Optical Illusions and Human Vision

Introduction to Optical Illusions

• Optical illusions can manipulate perception using light and perspective.
• They exploit the disconnect between sensation and perception.
• Some illusions create after-images by exploiting physiological glitches in vision.

Afterimage Illusion Example

• Staring at a certain pattern (i.e., a flag) can lead to seeing an afterimage when looking away.
• This occurs due to the photoreceptors in the eyes continuing to fire after looking away.
• The afterimage may appear in different colors (e.g., flag appears red-white instead of turquoise-black-yellow).

Complexity of Human Vision

• Approximately 70% of sensory receptors in the body are in the eyes.
• Vision involves nearly half of the cerebral cortex.

Basics of Light Perception

• Light is electromagnetic radiation traveling in waves.
• Hue: Determined by the frequency of light waves.
• Brightness: Determined by the amplitude of light waves.
• The visible spectrum is only a small part of the electromagnetic spectrum.

Eye Anatomy

• External Features: Eyebrows, eyelashes, eyelids, and lacrimal apparatus protect the eyes.
• Eyeball Structure: Irregularly spherical, 2.5 cm diameter, largely hollow with fluid.

Layers of the Eye

1. Fibrous Layer:
   • Sclera: The white part of the eye.
   • Cornea: Transparent front that lets in light.
2. Vascular Layer:
   • Choroid: Supplies blood to layers.
   • Ciliary Body: Muscle tissue around the lens.
   • Iris: Colored part of the eye, controls pupil size.
3. Inner Layer (Retina):
   • Contains photoreceptors, bipolar neurons, and ganglion neurons.

Photoreceptors

• Cones:
  • Detect fine detail and color.
  • Types: Red, green, blue sensitive.
  • Active in bright conditions.
• Rods:
  • More numerous, sensitive to light, grayscale vision.
  • Rule peripheral vision.

Neural Pathway of Vision

• Light hits posterior retina and is processed by photoreceptors, bipolar cells, and ganglion cells.
• Ganglion cell axons form the optic nerve, transmitting signals to the brain.

Optical Illusion Explanation

• Afterimages occur due to persistent firing of photoreceptors after exposure to strong stimuli.
• Cones can "tire" and stop responding after prolonged exposure to bright colors.
• Different cones (red, green, blue) respond variably, creating afterimages with different perceived colors.

Conclusion

• Human vision is complex and fallible, but understanding these aspects allows insight into the system.
• Summary of eye structure and the vision pathway highlights the importance of rods and cones.

Credits

• Special thanks to contributors and supporters including Thomas Frank and the Crash Course team.
• Created by the Crash Course team with contributions from various experts and artists.