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ANATOMY OF THE VISUAL PATHWAY

The optic nerve is the output of the retina (Figs. 6–1 and 6–2). 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.

FIGURE 6–1

Anatomy of the visual pathways. The left visual field (blue) and right visual field (orange) are traced through the pathways, and the pop-out zooms in on the optic chiasm to demonstrate which visual field information crosses in the chiasm.

FIGURE 6–2

Anatomy of the visual pathways. The left visual field (blue) and right visual field (orange) are traced through the pathways with the superior visual fields darker and the inferior visual fields lighter as they are traced through the optic radiations to the primary visual cortex.

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

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