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A lesion to the optic chiasm typically results in bitemporal hemianopsia, which is characterized by a loss of vision in the outer (temporal) half of the visual field in both eyes. This occurs because the optic chiasm is the point where the optic nerves from each eye cross; specifically, the nasal fibers, which carry the visual information from the temporal visual fields, are affected. When these fibers are compromised, the brain does not receive the proper information from the peripheral aspects of the visual field, leading to the typical “tunnel vision” appearance where central vision remains intact but peripheral vision is lost.

Understanding this defect helps in the diagnosis and management of various conditions that can affect the chiasm, such as pituitary tumors, which are known to cause this type of visual loss due to their growth in the sella turcica, which pressure the chiasm. Other types of visual field defects, such as homonymous hemianopsia, quadrantanopia, and scotomas, are associated with lesions in different locations along the visual pathway, further illustrating the importance of the optic chiasm's unique role in visual perception.