Question

Homonymous hemianopia may occur due to lesion involving

• A. Optic nerve
• B. Optic chiasm
• C. Optic tract
• D. Bilateral foveal lesions

Hint:

Fibers from matching halves of both eyes are already grouped together just behind the chiasm.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to identify which site along the visual pathway, when damaged, produces a homonymous hemianopia, meaning loss of the same half of the visual field in both eyes.

Step 2: Key Formula or Approach:
The visual field defect produced by a lesion depends on exactly where along the pathway, from optic nerve to optic chiasm to optic tract to the cortex, the fibers are damaged, since fibers from each eye cross or stay uncrossed at different points.

Step 3: Detailed Explanation:
A lesion of the optic nerve, before the chiasm, damages only the fibers from that one eye, causing complete or partial vision loss in that single eye alone, not a matching field defect in both eyes.
Bilateral foveal lesions would cause central vision loss in both eyes rather than a hemianopia affecting one whole side of the visual field in each eye.
A lesion of the optic tract, which lies just behind the chiasm, contains fibers from the corresponding halves of both eyes that have already crossed and rejoined, so damage here classically causes a homonymous hemianopia, loss of the same side of the visual field in both eyes.
A lesion of the optic chiasm itself, where fibers from the nasal half of each retina cross over, classically damages only the crossing fibers, causing a bitemporal hemianopia (loss of the outer visual field in both eyes), which is different from a homonymous hemianopia.

Step 4: Final Answer:
So, based on the standard organization of the visual pathway, it is a lesion of the optic tract that produces a homonymous hemianopia, since the chiasm is where fibers cross to classically produce bitemporal, not homonymous, field loss.