A 54-year-old man with a tumour in the deep white matter of the left temporal lobe develops a right homonymous superior quadrantanopia. Which bundle of optic radiation fibres has been interrupted?

• A Meyer's loop, sweeping around the inferior horn of the lateral ventricle ✓
• B Baum's loop, passing through the parietal lobe
• C Fibres passing directly through the splenium of the corpus callosum
• D Fibres running in the tapetum toward the occipital pole

Explanation

Fibres representing the contralateral superior quadrant sweep forward around the inferior horn of the lateral ventricle as Meyer's loop within the temporal lobe before turning back to the calcarine cortex. Temporal lobe lesions interrupt these fibres first, giving the classic pie-in-the-sky superior quadrantanopia. Baum's loop carries inferior quadrant fibres through the parietal lobe, so parietal lesions cause inferior quadrantanopia instead. The tapetum and splenium relate to interhemispheric callosal fibres, not the geniculocalcarine pathway.

Reference: Gray's Anatomy for Students, 4th ed.

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Written and medically reviewed by the StethoPrep medical team.