Retrochiasmal Segment

Video overview

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)