Central Visual System (Part 2)

Overview

• Final discussion on the LGN and striate cortex
• Higher-order visual information processing in visual cortex, parietal, and temporal lobes

LGN Function and Discovery

• Hubel and Wiesel's Nobel Prize-winning Experiments
  • Injected radioactive amino acids into one eye of a monkey
  • Tracked the pathway from retinal ganglion cells to LGN and striate cortex using trans-neural autoradiography
  • Discovered that alternating bands in the LGN correspond to input from separate eyes
  • Identified ocular dominance columns in layer 4 of striate cortex
• Ocular Dominance Columns and Layer 4
  • LGN and layer 4 of striate cortex show monocular input
  • Above layer 4C, neurons become binocular (e.g., layers 3, 2, and 1)
  • Layers project to different cortical areas: parietal, temporal, superior colliculus, etc.

Cytochrome Oxidase Blobs

• Cytochrome Oxidase: Enzyme used for cell metabolism
• Located in ocular dominance columns
• Receives koniocellular input from LGN, specializing in color perception

Monocular and Binocular Receptive Fields

• Monocular Fields (within striate cortex):
  • Located in LGN and layer 4C
  • 4C Alpha: Insensitive to wavelength
  • 4C Beta: Center surround color opponency
• Binocular Fields (above layer 4C):
  • Layers 4B, 4A, 3, 2, and 1

Higher-Order Visual Processing

• Orientation and Directional Selectivity
  • Neurons responsive to specific orientations and directions (studies by Hubel and Wiesel)
• Types of Cells
  • Simple Cells: Selective to orientation with distinct on/off regions
  • Complex Cells: Binocular, less distinct on/off regions, selective to orientation and movement
  • Blob Receptive Fields: Specialize in color analysis

Visual Pathways

• Magnocellular Pathway
  • Analyzes motion
  • Large receptive fields
• Blob Pathway: Specializes in color analysis
  • Receives input from koniocellular layers
• Parvocellular (Parvo Inter-Blob) Pathway
  • Analyzes shape of objects
  • Small receptive fields, good at edge detection

Unified Visual Processing

• 2x2 mm Chunk of Striate Cortex: Contains all cells needed for color, motion, and shape analysis

Two Visual Streams

• Dorsal Stream (Parietal Lobe)
  • Analyzes motion, integrates with somatosensory cortex for proprioception
  • Involves higher-level motion perception and coordination
  • Case Study: Akinitopsia (lack of motion perception) from lesions
• Ventral Stream (Temporal Lobe)
  • Object and facial recognition
  • Interaction with hippocampus for memory storage
  • Case Study: Prosopagnosia (difficulty recognizing faces) from damage to facial fusiform gyrus

Syndromes and Lesions

• Achromatopsia: Lack of color vision due to damage in V4 or genetic mutations
• Prosopagnosia: Difficulty recognizing faces due to unilateral stroke

Conclusion

• Completion of visual system chapter
• Advice for further study and seeking help if necessary