Processing and Relaying of Visual Information

Processing and Relaying of Visual Information

Visual Pathway to the Brain

  • Optic Nerve Formation: Axons of retinal ganglion cells form the optic nerve.
  • Optic Chiasma: Medial optic nerve fibers from each eye cross over at the optic chiasma, then join lateral optic nerve fibers and continue as optic tracts.
    • The left optic tract carries a complete representation of the right half of the visual field, and vice versa.
  • Sensory Information Pathway: Sensory information passes through various structures to reach the primary visual cortex in the occipital lobes.
    • Conscious perception of visual images occurs in the primary visual cortex.
  • Information Distribution: Information is also passed to temporal, parietal, and frontal lobes.
    • Objects are identified and their location in space is determined in these lobes.

NOTE: Structures to know include the optic nerve, optic chiasma, optic tract, and primary visual cortex.

Visual Fields and Pathways

  • The visual fields of the two eyes overlap considerably.
  • Fibers from the lateral portion of each retinal field do not cross at the optic chiasma.

Depth Perception

  • Binocular Vision: Both eyes view the same image from slightly different angles.
  • 3D Image Formation: The visual cortex fuses these slightly different images, resulting in a three-dimensional image (depth perception).
  • Requirement of Both Eyes: Depth perception requires input from both eyes.
    • If only one eye is used, depth perception is lost.
  • Compensatory Mechanisms: Without depth perception, a person must judge an object’s position based on learned cues (e.g., nearer objects appear larger).
  • Panoramic Vision in Animals: Many animals (e.g., pigeons, rabbits) have panoramic vision.
    • Their eyes are placed more laterally on the head, resulting in very little overlap of visual fields.
    • Each visual cortex receives input principally from a single eye and a totally different visual field.

Clinical – Homeostatic Imbalance

  • Loss of 3D Vision: Loss of an eye or destruction of one optic nerve eliminates 3D vision (depth perception).
    • Peripheral vision on the damaged side is also affected.
  • Visual Field Loss: If neural destruction occurs beyond the optic chiasma (to the optic tract, visual cortex, etc.), then part or all of the opposite half of the visual field is lost.
    • Example: A stroke affecting the left visual cortex leads to blindness in the right half of the visual field.