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A visual defect characterized by blindness in one-half of the visual field that is comparable in both eyes. For example, with a left homonymous hemianopsia, when looking straight ahead the patient would not be able to see anything to the left of midline regardless of which eye was being tested. Such a visual field defect is the result of an interruption of the visual pathways anywhere after the partial crossing of the visual fibers in the optic chiasm to the occipital cortex. This then could involve the optic tract, the lateral geniculate nucleus, the optic radiations, or the primary visual cortex. Two potential exceptions to this rule might be noted. One is that if the lesion producing the hemianopsia is in the visual cortex, the patient might retain a small amount of foveal vision in the very central part of the left visual field, a phenomenon known as macular sparing(probably due to overlapping vascular distributions for this critical...
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