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