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The visual system is unique as much of visual processing occurs outside the brain within the retina of the eye. The previous chapter described how the light-sensitive receptors of the eye convert the image projected onto the retina into spatially distributed neural activity in the first neurons of the visual pathway (i.e., the photoreceptors). Within the retina, the receptors synapse with bipolar and horizontal cells, which establish the basis for brightness and color contrasts. In turn, the bipolar cells (the 2° visual afferent) synapse with retinal ganglion cells and amacrine cells, which enhance contrast effects that support form vision and establish the basis for movement detection. The information from the eye is carried by the axons of the retinal ganglion cells (the 3° visual afferent) to the midbrain and diencephalon. This chapter will provide more information about visual pathway organization and the visual processing that occurs within the brain.

The Visual Pathway from Retina to Cortex

As noted previously in the somatosensory sections, all sensory information must reach the cerebral cortex to be perceived and, with one exception, reach the cortex by way of the thalamus. In the case of the visual system, the thalamic nucleus is the lateral geniculate nucleus and the cortex is the striate cortex of the occipital lobe.

The Optic Nerve

Figure 15.1 The visual pathway with the course of information flow from the right (green) and left (blue) hemifields of the two eye's visual fields.

The axons of the 3° visual afferents (the retinal ganglion cells) form the optic nerve fiber layer of the retina on their course to the optic disc. At the optic disc, the 3° visual afferents exit the eye and form the optic nerve. The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina (i.e., the nasal hemiretina) decussate in the optic chiasm to the opposite optic tract (Figure 15.1). Consequently, each optic tract contains retinal ganglion cell axons that originate in the nasal half of the contralateral retina and the temporal half of the ipsilateral retina. Recall that the ipsilateral temporal hemiretina and the contralateral nasal hemiretina have projected on them the images of corresponding halves of their visual fields. For example, the temporal (left) hemiretina of left eye and the nasal (left) hemiretina of right eye both have projected on them the right halves of their respective visual fields. Consequently, each optic tract has within it axons representing the contralateral half of the visual field.

The axons in the optic tract terminate in four nuclei within the brain (Figure 15.2):

• the lateral geniculate nucleus of the thalamus - for visual perception;
  
• the superior colliculus of the midbrain - for control of eye movements;
  
• the pretectum of the midbrain - for control of the pupillary light reflex; and
  
• the suprachiasmatic nucleus of the hypothalamus - for control of diurnal rhythms and hormonal changes.

Figure 15.2 The inferior surface of the brain illustrating the visual pathway. The termination sites of the retinal ganglion cell axons in three nuclei that are not considered a part of the visual pathway are also illustrated. They include the hypothalamus, pretectum and the superior colliculus.

The Lateral Geniculate Nucleus

The vast majority of optic tract fibers terminate on neurons in the lateral geniculate nucleus (LGN) of the thalamus (Figure 15.3A).

Like the retina, the lateral geniculate nucleus is a laminated structure, in this case, with six principal layers of cells (Figure 15.3B).

• The largest cells form the deepest two (magnocellular) layers
  
• Smaller cells form the upper four (parvocellular) layers
  
• Thin layers of the smallest cells (i.e., the koniocellular neurons) are interposed between these principal layers.
  

The optic tract fibers (3° visual afferents) from each eye synapse in different layers of the LGN. Consequently, each LGN neuron responds to stimulation of one eye only.

Figure 15.3 Structures of the visual pathway (A). The neurons of the lateral geniculate nucleus form 6 layers that are visible when stained for Nissl substance (B). The magnocellular layers (1 and 2) appear darker as the cells in these layers are larger and contain more Nissl substance than the cells in the parvocellular layers (3 through 6).

The functional properties of LGN neurons are similar to those of retinal ganglion cells.

The LGN neurons are monocular (i.e., respond to stimulation of one eye only) and have concentric (center-surround) receptive fields. The LGN neurons are segregated into three major groups:

• The neurons in the magnocellular layers (mLGN cells)
  
  • process M-retinal ganglion cell inputs
    
  • behave like M-retinal ganglion cells
    
  • have relatively large center-surround receptive fields
    
  • are color insensitive
    
  • are most sensitive to movement of visual stimuli
    
• The neurons in the parvocellular layers (pLGN cells)
  
  • process P-retinal ganglion cell inputs
    
  • behave like P-retinal ganglion cells
    
  • have relatively small center-surround receptive fields
    
  • are color sensitive
    
  • are well suited for detecting contrasts that form the basis for shape/form discrimination.
    
• A third group, the koniocellular neurons (kLGN)
  
  • process P-retinal ganglion cell inputs
    
  • behave like P-retinal ganglion cells
    
  • have the smallest concentric receptive fields
    
  • have stronger color sensitivity than P-retinal ganglion cells
    
  • are well suited for detecting colors that aid in shape/form discrimination.
    

The axons of these different types of LGN neurons terminate in different layers or sublayers of the primary visual cortex.

Visual Cortical Areas

The primary visual cortical receiving area is in the occipital lobe. The primary visual cortex is characterized by a unique layered appearance in Nissl stained tissue.

Figure 15.4 Nearly the entire caudal half of the cerebral cortex is dedicated to processing visual information. A lateral view of the left cerebral hemisphere (A). A view of the medial surface of the right hemisphere (B). The primary motor cortex (i.e., the precentral gyrus), and the primary somatosensory receiving area (i.e., the postcentral gyrus) are represented in red and blue, respectively. The numbers provide the Brodmann Area designation.

Consequently, it is called the striate cortex. It includes the calcarine cortex, which straddles the calcarine fissure, and extends around the occipital pole to include the lateral aspect of the caudal occipital lobe (Figure 15.4, Area 17).