The optic nerve is the output of the retina. Therefore, each optic nerve carries all of the visual information from the eye from which it emerges. Since the left hemisphere moves the right side of the body and the right hemisphere moves the left side of the body, it makes sense that the left hemisphere should receive the visual information from the right half of the world and the right hemisphere should receive the visual information from the left half of the world. Therefore, some of the information in each optic nerve must cross so that the brain can work with the left and right visual fields rather than merely what is seen by the left and right eyes: The left hemisphere must receive right visual field information from both the left eye and the right eye; the right hemisphere must receive left visual field information from both the left eye and the right eye. The crossing of visual field information to convert the visual world from left eye–right eye organization to left field–right field organization occurs at the optic chiasm. Posterior to the optic chiasm, visual information is organized into fields: the left visual field is processed in the right hemisphere, and the right visual field in the left hemisphere.

Which visual information from each eye needs to cross to convert the organization of visual information from eyes to visual fields? Imagine viewing a simple rectangle that is half tan and half blue (Fig. 6–1). The right visual field (blue) projects onto the medial (nasal) retina in the right eye and the lateral (temporal) retina in the left eye. The left visual field (tan) projects to the medial (nasal) retina in the left eye and the lateral (temporal) retina in the right eye. The medial (nasal) retinas are, therefore, receiving the lateral (temporal) visual fields, and the lateral (temporal) retinas are receiving the medial (nasal) visual fields.

All visual information from the right visual field must end up in the left hemisphere, and all visual information from the left visual field must end up in the right hemisphere. The right visual field information in the left optic nerve (from the lateral retina) is already on the correct (left) side of the brain, but the right visual field information in the right optic nerve (from the medial retina) must cross from the right optic nerve to the left side of the brain to join it. The left visual field information in the right optic nerve (from the lateral retina) is already on the correct side of the brain, but the left visual field information in the left optic nerve (from the medial retina) must cross from the left optic nerve to the right side to join it.

In summary, the information seen by the lateral (temporal) retina of each eye is from the contralateral visual field, and so it is already on the correct side of the brain in the optic nerve and does not need to cross. The information in the medial (nasal) retina in each eye is from the ipsilateral visual field, so that information needs to cross at the chiasm. Therefore, at the chiasm, the information from the medial (nasal) retinas (representing the lateral [temporal] visual fields) crosses, and the information from the lateral (temporal) retinas (representing the medial [nasal] visual fields) remains ipsilateral.

Posterior to the optic chiasm, the visual world is separated into left and right visual fields, with the left visual field represented in the right hemisphere, and the right visual field represented in the left hemisphere. This information travels in the left and right optic tracts from the chiasm to the left and right lateral geniculate nucleus (LGN) of the thalamus. From each LGN, information travels to the visual cortex of the occipital lobes in a superior radiation and an inferior radiation on each side. Just as the left visual field goes to the opposite (i.e., right) side of the brain and the right visual field goes to the opposite (i.e., left) side of the brain, the inferior visual fields travel in the superior radiations to the superior bank of the calcarine cortex in the posterior occipital lobe, and the superior visual fields travel in the inferior radiations to the inferior bank of the calcarine cortex. The superior radiations travel through the parietal lobes to the occipital lobes, and the inferior radiations travel through the temporal lobes to the occipital lobes. The inferior radiations loop anteriorly before proceeding posteriorly, and this anterior loop is called Meyer’s loop.

Understanding the flow of visual information allows for the localization of visual deficits to particular regions of the visual pathway (see Fig 6–1).

A visual deficit limited to just one eye (i.e., the other eye sees the whole world normally when the problematic eye is closed) must be due to either ocular pathology (e.g., lens, anterior chamber, retina) or pathology of the optic nerve.

A visual deficit limited to a particular visual field in both eyes (i.e., the same deficit in each eye—a homonymous deficit) localizes posterior to the optic chiasm: optic tract, LGN, optic radiation(s), or occipital lobe (on the side contralateral to the homonymous deficit).

• 
  

  An optic tract or LGN lesion will cause a contralateral homonymous hemianopia. Lesions of the optic tract may cause incongruous visual field deficits that are on the same side in each eye (homonymous) but of different shapes in each eye. Isolated lesions in the optic tract or lateral geniculate nucleus are rare in practice.

  
  
• 
  

  Lesions affecting an individual optic radiation or individual bank of the calcarine cortex will cause a contralateral visual deficit in one quadrant in both eyes (quadrantanopia):

  
  
  
  
  • 
    

    A superior quadrantanopia localizes to the contralateral inferior radiation and/or inferior bank of the calcarine cortex.

    
    
  • 
    

    An inferior quadrantanopia localizes to the contralateral superior radiation and/or superior bank of the calcarine cortex.

  
  
• 
  

  Lesions of one occipital lobe that affect both the superior and inferior portions of the calcarine cortex lead to a contralateral homonymous hemianopia, as would a lesion of the optic tract or lateral geniculate nucleus on that side. Infarction of one occipital lobe (due to stroke in the posterior cerebral artery territory) may produce a homonymous hemianopia with macular sparing (preserved central vision). This may be due to the macula receiving some blood supply from the middle cerebral artery and/or the macula being bilaterally represented since it is at the center of vision and, therefore, does not fall into one “field.”