Notes on Color Vision Lecture (Part 5)

Introduction

• Exploration of how color output from cones is processed into recognizable colors.
• Staring at an oddly colored flag and then at a white space results in an afterimage of the correctly colored flag.
• The afterimage phenomenon provides clues about color vision.

Overview of Color Processing

• Initial Sensation of Color
  • Begins with cones in the retina.
  • Cones sense different wavelengths of light and generate nerve impulses.
  • First processing happens via ganglion cells in the retina.
• Pathway to the Brain
  • Nerve impulses are sent to the brain for further processing into color opponent channels leading to recognizable colors.

Review of Cones

• Structure of the Retina
  • Retina contains three layers of cells:
    • Ganglion Cells (top layer)
    • Bipolar Cells (middle layer)
    • Photoreceptors (bottom layer: rods and cones)
  • Cones function in bright light and provide color vision; rods provide grayscale vision.
  • Estimates suggest we can distinguish over a million colors.

Historical Theories of Color Vision

• Thomas Young and Hermann von Helmholtz's Theory
  • Proposed three receptors (red, green, blue) for color perception.
  • Each receptor sends separate signals to the brain.
• Cone Sensitivities
  • Cones are sensitive to specific parts of the light spectrum:
    • S Cones (Blue): Short wavelength
    • M Cones (Green): Middle wavelength
    • L Cones (Red): Long wavelength
• Color Discrimination
  • More than one cone type allows for fine color discrimination through combined nerve impulses.

Retinal Circuitry

• Ganglion Cells and Receptive Fields
  • Several photoreceptors connect to ganglion cells that aggregate responses for a specific area.
  • Receptive fields have a center-surround organization:
    • On-center: Excitation from center.
    • Off-surround: Inhibition from surrounding area.
• Cone-Opponent System Arrangements
  • Two main arrangements identified:
    1. Red center with green surround
    2. Green center with red surround
    3. Blue center with yellow surround
    4. Yellow center with blue surround
• Opponent colors are red vs. green and blue vs. yellow.
• Black vs. White: Different channel, not affecting color perception.

Opponent Color Theory

• Proposed by Ewald Hering to address shortcomings of Young and Helmholtz.
• Opponent colors are mutually exclusive combinations:
  • Cannot perceive combinations like bluish-yellow or greenish-red.
• Three Channels of Color Processing
  • Luminance (black vs. white)
  • Blue vs. yellow
  • Red vs. green

Processing Stages

• Initial Separation: Begins at the retina with cone outputs creating opponent color channels.
• Final Processing: Occurs in the cortex to match our actual color perception, correcting previous output discrepancies.

Unique Hues

• Four unique, psychologically elementary colors: blue, green, yellow, red.
• Each unique hue occurs where one channel is neutral, leaving others unopposed:
  • Unique yellow at 580 nm (red and green cancel)
  • Unique blue at ~475 nm
  • Unique green at ~500 nm
  • Unique red is outside the spectrum.

Estimates of Color Visibility

• Distinction of about 180 pure spectral hues.
• Professor Neitz's calculation suggests:
  • With one cone type: 200 gray levels.
  • With two cone types: 10,000 colors (50 x 200).
  • With three cone types: 1 million colors (10,000 x 100).
  • Full color map suggests over 2 million possible colors.

Conclusion

• Color perception starts with cones, processed by retinal ganglion cells, creating opponent channels.
• Final color processing occurs in the cortex to align with perception.
• Upcoming videos will cover color vision in different animals and primates.
• For further reading, references provided.