56. Visual Pathways

Question 1

What retinal fibers correlate to the left visual field?

Answer 1

Left nasal fibers and Right temporal fibers

Question 2

damage to optic nerve results in what visual field deficit?

Damage to optic chiasma causes what? Why?

Damage to optic tract causes what?

Answer 2

Optic Nerve: blindness of ipsilateral eye

Optic Chiasm: Bitemporal Hemianopia (lose both eye nasal fibers because this is where they decussate = tunnel vision)

Optic Tract: Contralateral Homonymous Hemianopia (lose fibers that make one visual field)

Question 3

What parts of the visual field are located within the dorsal LGN?

What happens to the visual field with a vascular lesion in the lateral choroidal artery? anterior choroidal artery?

Answer 3

Medial LGN: lower contralateral visual field (superior retina)
Lateral LGN: upper contralateral visual field (inferior retina)

LChoroidalA: lose central dLGN = wedge scotoma in middle of contralateral visual field
AChoroidalA: lose medial/lateral dLGN = 2 wedge scotomas (high and low) in contralateral visual field

Question 4

What are the different types of Optic Radiations?

Answer 4

1. Temporal Optic Radiations: Meyer’s Loop
   - superior visual field info (from lateral LGN)
   - damage = Pie-in-the-Sky upper contralateral quandrantanopia
   - projects to lower bank of calcarine fissure
2. Parietal Optic Radiations
   - inferior visual field info (medial LGN)
   - damage = lower contralateral quadrantanopia
   - projects to upper bank of calcarine fissure

Question 5

Cortical Visual Fields: how does the calcarine sulcus process visual field info?

Why does damage here lead to macular sparing?

What does damage to one calcarine cortex look like?

Answer 5

Upper Bank: lower visual field info (superior retinal fibers) from Parietal Optic Radiations
Lower Bank: upper visual field info (inferior retinal fibers) from temporal optic radiations

Macular Sparing: magnification in calcarine cortex near center of vision (distorted homunculus to emphasize fovea) - occipital damage does not affect center of visual field due to large space in cortex and dual blood supply (MCA, PCA)

damage: Bilateral Contralateral Homonymous Hemianopia with macular sparing

Question 6

Cortical Processing

• how do Retinal Ganglion Cells and LGN cells get activated?
• what is a simple cell, how does it function?
• what is a complex cell and how does it function?
• what is a “Hand Cell”?

Answer 6

RGC/LGN - use center-surround receptive fields (stim in center, inhibit around center); multiple cells with receptive fields line up on retina and project to single Simple Cell in visual cortex
Simple Cell - respond maximally to orientation of light (bar) covering all its ganglion receptive fields; multiple simple cells with same orientation but varying position project to Complex Cell
Complex Cell - detects orientation of light/objects (not thrown off by slight movements) - converge on Hypercomplex cells in inferotemporal visual cortex for meaningful objects
Hand Cell: along ventral stream of temporal pathway - processes visual form of hand (feature detector, shape of object more important than orientation)

Question 7

What is prosopagnosia?

When do signals from each eye become fully integrated?

How do our eyes perform stereoscopic depth perception?

Answer 7

Inability to recognize faces - due to damage to fusiform gyrus (highest center for ventral stream of vision)

Integrate beyond Layer 4 of Calcarine Cortex (segregated in optic tracts, LGN, radiations)

Binocular Disparity Cell: fires when images do not sync up (objects project to both nasal fibers or both temporal fibers); Zero Disparity Cell: fires when images do sync up

Question 8

Dorsal Stream

• other names
• path from LGN
• damage

Answer 8

Parietal, “Where”, Action, Magnocellular

LGN - V1 (calcarine cortex) - MT - BG/Pons/Cerebellum/Motor Cortex

Damage = Optic Ataxia - can see the hole, but cannot maneuver hand through it

Question 9

Ventral Stream

• other names
• path from LGN
• damage

Answer 9

Temporal, “what”, Perception, Parvocellular

LGN - V1 - V4 - Temporal Lobe/Amygdala/Limbic System/Fusiform Gyrus

Damage = Visual Agnosia = cannot match line orientation

Question 10

What are extrastriate lesions?

What symptoms can they cause?

Answer 10

“Psychic Blindness” outside the visual path

Temporal Lesion: loss of recognition/object memory
Parietal Lesion: visual inattention/neglect
Occipito-temporal Lesion (fusiform gyrus): prosopagnosia