Notes on Brain and Behavior - Visual Processing

Typical Receptive Fields for V1 Cells

  • Simple Cells in V1

    • Respond to specific orientations of light.

  • Complex Cells

    • Maximally excited by bars of light moving in a specific direction through the receptive field.

  • Hypercomplex Cells

    • Similar to complex cells but have a strong inhibitory area at one end of their receptive fields, refining their directionality and response.

Neural Circuit in the Primary Visual Cortex (V1)

  • Sensory inputs enter the cortical column at Layer VI (bottom) and synapse on stellate cells in Layer IV.
  • Pyramidal Cells in Layers III and V send outputs to other columns or structures, facilitating vertical information flow.

Processing Shape in the Temporal Cortex

  • Cell Activation

    • Neurons are activated by complex visual stimuli such as faces, hands, etc.

  • Stimulus Equivalence

    • Recognizing an object as the same across different viewing orientations.

  • Tanaka’s Research (1993)

    • Found that complex features are necessary for activation of neurons in Area TE (e.g., orientation, size, color, texture).
    • Neurons with similar responsiveness cluster in columns representing categories of shapes.

Seeing Color

Subtractive Color Mixing
  • Achieved by mixing primary colors of paint (red, blue, yellow). This method creates color by absorbing certain wavelengths.
Additive Color Mixing
  • Involves adding light to create colors; the more light, the lighter the color.
Trichromatic Theory
  • Explains color vision via the coding of three primary colors (red, green, blue).
  • The perceived color is determined by relative cone responses.
  • Related to color blindness but has limitations in explaining afterimages.
Opponent-Process Theory
  • Emphasizes color opposition (red vs. green; blue vs. yellow).
  • Neurons in retinal ganglion cells respond to color pairs.
  • Center-surround receptive fields exhibit excitatory and inhibitory responses to color stimuli.

Opponent Processes

  • Neurons in cortical V4 respond to perceived colors rather than specific wavelengths.
  • They maintain color constancy, ensuring perceived colors remain consistent despite illumination changes.

Neuronal Activity in the Dorsal Stream

  • Posterior Parietal Cortex
    • Processes visual information for actions (the “how” stream).
    • Neurons are inactive if the subject is under anesthesia but will activate when observing object manipulation.

Pathway Injuries and Impact on Vision

  • Monocular Blindness

    • Caused by damage to retina/optic nerve in one eye, leading to complete loss of sight in that eye.

  • Homonymous Hemianopia

    • Blindness of an entire left or right visual field.

  • Quadrantanopia

    • Blindness of one quadrant in the visual field.

  • Scotoma

    • Small blind spot due to lesions or migraines in the visual cortex.

Injuries in the What Pathway

  • Agnosia

    • A general term meaning "not knowing," which can be further specified.

  • Visual-Form Agnosia

    • Inability to recognize objects or their drawings.

  • Color Agnosia (Achromatopsia)

    • Inability to recognize colors.

  • Face Agnosia (Prosopagnosia)

    • Inability to recognize faces or specific features.

Visual Guidance and Action

  • Visual attention and motor actions can occur subconsciously, as the brain coordinates hand positioning without conscious thought, especially when reaching for objects like pens or mugs.

Injury to the How Pathway

  • Optic Ataxia
    • Deficit in visual control affecting reaching and movement due to damage in the parietal cortex, while object recognition remains intact.