The figure below shows the visual pathway, and the two circles are the visual fields – these show (in black) where the patient could not see anything. Visual fields are tested for each eye separately; the patient could not see objects located to their left with their left eye, or objects located to the right with their right eye. Thus, the temporal visual field was affected in both eyes. On the visual pathway, which of the four locations (labelled A to D) would a lesion be located that would give rise to the visual field deficits?

Question

BlackTom AI · Accepted Answer

Temporal visual fields are carried by the nasal halves of each retina; nasal retinal fibers decussate (cross) at the optic chiasm. A lesion that interrupts those crossing fibers therefore produces loss of the temporal visual field in both eyes (bitemporal hemianopia).

Step‑by‑step reasoning:
1. Map the tested deficits to retinal locations: the patient cannot see objects to their left with the left eye → loss of the left eye’s temporal field. The patient cannot see objects to their right with the right eye → loss of the right eye’s temporal field. Together this is loss of both temporal visual fields (bitemporal loss).
2. Determine which retinal fibers subserve temporal fields: temporal visual field → nasal retina in each eye.
3. Identify where nasal retinal fibers travel: nasal retinal fibers cross at the optic chiasm while temporal retinal fibers remain uncrossed.
4. Localize the lesion: damage that selectively affects the crossing nasal fibres of both eyes must be at the optic chiasm. In the figure, location B is at the optic chiasm.

Why B is correct:
- A lesion at B (optic chiasm) interrupts the decussating nasal retinal fibers from both eyes, producing the described bitemporal field loss.

Why the other options are incorrect:
- A (an optic nerve lesion) would produce monocular visual loss (loss of both nasal and temporal fields of one eye), not bitemporal loss.
- C (optic tract) lesions produce a contralateral homonymous hemianopia (loss of the same visual field in both eyes, e.g., left visual field from both eyes), not temporal fields in each eye.
- D (optic radiation/occipital cortex) also causes contralateral homonymous field defects (often quadrantanopia or hemianopia), not bilateral temporal field loss.

Conclusion: The lesion is at the optic chiasm (location B), which matches the provided correct answer B.

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