Vision: Cortical Processes

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Vision Cortical Processes

• Lecture 18

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Primary Visual Cortex - V1

• Striate cortex
• along calcarine sulcus
• Hubel and Wiesel - Nobel Prize
• Single unit recording
• Feature detection theory
• Simple, complex, hypercomplex cells

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Feature Detection Theory

• Objects ---gt simple features
• Convergence of info from retina
• to Primary Visual Cortex

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Simple Cells

• Lines and edges, not center-surround
• Fixed angular orientation
• Fixed location

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• LGN center-surround
• Converge onto simple cells

V1
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• Bar of light
• Increased APs in simple cell

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Wrong Orientation

• Little effect on simple cell APs

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Wrong Location

• No effect on simple cell APs

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Complex Cells

• Fixed angular orientation
• Location not important
• Often respond best to movement in preferred
  direction
• Convergence of simple cells

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Hypercomplex Cells

• Complex cells converge
• on to Hypercomplex cells
• More complex images as visual information
  converges

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V1 Organization

• 6 layers
• Most input ---gt Layer 4
• 1st binocular receptive fields
• a cell gets input from both eyes
• ocular dominance
• Modular organization
• Color processing by blob cells
• Wavelength specific

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R L R L
B L O B
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Color Perception

• V1(blobs) ---gt V2 ---gt V4
• V4 medial occipital lobe
• lingual fusiform gyri
• V4 damage ---gt Achromatopsia
• unilateral contralateral visual field
• bilateral no color vision at all
• no concept of color

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Calcarine Sulcus
V4
lingual gyrus
fusiform gyrus
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2 Pathways

• Simultaneous processing
• Parallel Distributed Processing
• Dorsal Stream---gt Posterior Parietal
• Ventral Stream ---gt Inferotemporal

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Divergence of form info
Posterior Parietal
Where
V1
What
Inferior Temporal
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Form Perception

• LGN - Parvocellular System
• V1 (Interblobs) ---gt V2 ---gt V3
• Inferotemporal Cortex
• complex stimuli - grandmother cells
• Visual Agnosia
• Prosopagnosia

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Movement Perception

• Rods
• LGN - Magnocellular system
• V1 ---gt V2 ---gt V5
• V5 middle temporal lobe (MT)
• Damage - akinetopsia
• unable perceive motion

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What do we see?

• Objects in visual awareness are not things in the
  real world...
• but rather representations of those things
• Constructed image
• by the brain
• Interpretation

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High-level Visual Processing

• V1
• V2 -input from V1
• divergence of color, form, location
• V4 - color - lingual fusiform gyri
• V3 - shape (form) - inferotemporal
• V5 - motion - midtemporal
• PPC - location - posterior parietal cortex