Final answer:
The optic chiasm, where nerve fibers from both eyes cross paths, can be affected by conditions such as bilateral hemianopia, often caused by a pituitary tumor compressing the nerve fibers responsible for peripheral vision.
Step-by-step explanation:
Defect at the Optic Chiasm
A defect that commonly occurs at the optic chiasm is known as a loss of peripheral vision or bilateral hemianopia. This condition arises when the axons from the medial retinae, which transmit visual information from the peripheral field of each eye, are damaged or compressed. One typical cause of compression at this location is a pituitary tumor. Since the pituitary gland is situated in the sella turcica, directly below the optic chiasm, any growth or enlargement can press upon the crossing nerve fibers, interrupting the normal flow of visual information. This can lead to a characteristic loss of vision, where the individual loses sight of objects on the far right and left sides, while the central vision remains intact.
It is important to note that at the optic chiasm, information from the right visual field (seen by both eyes) is transferred to the left hemisphere of the brain, and information from the left visual field is sent to the right hemisphere. This decussation, or crossing over, is vital for the brain's ability to process the visual field appropriately. However, when a lesion is present at the optic chiasm, it can specifically affect the peripheral visual field, sparing the superior and inferior portions, which leads to a narrower field of vision sometimes mistaken for tunnel vision but is actually bilateral hemianopia.