Biological Basis of Vision

Biological Basis of Vision

• Rods: Function in low light, detecting light intensity (grayscale), not color. Highly sensitive, ideal for night vision.
• Cones: Operate in bright light, enabling color perception. Three types: short-wavelength (blue), medium-wavelength (green), and long-wavelength (red).
• Blind Spot: Location on the retina where the optic nerve exits, lacking photoreceptors. The brain fills in missing details using surrounding visual information.
• Scotopic Vision: Rod-dominated, active in dim light (e.g., moonlight).
• Photopic Vision: Cone-dominated, daylight vision, full color.
• Mesopic Vision: Mixed rod and cone activity, intermediate light (e.g., dusk).

Visual Adaptation and Dynamic Range

• Dynamic Range: Human visual system perceives luminance levels from dim starlight (0.001 cd/m²) to bright sunlight (100,000 cd/m²). Adaptation allows sensitivity to both dark and bright environments.
• Purkinje Effect: In low light, rod peak sensitivity shifts to shorter wavelengths (blue). Red objects appear dimmer than blue objects of equal brightness in low-light.
• Low Luminance (Dark): Poor spatial resolution, limited color perception (grayscale).
• High Luminance (Bright): Increased spatial resolution, vibrant, saturated color perception.

Color Perception

• Color Signal Formation: Color reaching the eye is a pointwise multiplication of the illuminant and reflectance spectra. Cones process this signal to perceive colors.
• Opponent Colors: Colors processed in opposing pairs: red-green, blue-yellow. Brain doesn't perceive reddish-green or bluish-yellow simultaneously.
• Trichromacy: Any color can be produced by mixing three primary colors (red, green, blue).
• Metamerism: Different spectra can produce the same perceived color under specific lighting.

Color Spaces

• CIE RGB: Matches cone type sensitivities (L, M, S).
• CIE XYZ: Derived from RGB through linear transformation, adding luminance (Y) and chromaticity (XY). Distance in this space does not correlate with perceptual difference.
• CIE Lab: Perceptually uniform color space; distances correspond to human perception. Based on opponent channels—brightness (L) and color differences (A: red-green, B: blue-yellow).
• Chromaticity Diagram: Visualizes all human-visible colors. Pure colors are on the horseshoe boundary. “Line of Purples” connects red and violet ends.

Visual Processing Pathways

• Retina to Brain: Retina encodes light into neural signals, relayed through the LGN (Lateral Geniculate Nucleus) to the primary visual cortex (V1).
• Dorsal Stream ("Where"): Pathway V1 → V2 → MT → MST → Parietal Cortex. Processes motion, spatial awareness, visually guided actions.
• Ventral Stream ("What"): Pathway V1 → V2 → V4 → Inferior Temporal Cortex. Processes object recognition, color, and long-term memory links.

Contextual and Adaptation Effects

• Contextual Effects: Surroundings influence perception (e.g., Ebbinghaus Illusion, Adelson's Checker-Shadow Illusion).
• Adaptation Effects: Prolonged stimulus exposure reduces sensitivity.
• Von Kries Law: Models color constancy: Each cone type adjusts its gain to maintain stable color perception under varying lighting.

Brightness Perception

• Weber's Law: Smallest perceivable change (JND) is proportional to the magnitude of the stimulus.