occipital lobe

**Anatomy** - Location: posterior cerebrum, behind the parieto-occipital sulcus; rests on the tentorium cerebelli. - Primary visual cortex (V1, Brodmann 17, striate cortex) lies along the calcarine sulcus. The cuneus (above the sulcus) receives input from the contralateral INFERIOR visual field. The lingual gyrus (below) receives input from the contralateral SUPERIOR visual field. - Visual association cortex: Brodmann 18 (V2) and 19 (V3–V5).

**Visual pathway (high-yield)** Retina → optic nerve → optic chiasm (nasal fibers cross) → optic tract → lateral geniculate nucleus (thalamus) → optic radiations → V1. - Meyer's loop (temporal lobe): carries INFERIOR retinal fibers (superior visual field) → lesion gives "pie in the sky" (contralateral superior quadrantanopia). - Parietal radiations: carry SUPERIOR retinal fibers (inferior visual field) → lesion gives "pie on the floor" (contralateral inferior quadrantanopia). - Remember "PITS": Parietal-Inferior, Temporal-Superior (field defects).

**Blood supply** - Posterior cerebral artery (from the vertebrobasilar system). - Macula is represented at the occipital pole, which has dual supply from the PCA and MCA branches (collateral). This causes **macular sparing** in PCA strokes.

**Clinical correlations** - PCA stroke: contralateral homonymous hemianopia WITH macular sparing. Alexia without agraphia occurs with a left PCA infarct involving the splenium of the corpus callosum. - Bilateral occipital lesions: cortical blindness with preserved pupillary light reflex (the pathway to the pretectal nuclei is intact) and often **Anton syndrome** (denial of blindness). - Balint syndrome: bilateral parieto-occipital lesions → simultanagnosia, optic ataxia, oculomotor apraxia. - Visual agnosias: prosopagnosia (fusiform gyrus, inferior occipitotemporal) and object agnosia from association cortex lesions. - Dorsal ("where") stream: occipital → parietal. Ventral ("what") stream: occipital → temporal. - Occipital lobe seizures: simple visual hallucinations (flashing lights, colors, spots). Also seen in migraine with aura (scintillating scotoma, cortical spreading depression). - Posterior reversible encephalopathy syndrome (PRES): vasogenic edema in the parieto-occipital regions, causing visual disturbances, headache, and seizures (hypertension, eclampsia, immunosuppressants). - Cortical blindness can also follow hypoperfusion, e.g., watershed infarcts of the MCA/PCA border zone after hypotension. - Herniation/other: Kernohan and other herniation syndromes can compress the PCA → occipital infarct (e.g., transtentorial herniation). - Charles Bonnet syndrome: visual hallucinations in visually impaired patients with preserved insight. - Riddoch phenomenon: preserved perception of motion in a blind field (V5/MT).

**Key test strategy** Congruous homonymous defects (identical in both eyes) point to occipital lesions. Incongruous defects suggest optic tract or more anterior lesions. Macular sparing strongly suggests an occipital (PCA) lesion.