What happens to nasal retinal fibers at the optic chiasm?

• A. They cross (decussate) to the opposite side.
• B. They stay on the same side.
• C. They project directly to the LGN from the retina.
• D. They originate from the temporal retina.

Answer: (A)

At the optic chiasm, the nasal portion of each retina sends its fibers across to the opposite side. This crossing (decussation) rearranges visual input so that information from the opposite visual field is processed in the opposite hemisphere. In contrast, fibers from the temporal retina do not cross and stay on the same side. After crossing, the nasal fibers continue in the contralateral optic tract toward the lateral geniculate nucleus, feeding into the visual pathway that ultimately reaches the cortex. This crossing is what allows binocular, contralateral representation of the visual fields.