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Optic Chiasm

The optic chiasm or optic chiasma is the part of the brain where the optic nerves (CN II), coursing backward from each eye partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus. The temporal retina of each eye, the most lateral half of the retina, provides information about light entering the eye from the nasal side, the opposite side. On the contrary, in each eye, the nasal retina provides information about light entering the eye from the same side of the body as the eye. Pathways The images on the nasal sides of each retina cross over to the opposite side of the brain via the optic nerve at the optic chiasm (decussation of medial fibers). The temporal images, on the other hand, stay on the same side. This allows the images from either side of the field from both eyes to be transmitted to the appropriate side of the brain, combining the sides together. This allows for parts of both eyes that attend to the right visual field to be processed in the left visual system in the brain, and vice versa. This is linked to skin sensation which also reaches the opposite side of the body, after reaching the diencephalon (rear forebrain). This decussation (crossing) is an adaptive feature of frontally oriented eyes and therefore having binocular vision. (Some animals, with laterally positioned eyes, have little binocular vision, so there is a more complete crossover of visual signals.) Beyond the optic chiasm, with crossed and uncrossed fibers, the optic nerves become optic tracts. The signals are passed on to the lateral geniculate body, in turn giving them to the occipital cortex (the outer matter of the rear brain).