## Anatomical Localization of Visual Field Defects **Key Point:** Homonymous hemianopia (same half of visual field lost in both eyes) with preserved pupillary light reflex is pathognomonic for a post-geniculate lesion — optic tract, lateral geniculate nucleus, optic radiations, or visual cortex. ### Why Post-Geniculate? The optic pathway has a critical anatomical transition: | Structure | Visual Field Defect | Pupil Response | Key Feature | |-----------|-------------------|---------------|--------------| | Optic nerve | Monocular vision loss | Afferent pupil defect | One eye only | | Optic chiasm | Bitemporal hemianopia | Normal | Midline crossing fibers | | Optic tract | Homonymous hemianopia | **Normal** | Post-geniculate | | Retina | Monocular field loss | Afferent defect | One eye only | **High-Yield:** The pupillary light reflex pathway (retina → optic nerve → pretectal nucleus → Edinger-Westphal nucleus) is **separate** from the visual pathway. A post-geniculate lesion damages vision but spares the pupillary reflex arc. ### Clinical Correlation **Clinical Pearl:** Homonymous hemianopia + normal pupils = **stroke, tumor, or trauma affecting optic tract, LGN, optic radiations, or occipital cortex**. The vertical midline is respected because the optic tract and all downstream structures process the contralateral visual hemifield as a unit. **Mnemonic:** **TRACT** — **T**he **R**etrogeniculate **A**natomy **C**auses **T**he preserved pupil (optic tract, radiations, cortex all spare the pupil pathway). 
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