Updated: 2/24/2018

Visual Pathway

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Overview
 

 
Introduction
  • When light passes through the lens it reaches the
    • the retina, where the formed image is
      • inverted and reversed
        • for example, upper visual space information will be projected to the lower retina
    • axons leave the retina into the optic nerve
      • fibers pass through the optic chiasm and subsequently in the optic tract
        • there is fiber crossing in the optic chiasm
          • neuronal fibers from the left side of the retina (left hemi-retina) will end up in the
            • left optic tract
          • neuronal fibers from the right hemi-retina will end up in the
            • right optic tract
    • axons in the optic tracts will synapse in the lateral geniculate nucleus (LGN) of the thalamus
      • fibers from the LGN makes it way to the visual cortex as optic radiations
        • inferior optic radiations form the Meyer's loop
          • which carries information from the inferior retina (and thus the superior visual field)
          • note that these radiations pass into the temporal lobe and therefore
            • temporal lobe lesion result in a
              • contralateral homonymous superior quadrantopia ("pie in the sky")
        • superior optic radiations pass by the parietal lobe and therefore
          • lesions of the parietal lobe results in a
            • contralateral homonymous inferior quadrantopia ("pie on the floor")
    • optic radiations eventually synapse in the primary visual cortex
      • superior optic radiations project to the
        • superior bank of the calcarine fissure
      • inferior optic radiations will project to the
        • lower bank of the calcarine fissure
  • Summary of conscious vision perception
    • retina → optic nerve → optic chiasm → optic tract → lateral geniculate body → optic radiation to primary visual cortex
  • Additional pathways
    • retina → optic nerve → optic chiasm → optic tract → pretectal area and superior colliculus
      • the pretectal area is important for the
        • pupillary light reflex
          • swinging flashlight test is used to diagnose a relative afferent pupillary defect
            • normal 
              • both pupils constrict quickly and equally when either is exposed to direct light
              • pupils do not change in size when light moves quickly between eyes
            • abnormal (lesion in the afferent pathway)
              • sensory stimulus from affected pathway to the midbrain is reduced
              • unaffected pupil will dilate from its constricted state when light is moved from the unaffected to the affected eye 
              • common causes include unilateral optic nerve lesions and severe unilateral retinal disease
      • the superior collicus and pretectal area is important for
        • eye movement towards visual stimuli
Common Lesions 
 
Select Visual Field Defects
Visual Field Deficit
Etiology
Central scotoma
  • Macular degeneration
  • Retinal
    • infarction
    • hemorrhage
    • degeneration
    • infection
Monocular vision loss
  • Optic neuritis
  • Anterior ischemic optic neuropathy
  • Optic glioma
Bitemporal hemianopia
  • Pituitary adenoma
  • Craniopharyngioma
  • Hypothalamic glioma
Contralateral homonymous hemianopia
  • Optic tract lesions (rare) secondary to
    • malignancy
    • demyelination
    • infarction
  • Optic radiation lesions
  • Lesion involving the entire primary visual cortex
Contralateral superior quadrantanopia ("pie in the sky")
  • Temporal lobe lesions 
  • Lesions involving the lower bank of the calcarine fissure
Contralateral inferior quadrantanopia ("pie on the floor")
  • Parietal lobe lesions such as
    • infarction of the superior division of the middle cerebral artery
  • Lesions involving the superior bank of the calcarine fissure
Homonymous heminanopia with macular sparing
  • Posterior cerebral artery infarction
 

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