Retrochiasmal Segment

    • Visual pathway segment that conveys signals from the optic tract to the primary visual cortex
    • Completes the process, started at the optic chiasm, of hemifield representation of vision in cerebral hemispheres
    • Optic tract
      • Conveys signals from the opposite visual hemifield to the lateral geniculate body
    • Lateral geniculate body
      • Contains synapses for signals coming from the optic tract
      • Maintains segregation of signals from each eye, as signals from the ipsilateral eye terminate in layers 2,3, and 5, while signals from the contralateral eye terminate in layers 1,4, and 6
      • Modifies signals by means of descending attentional and limbic input from the cerebrum
    • Optic radiations
      • Contain axons exiting from the lateral geniculate bodies
      • Some axons loop around the anterior temporal horn of the lateral ventricle as Meyer’s Loop
      • Meyer’s Loop axons rejoin the rest of optic the radiations, which form a wide band along the border of the lateral ventricle, eventually dividing into superior and inferior forks at the atrium of the lateral ventricle
      • Superior fork of the optic radiations enters the superior portion of the primary visual cortex
      • Inferior fork of the optic radiations enters the inferior portion of the primary visual cortex
    • Primary Visual Cortex
      • Signals coming from the central 5-10 degrees of the visual field terminate in the posterior visual cortex
      • Signals coming from between 10 degrees and 60 degrees eccentric to fixation terminate in the intermediate primary visual cortex
      • Signals coming from beyond 60 degrees (“unpaired temporal crescent”) terminate on the anterior visual cortex
    • Optic tract lesions
      • May produce either complete homonymous hemianopia (total damage) or incomplete but incongruous homonymous hemianopia (subtotal damage)
    • Lateral geniculate body lesions
      • Mass lesions usually cause damage that extends beyond these small structures, destroying the entire lateral geniculate body, and causing complete homonymous hemianopias
      • Anterior choroidal artery occlusion or lateral choroidal artery occlusion may produce hourglass homonymous hemianopias
      • Tip: inflammation and infarction may target BOTH lateral geniculate bodies in isolation
    • Meyer's loop lesions
      • Produce superior wedge-shaped homonymous hemianopia, also called “pie-in-the-sky” defects
      • Common cause is temporal lobectomy for intractable seizures
    • Optic radiation lesions
      • Produce complete homonymous hemianopias or incomplete and congruous homonymous hemianopias
    • Primary visual cortex lesions
      • Produce complete homonymous hemianopias, incomplete congruous homonymous hemianopias, or the following other incomplete homonymous hemianopias
        • Superior homonymous quadrantanopia (lesion damages only inferior visual cortex)
        • Inferior homonymous quadrantanopia (lesion damages only superior visual cortex)
        • Homonymous paracentral scotomas (lesion restricted to posterior visual cortex)
        • Macular-sparing homonymous hemianopia (lesion restricted to midportion and anterior portion of visual cortex
        • Temporal crescent-sparing homonymous hemianopia (lesion spares anterior visual cortex)
        • Temporal crescent scotoma (lesion restricted to anterior visual cortex)

    Vision Pathways

    Overview Videos All Segments Considered Optical Segment Prechiasmal Segment Chiasmal Segment Retrochiasmal Segment Occipito-Parietal Segment Occipito-Temporal Segment