral geniculate nuclei (LGNs): Axons of retinal
ganglion cells synapse there
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The Lateral Geniculate Nucleus (cont’d)
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• Two types of layers in LGN:
Magnocellular vs. Parvocellular
The Lateral Geniculate Nucleus (cont’d)
• Each layer – only receives input from one eye
(hemiretina )
– Magnocellular layers (1&2)
receive inputs from Parasol
RGC
• Layers 2,3,5
– Ipsilateral: Referring to the same side of the body
(or brain)
– Parvocellular layers (3-6)
receive inputs from Midget
RGC
• Layers 1,4,6
– Contralateral: Referring to the opposite side of
the body (or brain)
• Left LGN input from right visual
field (left half of each retina)
• Right LGN input from right visual
field (right half of each retina)
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Lateral Geniculate Nucleus (LGN)
• Part of Thalamus
• 80% RGCs project to LGN
– other 20% to Superior Colliculus
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Layers of LGN -- Retinotopic organization
• Adjacent areas in LGN receive input from adjacent
areas of retina
• Each layer -- complete map of hemiretina
• Function: Plays a role in Attention
– ~80% input areas other than retina (higher visual
areas)
– May help to control the flow of visual information
• Unattended regions may be filtered
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Receptive Field Properties
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• Definition of receptive field?
Parvo Layers
– smaller r.f.’s
– broadband & color opponent
– slow - sustained response
• R. F. properties of LGN neurons
– circular
– surround antagonism
– size increases as we move to periphery
– Parvo: .1-.2 º in fovea / 3-5º in periphery
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LGN Role in Perception
Parvo vs. Magno Layers of LGN: RF Properties
Magno Layers
3 x larger r.f.
broadband only
rapid - transient
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• Magno and Parvo Layers
– Specialized for different types of visual information
– Parvo: Form analysis and object recognition
– Magno: Motion, location, and temporal analysis
• Schiller & Logothetis
– Chemically Lesioned Magno or Parvo layers of
LGN in Macaque monkey
– Tested perceptual skills
• Singleton detection task
– Make an eye movement to the stimulus that is
“different”
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Perceptual Tasks: Locate target based on:
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Task
Parvo Lesion
Color
Severe
Texture
Severe
Fine Shape
Severe
Coarse Shape Mild
3-D
Severe
Motion
None
Flicker
None
Magno Lesion
None
None
None
None
None
Moderate
Severe
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