As a result of light refraction by the cornea and lens, the right half of the visual field is projected to the left half of the retina of both eyes (the temporal half of the left retina and the nasal half of the right retina). The left half of the visual field is projected to the right half of the retina of both eyes. The temporal half of the left retina and the nasal half of the right retina therefore see the same image. Axons from ganglion cells in the left (temporal) half of the left retina pass to the left lateral geniculate nucleus of the thalamus. Axons from ganglion cells in the nasal half of the right retina cross over (decussate) in the x-shaped optic chi-asma, also to synapse in the left lateral geniculate body. The left lateral geniculate, therefore, receives input from both eyes that relates to the right half of the visual field (fig. 10.43).
The right lateral geniculate body, similarly, receives input from both eyes relating to the left half of the visual field. Neurons in both lateral geniculate bodies of the thalamus in turn project to the striate cortex of the occipital lobe in the cerebral cortex (fig. 10.44). This area is also called area 17, in reference to a numbering system developed by K. Brodmann in 1906. Neurons in area 17 synapse with neurons in areas 18 and 19 of the occipital lobe (fig. 10.44).
Approximately 70% to 80% of the axons from the retina pass to the lateral geniculate bodies and to the striate cortex. This geniculostriate system is involved in perception of the visual field. Put another way, the geniculostriate system is needed to answer the question, What is it? Approximately 20% to 30% of the fibers from the retina, however, follow a different path to the superior colliculus of the midbrain (also called the optic tec-tum). Axons from the superior colliculus activate motor pathways leading to eye and body movements. The tectal system, in other words, is needed to answer the question, Where is it?
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