Week 30 Visual Pathway and Processing

• What are the key structures of the visual pathway (from front to back)?

  Optic nerve → Optic chiasm → Optic tract → Lateral Geniculate Nucleus (LGN) → Optic radiations → Visual cortex.
• What are the associated structures of the visual pathway?

  Pituitary gland, Edinger-Westphal nucleus, Circle of Willis.
• 47% of temporal fibres at optic chiasm are uncrossed
• Which layers of the LGN are parvocellular?

  3, 4, 5 and 6
• What would be processed via the ventral stream?

  Object shape
• If a visual field defect shows macular sparing, where in the visual pathway would the lesion be ?

  Visual cortex
• Where are the frontal eye fields located?

  Pre-motor cortex
• How many neurons are involved in the visual pathway?

  Three neurons.
• Where do synapses occur in the visual pathway?

  1. Between bipolar and ganglion cells in the retina.

  2. At the LGN in the thalamus.
• Where does the optic nerve extend from and to?

  From the retina to the optic chiasm.
• What is the optic nerve head (ONH)?

  The exit point for retinal ganglion cell axons, visible in imaging techniques like OCT and ophthalmoscopy.
• How are nerve fibres arranged at the beginning of the optic nerve?

  Based on their retinal origin, with macular fibres centrally located as they travel.
• What happens at the optic chiasm?

  • Temporal retinal fibres remain uncrossed.

  Nasal retinal fibres cross to the opposite side.
• What is the significance of the optic chiasm in visual field processing?

  It ensures that all information from the right visual field is processed together in the left hemisphere, and vice versa.
• What percentage of fibres cross at the chiasm in neurotypical individuals?

  ~53%.
• How does a lesion at different locations in the visual pathway affect vision?

  Pre-chiasmal (optic nerve): Affects only one eye.

  • Chiasmal (optic chiasm): Causes bitemporal hemianopia (loss of peripheral vision).
  • Post-chiasmal (optic tract & beyond): Causes homonymous hemianopia (same-side visual field loss in both eyes).
• Where does the optic tract extend from and to?

  From the posterior optic chiasm to the LGN.
• How are fibres in the optic tract organised?

  By visual field rather than eye (e.g., right visual field fibres travel in the left optic tract).
• What is the LGN?

  A relay centre in the thalamus where optic tract fibres synapse.
• How is the LGN structured?

  Six layers:

  • Ipsilateral (uncrossed) fibres: Layers 2, 3, 5.
  • Contralateral (crossed) fibres: Layers 1, 4, 6.
• What functions do different LGN layers serve?

  Magnocellular (Layers 1 & 2): Contrast, low light, visual search.

  Parvocellular (Layers 3-6): Colour, fine detail.
• What are optic radiations?

  Nerve fibres from the LGN that project to the visual cortex
• How are the optic radiations divided?

  1. Meyer’s Loop (Temporal Lobe): Carries inferior retinal fibres (superior visual field).

  • Lesions → Superior quadrantanopia.

  1. Parietal Lobe Pathway: Carries superior retinal fibres (inferior visual field).

  • Lesions → Inferior quadrantanopia.

  1. Posterior Optic Radiations (Occipital Lobe): Combined superior & inferior fibres.

  • Lesions → Homonymous hemianopia.
• What is the final destination of visual information?

  The primary visual cortex (V1).
• How is V1 organised?

  Macular fibres occupy the largest area.

  • Information is arranged in retinotopic maps.
• What are common visual cortex lesion patterns?

  Congruous defects: Same pattern in both eyes.

  • Macular sparing: Macula remains intact despite hemianopia.
• How do strokes cause visual defects?

  Middle cerebral artery infarct: Affects parietal lobe fibres → Inferior visual field loss.

  Posterior cerebral artery infarct: Affects temporal lobe fibres → Superior visual field loss.
• What is hemianopia?

  Loss of half the visual field.
• What are the types of hemianopia?

  Bitemporal: Loss of peripheral vision in both eyes.

  Binasal: Loss of central visual fields.

  Homonymous: Same-side loss in both eyes.
• What is quadrantanopia?

  Loss of a quarter of the visual field.
• What does macular sparing indicate?

  Preservation of central vision despite a hemianopia, possibly due to redundancy in cortical representation.
• How do retinal ganglion cells detect edges?

  By responding to changes in light and contrast
• What are the two types of ganglion cell receptive fields?

  On-centre: Stimulated by light in the centre, inhibited by surrounding light.

  Off-centre: Inhibited by light in the centre, stimulated by surrounding light.
• How many layers does V1 have?

  6 layers
• Where does input from the LGN arrive?

  Layer 4 (divided into sub-layers 4a, 4b, 4cα, 4cβ).
• What is the function of the FEF (Frontal Eye Fields)?

  Controls eye movements via signals to the midbrain and brainstem.
• Where are the frontal eye fields located?

  In the pre-motor cortex.
• Which pathway dominates each stream?

  Dorsal stream: Magnocellular pathway (whole retina).

  Ventral stream: Parvocellular pathway (fovea).
• What are the two major visual processing streams?

  Dorsal stream ("How"): Motion, object position, visually guided actions.

  Ventral stream ("What"): Object recognition, form, long-term memory.
• What is the difference between magnocellular and parvocellular inputs in V1?

  Magnocellular (4cα): Motion, contrast.

  Parvocellular (4cβ): Colour, detail.