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ANATOMY OF THE VISUAL PATHWAY
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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.
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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.
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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 ...