Visual Information Processing-1

Visual information is relayed from the retina to the brain.
Axons of the ganglia form the optic nerve, specifically from the retinal ganglia.

- The retinal ganglia combine at the back of the eye to form the optic nerve.
- The optic nerve crosses at the optic chiasm and continues as optic tracts.
Each optic tract carries information from the same half of the visual field.

- Contains fibers from the temporal aspect (ear side) of one eye and the nasal aspect (nose side) of the opposite eye.

The retinal ganglia cells merge into the optic nerve at the back of the eyeball.
After crossing at the optic chiasm, the structure continues as optic tracts.
These optic tracts relay signals to neurons in the thalamus, specifically the lateral geniculate nuclei (LGN).
From the thalamic neurons:
- Signals are transmitted to the primary visual cortex, located in the occipital lobe.
- This is where the conscious perception of visual images occurs.
Signals travel through the internal capsule to form optic radiation fibers within the cerebral white matter.

- These fibers project into the primary visual cortex and occipital lobes.

Optic fiber tracks branch to the midbrain, which controls the extrinsic eye muscles.

Damage to the optic chiasm affects vision:
- Peripheral vision may be preserved, but central vision will be compromised.
An illustration of damage effects:
- With both eyes functioning, there would be a loss of central vision (depicted as blue and yellow) and light blue and light yellow regions may still be visible.

Depth perception results when both eyes view the same image from slightly different angles.

  - Overlapping visual fields of each eye allow for depth perception.
  - The visual cortex fuses these images to create a three-dimensional perception.
  - If one eye is lost or the optic nerve is damaged, depth perception is lost entirely.
Peripheral vision is also affected on the side of damage:
- If neural structures beyond the optic chiasm are damaged, part or all of the opposite half of the visual field may be lost.
Example: A stroke affecting the left visual cortex can lead to blindness of the right half of the visual field.

Visual processing begins with light impacting photoreceptors (rods and cones), leading to hyperpolarization of the cells, making them more negative.
Hyperpolarization triggers bipolar neurons to send signals to ganglion cells.

  - Retinal cells split into channels processing different aspects:
    - Color and brightness
    - More complex information: angle, direction, and speed of movement.

Lateral inhibition decodes visual information at the edges of objects, enhancing contrast and detail.
Ganglion cells pass processed information to the thalamus, which enhances depth perception using the cone inputs that represent color.

The primary visual cortex has a topographical map of the retina.

  - Neurons respond specifically to:
    - Dark and bright edges
    - Orientation of objects
The primary visual cortex is responsible for providing input related to:
  - Form
  - Color
  - Motion
Processed visual information then travels through:
  - Temporal lobes (sides)
  - Parietal lobes (top)
  - Frontal lobes (front)
This pathway allows for the identification of objects and their spatial location.

References to visual processing can also connect to discussions of other special senses, such as the chemical senses.