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Updated: Aug 24 2022

Visual Pathway

  • 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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