The Visual System

Goals

• Understand the anatomy and physiology of the eye and retina.
• Explore neural coding mechanisms such as lateral inhibition and color opponency.
• Discuss the major pathways from the retina and their roles in visual processing.
• Examine how brain damage affects visual perception.

Introduction to Vision

• Vision transduces electromagnetic energy within the visible light spectrum into neural signals.
• Unique Aspects:
  • Humans perceive light in the visible spectrum (400–700 nm).
  • Some animals see beyond this range (e.g., infrared in snakes, ultraviolet in birds).
• Key Visual Object Properties:
  • Luminance, color, motion, contrast, edges, contours, depth, and spatial location.

Anatomy and Physiology of the Eye

Major Structures

• Cornea: Transparent outer layer that bends light to focus it on the retina.
• Lens: Double convex structure that fine-tunes focus; inverts the image on the retina.
• Iris: Regulates the amount of light entering through the pupil.
• Pupil: Opening controlled by the iris; adapts to light intensity.
• Retina:
  • Photoreceptors transduce light into neural signals.
  • Contains specialized regions like the fovea for high-acuity vision.
• Blind Spot: Area where the optic nerve exits, lacking photoreceptors.

Inverted Retina

• Vertebrate retinas have photoreceptors at the back, beneath layers of neurons and axons.
• Compensations:
  • Fovea clears away these structures, optimizing acuity.
  • Dual blood supply supports photoreceptors’ high energy demands.

Photoreceptors

• Rods:
  • ~100 million.
  • Highly sensitive to dim light (scotopic vision).
  • Low spatial acuity, large receptive fields.
• Cones:
  • ~6 million.
  • Sensitive to bright light and color (photopic vision).
  • High spatial acuity, small receptive fields.
  • Types: S (short, blue), M (medium, green), L (long, red).
• Special Ganglion Cells:
  • Contain melanopsin for detecting overall light intensity (not involved in form vision).

Retinal Processing and Neural Coding

Compression Problem

• The retina processes 106 million photoreceptors but has only 1 million optic nerve axons.
• Strategies to address compression:
  • Lateral inhibition: Enhances contrast and edges.
  • Center-Surround Organization:
    • On-Center, Off-Surround Cells:
      • Activated by light in the center, inhibited by light in the surround.
    • Off-Center, On-Surround Cells:
      • Opposite response pattern.

Color Vision

• Trichromatic Theory (Young-Helmholtz):
  • Three cone types (S, M, L) respond to different wavelengths.
  • Intensity coding within cones loses specific wavelength information.
• Opponent Process Theory (Hering):
  • Colors are represented as oppositional pairs (red-green, blue-yellow).
  • Explains afterimages and the inability to perceive certain color combinations (e.g., reddish-green).

Visual Pathways

Retinal Ganglion Cell Types

1. Parasol (M Pathway):
   • Large receptive fields, high sensitivity, fast response.
   • Processes motion and luminance.
2. Midget (P Pathway):
   • Small receptive fields, high acuity.
   • Processes color and fine details.
3. Bistratified (K Pathway):
   • Intermediate sensitivity, processes blue-yellow color signals.
4. Photosensitive Ganglion Cells:
   • Detect absolute light intensity for circadian rhythms and pupil responses.

Pathways from Retina

• Optic Nerve (Cranial Nerve II):
  • Transmits visual information to the brain.
• Targets:
  • Lateral Geniculate Nucleus (LGN): Primary thalamic relay to the visual cortex.
  • Superior Colliculus: Coordinates eye and head movements.
  • Suprachiasmatic Nucleus: Regulates circadian rhythms.

Cortical Processing

• Primary Visual Cortex (V1):
  • Located in the occipital lobe.
  • Organized retinotopically (spatial mapping of the retina).
• Higher Visual Areas (V2, V3, etc.):
  • Integrate and process more complex visual information.
• Dorsal and Ventral Streams:
  • Dorsal (“Where”) Pathway:
    • Spatial localization and motion processing.
  • Ventral (“What”) Pathway:
    • Object and face recognition.

Challenges and Adaptations

1. Inverse Problem:
   • 3D world projects onto a 2D retina.
   • Depth cues (e.g., retinal disparity) are critical for spatial understanding.
2. Compression Problem:
   • Vast range of light intensity (~11 log units) reduced to a smaller dynamic range.
   • Differences in intensity rather than absolute values are transmitted.
3. Constructive Perception:
   • The brain interprets and “fills in” missing information, leading to visual illusions.

Summary

This lecture provided a foundation for understanding vision, covering the eye’s structure, photoreceptor function, and pathways to the brain. Key concepts such as lateral inhibition, color opponency, and the integration of dorsal and ventral streams highlight the complexity of visual processing. These principles set the stage for further discussions on higher-order visual cognition.