TAS

Visual Field Defect: Left Upper Quadrantanopia – Answer & Explanation

Correct Answer: d. Right lower optic radiation (Meyer’s loop)

Explanation:

• Left upper quadrantanopia (defect in the left upper visual field) indicates a lesion affecting fibers carrying information from the right inferior retina (which corresponds to the left upper visual field).

• The optic radiation (part of the visual pathway after the optic tract) has two parts:

• Upper fibers (Baum’s loop) → Carry superior retinal (lower visual field) info → Pass through parietal lobe.

• Lower fibers (Meyer’s loop) → Carry inferior retinal (upper visual field) info → Loop into the temporal lobe.

• A lesion in the right lower optic radiation (Meyer’s loop) causes a left upper quadrantanopia (“pie in the sky” defect).

Why Not Other Options?

• a. Right lower optic tract → Would cause a left homonymous hemianopia (not quadrantanopia).

• b. Left lower optic tract → Would cause a right homonymous hemianopia.

• c. Left lower optic radiation → Would cause a right upper quadrantanopia.

• e. Geniculate nucleus → Typically causes a homonymous hemianopia, sometimes with sparing of the temporal crescent.

Key Points:

• Optic radiation lesions → Quadrantanopia (Meyer’s loop for upper, Baum’s loop for lower).

• Optic tract/LGN lesions → Complete homonymous hemianopia.

• Meyer’s loop (temporal) → “Pie in the sky” (upper quadrant defect).

• Baum’s loop (parietal) → “Pie on the floor” (lower quadrant defect).

Mnemonic:

• “Meyer’s Loop goes to the Temporal lobe, causing a defect in the sky.”

• “Parietal (Baum’s) lesions knock the vision down to the floor.”

Exam Tip:

• If a question gives quadrantanopia, think optic radiation (not tract/LGN).

• Upper field defect? → Temporal lobe (Meyer’s loop).

• Lower field defect? → Parietal lobe (Baum’s loop).

Summary Table