Vision and Color Processing Notes

Vision and Color Processing

Theories of Color Vision

  • Trichromatic Theory (Young and Helmholtz, 1800s)

    • Proposes existence of 3 types of photoreceptors: sensitive to blue, green, and red light.

    • Each cone type contains a different pigment (

    • S: Short wavelength (blue)

    • M: Medium wavelength (green)

    • L: Long wavelength (red)

    • Light absorption varies leading to unique spectral sensitivities.

    • Color blindness occurs with defects in cone pigments (e.g., protanopia, deuteranomaly).

  • Opponent Process Theory (Hering, 1892)

    • Complements Trichromatic Theory by explaining color experiences.

    • Suggests processing occurs in opposing pairs:

    • Red vs. Green

    • Blue vs. Yellow

    • Black vs. White (luminance)

    • Explains phenomena such as negative afterimages and "impossible colors" like reddish-greens.

Receptive Fields in the Retina

  • Definition of Receptive Field (RF):

    • Region in visual field that influences a neuron's activity when light is present.

  • Types of Cells:

    • Rods and Cones:

    • Rods are sensitive to light changes, cones detect color.

    • Bipolar and Ganglion Cells:

    • Have center-surround RF organization (excitatory center surrounded by inhibitory area).

  • Size Variation:

    • Cones have smaller RFs primarily in the fovea, while rods in peripheral regions have larger RFs.

Pathway from Retina to Brain

  • Optic Tract:

    • Visual information from the retina travels via the optic nerve, crossing over at the optic chiasm.

  • Lateral Geniculate Nucleus (LGN):

    • Receives retina inputs.

    • Organized in 6 layers:

    • Magnocellular (M-pathway): Responds to motion (mostly rod inputs).

    • Parvocellular (P-pathway): Deals with color and fine details (mostly cone inputs).

    • Koniocellular cells: Adjacent to LGN layers, receive input from S cones.

Primary Visual Cortex (V1)

  • V1 Functionality:

    • The primary visual cortex (striate cortex) processes visual data from LGN:

    • Organized in 6 layers, retinotopic organization preserves spatial relationships.

  • Neurons in V1:

    • Majority possess rectangular RFs, shaped towards edges and movements, categorized as:

    • Simple Cells: Exhibit “on” and “off” regions, sensitive to orientation.

    • Complex Cells: Larger receptive fields, sensitive to motion, do not have distinct “on/off” regions.

  • Hierarchy of Response:

    • Complex cells receive inputs from simple cells, responding based on combined signals.

Visual Association Cortex: Dorsal and Ventral Streams

  • Dorsal Stream (Where/How):

    • Involved in spatial location and movement detection.

    • Damage can lead to conditions like Balint’s syndrome (simultagnosia).

  • Ventral Stream (What):

    • Responsible for form and color perception.

    • Damage results in specific impairments like achromotopsia (loss of color perception) and prosopagnosia (inability to recognize faces).

Perception and Constancies

  • Contextual Influences:

    • Perceptual constancies maintain appearance’s consistency across conditions:

    • Brightness Constancy: Perceived brightness stays the same despite lighting changes.

    • Color Constancy: Keeps perceived color constant under varying lighting.

  • Cognitive Influences on Perception (Bottom-up vs Top-Down):

    • Bottom-up: Perception shaped directly by sensory input.

    • Top-down: Previous experience and contextual cues influence perception, sometimes leading to visual illusions (e.g., the hollow mask illusion).