Lecture 13-Visual Perception: Color Vision

Physiological Psychology - Visual Perception: Color Vision

Arrangement of Rods and Cones on Retina

  • Rods and Cones Overview:
    • Rods and cones are photoreceptors in the retina which convert light into neural signals.
    • Arranged towards the back of the retina; bipolar cells connect them to retinal ganglion cells.
    • Fovea: Central part of the retina with high acuity where cones are concentrated.
    • Blind Spot: Area without photoreceptors where optic nerve exits the eye.
    • Some animals have two foveas for enhanced vision.

Differences Between Rods and Cones

  • Rods:
    • High sensitivity in low light but low acuity; responsible for night vision.
  • Cones:
    • Three types (S, M, L) with different responses to light:
    • S (Short): Responds best to blue light.
    • M (Medium): Responds best to green light.
    • L (Long): Responds best to red light.
    • Have varying degrees of convergence onto ganglion cells affecting acuity and sensitivity.

Receptor Potentials and Transduction

  • Dark Current: In dark conditions, rods release neurotransmitters steadily due to open Na+ channels.
  • Isomerization of Retinal: Light causes retinal to change shape, separating from opsin and closing Na+ channels, initiating generation of a receptor potential.
  • Bipolar cells respond differently to Glutamate release:
    • On-Center Cells: Depolarize with less glutamate.
    • Off-Center Cells: Depolarize with more glutamate.

Coding of Color

  • Trichromatic Theory: Color perception is based on the activity of three types of cones (S, M, L).
    • Physiological evidence supports this theory by measuring absorption spectra of visual pigments, revealing peak responses at specified wavelengths:
    • 419nm (S, short)
    • 531nm (M, medium)
    • 558nm (L, long).
    • Color does not require rods; must involve relative comparisons across cone types.

Cone Response and Color Perception

  • Color perception arises from combined responses of all three types of cones.
  • Metamers: Different wavelengths can produce the same perceptual color under certain circumstances as shown in unusual color-matching experiments.

Color Blindness

  • Commonly occurs when an individual has two types of cone pigments instead of three.
  • Most frequently leads to red-green color confusion.

Monochromatism

  • Rare condition characterized by:
    • No functioning cones; only rods present.
    • Perception limited to shades of gray.
    • Very sensitive to bright light; poor visual acuity.

Dichromatism

  • Three types of dichromatism, each missing one of the cone pigments:
    • Protanopia: Missing long-wavelength pigment.
    • Deuteranopia: Missing medium-wavelength pigment.
    • Tritanopia: Rare, missing short-wavelength pigment.

Limits of Trichromatic Theory

  • Fails to explain:
    • Why reddish-green and bluish-yellow combinations are not perceived.
    • The phenomenon of negative afterimages and simultaneous color contrast.

Opponent-Process Theory

  • Proposes that color perception occurs in systems of paired opposites:
    • Red-Green, Blue-Yellow, White-Black.
    • Two types of input to visual pathway neurons: excitatory (perceives one color) and inhibitory (perceives opposite color).
    • Explains color afterimages and simultaneous color contrast phenomena.

Neurophysiological and Psychophysical Evidence

  • Neurons in the lateral geniculate nucleus (LGN) demonstrate increased firing to one color while decreasing to its opponent.
  • Hue cancellation experiments provide evidence supporting opponent-process theory's validity.

Unique Characteristics of Red

  • Red lacks a single wavelength counterpart for cancelling its perception, unlike blue, green, and yellow.
  • To eliminate red, green must be paired with some blue due to overlaps in color perception ranges.

Integration of Theories

  • Trichromatic and Opponent-Process Theories complement one another:
    • Trichromatic explains cone responses within the retina.
    • Opponent-process explains color processing in ganglion cells and higher visual brain areas.

Color in the Cortex

  • The perception of color involves multiple cortical regions responding to specific wavelengths and exhibiting opponent processes.
  • Effectively helps interpret images more clearly.
  • Color Constancy: Top-down processes influence perceived color and are shaped by experience and memory.

Visual Pathways

  • The processing pathway from the eye includes the optic nerves, optic chiasm, lateral geniculate body, and visual cortex, indicating organized processing of visual information.