optic tract

**Anatomy/Course:** Retinal ganglion cell axons form the optic nerve (CN II) -> optic chiasm (nasal retinal fibers cross) -> optic tract -> synapse in the LGN of the thalamus (majority) -> optic radiations -> primary visual cortex (V1, calcarine sulcus, occipital lobe). Each optic tract contains fibers from the ipsilateral temporal hemiretina and contralateral nasal hemiretina, so it represents the contralateral visual hemifield. A minority of fibers bypass the LGN: to the pretectal nucleus (pupillary light reflex), superior colliculus (saccades/visual orienting), and suprachiasmatic nucleus of the hypothalamus (circadian rhythm).

**Optic radiations (post-LGN, for comparison):** - Meyer's loop (temporal lobe, inferior retina/superior visual field): lesion -> contralateral superior quadrantanopia ("pie in the sky"). - Parietal radiations (superior retina/inferior visual field): lesion -> contralateral inferior quadrantanopia ("pie on the floor").

**Lesion of the optic tract:** - Contralateral homonymous hemianopia, typically INCONGRUOUS (asymmetric) because fibers from corresponding retinal points are not yet fully aligned. More congruous defects point more posteriorly (toward the occipital cortex). - No macular sparing (macular sparing suggests an occipital/PCA stroke with collateral MCA supply). - Contralateral RAPD (Wernicke's pupil) may occur, because the contralateral eye's nasal retina has more fibers (crossed fibers outnumber uncrossed, ~53:47) and the temporal visual field is larger. - Optic atrophy with "bow-tie" (band) atrophy of the optic disc in the contralateral eye can develop (from nasal fiber loss).

**Causes:** Tumors (craniopharyngioma, pituitary adenoma with suprasellar extension, hypothalamic/ suprasellar masses), aneurysms (posterior communicating or anterior choroidal artery), demyelination (MS), trauma, surgical injury, and stroke in the territory of the anterior choroidal artery (which supplies the optic tract, internal capsule posterior limb, and LGN partially).

**Blood supply:** Anterior choroidal artery (branch of ICA) and posterior communicating artery (optic tract); LGN is supplied by the anterior choroidal artery (lateral/ventral portion) and posterior (lateral) choroidal artery from the PCA.

**Board pearls:** - Lesion location -> deficit: optic nerve = ipsilateral monocular blindness; chiasm (midline) = bitemporal hemianopia; optic tract = contralateral homonymous hemianopia; Meyer's loop = contralateral superior quadrantanopia; parietal radiations = contralateral inferior quadrantanopia; occipital cortex = contralateral homonymous hemianopia with macular sparing. - LGN layers: layers 1, 4, 6 receive contralateral eye input; layers 2, 3, 5 receive ipsilateral eye input ("Crossed Rule: 1, 4, 6"). Layers 1-2 are magnocellular (motion/depth); layers 3-6 are parvocellular (color/form). - Pupillary light reflex fibers leave the optic tract before the LGN to reach the pretectal nuclei, which is why LGN or radiation lesions do not cause an RAPD, while tract lesions can.