Visual Perception and Illusions

The Eye & Visual Pathways

• The eyes and visual pathways in the brain facilitate visual perception.
• Understanding their structure and function is crucial to understanding how we see.
• Key topics include:
  • The moving eye
  • Art in Focus
  • The Artist’s Eye (and Brain)

History of Understanding Vision

• Ancient Greeks:
  • Euclid (~300 BCE): Emission theory – the eye emits radiation.
  • Aristotle (384-322 BCE): Intromission theory – light comes from objects to the retina.
• Ibn al-Haitham (~1000 CE): Light comes from the object to the eye.
• ~1500 CE: Optics and lenses studied, leading to the understanding of the eye as a refracting element with image formation on the retina.
• Full understanding came with:
  1. Snell’s law (1621): Describes light refraction.
  2. Gauss’s system (1855): Quantitative system to trace light rays (Gaussian Optics).
  3. Gullstrand’s model (~1909): Accurate optical model of the eye.

Evolution of Vision

• Almost all living things are sensitive to light.
• Evolution steps:
  1. Light detection
  2. Resolution (direction of light)
  3. Movement detection
  4. Color vision
• First eyes were simple light-sensitive cells.

Light Detection

• Early organisms had light-sensitive cells to distinguish light from dark.
• Example: Flatworms moving away from light.

Resolution

• Survival advantage in resolving differences in light and dark from various directions.
• Two possibilities:
  • Cell elongation
  • Layer invagination

Key Components of the Human Eye

• Cornea
• Iris
• Lens
• Vitreous humor
• Retina

Cornea & Lens

• Cornea and lens refract light to form an image on the retina.
• Cornea:
  • Most of the eye’s focusing power ($\approx +40D$).
  • Approximately 0.5mm thick centrally.
• Lens:
  • Less power ($\approx +20D$ when relaxed).
  • Variable power for near objects (accommodation).

Iris

• Regulates light entering the eye via pupil size.
• Two muscle sets control pupil size.

Retina

• Light is converted into neural signals by photoreceptors.
• Two types of photoreceptors:
  • Rods
  • Cones

Rods

• Night vision.
• One type only.
• No color vision.
• 120 million.
• Absent from fovea.

Cones

• Day vision.
• Three types.
• Allow color vision.
• 6 million.
• Densest in fovea.

How Sharp is our Vision?

• High cone density is needed for fine details.
• Rapid decrease in cone density away from fovea.
• Wide, “high resolution” vision is an illusion; high-detail vision area is small.

Extraocular Muscles

• Six extraocular muscles per eye:
  • One pair moves eyes horizontally (Lateral rectus & Medial rectus).
  • One pair moves eyes up & down (Superior rectus & Inferior rectus).
  • One pair rotates eyes (Superior oblique & Inferior oblique).

Blind Spot

• Zero cone density at the blind spot leads to zero vision during the day.
• Zero rod density at fovea leads to a blind spot at night.

Blind Spots

• Areas of blindness are “filled-in” by the brain.
• Blind spots in each eye don’t overlap (daytime).
• At night, blind spots at fovea overlap.

Visual Pathway

• Signals from photoreceptors travel through neural circuits to retinal ganglion cells.
• Projections from retinal ganglion cells travel to the brain via the optic nerve.

Summary

• The eye and visual pathways are key to visual perception.
• Anatomical features affect visual perception.
• Visual information is processed in specialized brain areas.

What are Illusions?

• Illusion = systematic discrepancies from simple measurements.
• Can occur in any sense or combined across senses.

Why do illusions exist?

• Perceptions are unconscious inferences based on ambiguous data.
• The visual system is optimized for common stimuli but may fail in unusual contexts.

Classification of Illusions

• Errors in language can suggest categories of perceptual illusions:
  • Ambiguity
  • Distortion
  • Paradox
  • Fiction

Gregory (1997)

• Optics: Optical disturbance between object & retina.
• Signals: Disturbed neural sensory signals.
• Rules & Objects: Cognitive; knowledge-based for making sense of neural signals.

Gestalt Principles of Grouping

• Proximity: Close objects form groups.
• Similarity: Similar objects form groups.
• Closure: Tendency to see closed forms.
• Continuity: Tendency to see continuous segments.
• Common Fate: Moving objects together.

Why are visual illusions important?

• Illusions provide insight into how the eye and brain function.

What is Light?

• Small portion of the electromagnetic spectrum visible by humans.
• Causes visual sensation via photoreceptor absorption.

Light Sources

• Primary Sources: Generate electromagnetic radiation.
• Secondary Sources: Reflect or scatter incident radiation.

Electromagnetic Radiation

• Wave motion type: transverse.

How fast is light?

• Speed of light in a vacuum = c ($c = 3 \times 10^8$ m/s).
• n = c/l (or l = c/n).
• Higher frequency = shorter wavelength.

The Visible Spectrum

• Small part of the electromagnetic spectrum (400-700 nm).

Dual nature of electromagnetic radiation

• Light can act as a continuous wave.
• Light can also act as a particle (photons or quanta).

Not all wavelengths are equally visible

• Eyes are most sensitive to wavelengths in the middle of the spectrum (yellow-green).
• Sensitivity curves:
  • Photopic (V(l)): bright conditions
  • Scotopic (V(l)’): dim conditions

What’s so special about light?

• Depends on spectrum and overlap with eye's spectral sensitivity (V(l) curves).

Spectrum of a light source

• Amount of energy or power at each wavelength.

Light Composition

• Monochromatic light: Narrow range of wavelengths.
• Broad-band light: Wide range of wavelengths.

Incandescent Sources

• Produce light because they are hot (e.g., sun, tungsten bulb).

Incandescent Sources

• Produce electromagnetic radiation because they are hot.
• K = Kelvin (absolute) scale for temperature = $°C + 273$
• Colour Temperature = what “white” looks like, or colour of illumination source (photography)

Summary

• Visual illusions help understand sensory information processing.
• The eye is sensitive to a small range of electromagnetic wavelengths.