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Retinotopic Organization in the Visual Pathway

Clinical Examples

The topographic (spatial) relationships of retinal neurons are maintained throughout the visual system, which preserves the retinotopic map of the visual world. That is, the retina is mapped onto the LGN and striate cortex in an organized (topographic) fashion. Consequently, neighboring parts of retina project to neighboring parts of LGN and neighboring parts of LGN project to neighboring parts of the striate cortex. This retinotopic organization in the visual pathway results in a spatial representation of the visual field in the LGN and visual cortex.

Spatial Representation of the Retinal Image

You should recall the following regarding the spatial representation of the retinal image within the visual pathway.

• The optic image on the retina is upside-down and left-right reversed.
  
• The monocular visual fields of the two eyes overlap partially to form the binocular visual field .
  
• The temporal hemiretina of one eye and the nasal hemiretina of the other eye have projected on them the images of corresponding halves of their visual fields (Figure 15.1). For example, the temporal (left) hemiretina of left eye and the nasal (left) hemiretina of right eye both have projected on them the right half of the visual fields of each eye.
  
• Beyond the optic chiasm, the corresponding visual hemifields of the two eyes are represented in the contralateral side of the visual pathway (Figure 15.1). For example, the left hemifield of both eyes are represented in the right optic tract, right lateral geniculate nucleus, right optic radiations and right striate cortex.
  
• The fibers of the optic radiation fan out into the temporal, parietal and occipital lobes on their course to the striate cortex. Those forming the sublenticular optic radiations carry information about the superior hemifield, whereas those forming the retrolenticular optic radiations carry information about the inferior hemifield (Figure 15.5). The optic radiation fibers traveling the most direct course back to the striate cortex carry information about the central visual field.
  
• There are many more receptor cells in the fovea and many more bipolar and ganglion cells in the macula than in the periphery of the retina. Consequently, the central visual field is disproportionately represented in the visual system. That is, more visual receptors, more optic nerve fibers and more LGN and cortical neurons are involved in processing and carrying information about that portion of the retinal image representing the center of the visual field.
  

Visual Field Defects

Visual field defects are areas of loss of vision in the visual field. Visual field defects are detected by perimetry testing, during which the patient fixates his eyes on a target and his ability to detect a small object in specific positions in space is determined.

Figure 15.11 illustrates perimetry test results for the two eyes of someone with normal vision. The bottom panel of Figure 15.11 is a simplified illustration of the monocular visual fields used in the following examples of visual field defects. A visual field defect provides clues to the structure(s) affected. That is, the area(s) of visual field loss and eye(s) exhibiting the visual field loss offer clues about the site of the damage. The following examples of visual field losses should help you determine how well you can utilize what you have learned thus far about the visual system.