50 Visual Pathway

Question 1

describe the visual pathway

Answer 1

• retina converts photo-energy to neural impulses
• the optic nerve transmits neural signals from retina to optic chiasm
• the nasal hemiretinal information decussates, and after the OC, the entire hemifield is represented on the contralateral side in the nervous system
• the optic tract transmits neural signals from OC to LGN and carries contralateral hemifield info.
  
  • LGN = lateral geniculate nucleus of the thalamus
• the optic radiation transmits neural signals from LGN to primary visual cortex
  
  • Meyer’s loop - projection through temporal lobe and represents upper visual field quadrants
  • Baum’s loop - projection through parietal lobe and represents lower visual field quadrants
• the primary visual cortex (V1) in the occipital lobe receives signals in layer IV

Question 2

describe Meyer’s loop

Answer 2

Question 3

describe Baum’s loop

Answer 3

Question 4

describe the location of the primary visual cortex (V1), the Broadmann area and the blood supply

Answer 4

• the major portion of V1 is represented on the medial aspect of the brain
  
  • located along the 2 banks (gyri) of the calcarine sulcus, one portion superior and the other portion inferior to the calcarine sulcus
• the primary visual cortex receives its major blood supply from calcarine branches originating from the PCA

Question 5

describe the retnotopic organization of the primary visual cortex

Answer 5

Question 6

describe the pathway for depth and motion

Answer 6

• the pathway for depth and motion starts with magnocellular neurons (M ganglion cells) in the retina
• these neurons synapse in two out of the six layers of the LGN
• the output from the LGN follows the M pathway along the optic radiation and synapse in layer 4 of the primary visual cortex
• from the primary visual cortex onwards, the M pathway forms the dorsal (parietal) pathway

Question 7

describe the pathway for form and color

Answer 7

• the pathway for form and color starts with parvocellular neurons (P ganglion cells) in the retina
• these neurons synapse in layers 3-6 of the LGN
• the output from the LGN follows the P pathway along the optic radiation and synapse in layer 4 of the primary visual cortex
• from the primary visual cortex onwards, the P pathway forms the ventral (inf. temporal) pathway

Question 8

describe cortical color blindness (achromatopsia)

Answer 8

• inability to distinguish colors or color hues may be due to cortical lesion in Brodmann’s Areas 18 or 37
• inf. temporal cortical lesions can cause color blindness
  
  • P pathway

Question 9

describe what can cause this

Answer 9

Question 10

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Question 11

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Question 12

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Question 13

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Question 14

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Question 15

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Question 16

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Question 17

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Question 18

describe the concept of macular sparing

Answer 18

• macular sparing is often associated with vascular lesions involving the PCA or its branches
• the reason for macular sparing is that in these cases the blood supply of the occipital pole of the cerebral cortex, which is the area representing macular vision, may stay intact due to sufficient blood flow originating from the MCA