Visual pathway

Also known as: optic pathway, optic chiasm, optic tract, lateral geniculate nucleus, LGN, optic radiation, Meyer loop, primary visual cortex, V1, striate cortex, visual field defects

The pathway from retina to cortex: optic nerve → optic chiasm (nasal fibres cross) → optic tract → lateral geniculate nucleus → optic radiation → primary visual cortex (V1), so each hemisphere sees the opposite half of the visual field.

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Overview

Retina and optic nerve. Images are inverted and reversed on the retina: the temporal visual field falls on the nasal retina and the superior field on the inferior retina. Ganglion-cell axons leave as the optic nerve (≈1.2 million axons, ≈45–50 mm long: intraocular, intraorbital, intracanalicular and intracranial parts), which is a CNS tract myelinated by oligodendrocytes and wrapped in meninges.

Optic chiasm. Above the diaphragma sellae and pituitary gland, fibres from the nasal hemiretinae cross (≈53%) while temporal fibres stay ipsilateral. Each optic tract therefore carries information from the contralateral visual hemifield. Inferonasal fibres cross in the anterior chiasm and loop briefly into the contralateral optic nerve (Wilbrand knee, an anatomical point now debated).

Lateral geniculate nucleus (LGN). About 90% of tract fibres synapse in the LGN of the thalamus, a six-layered nucleus: layers 1–2 magnocellular (motion, contrast; from parasol RGCs), 3–6 parvocellular (fine detail, red–green colour; from midget RGCs), with koniocellular cells between layers (blue–yellow). Contralateral eye → layers 1, 4, 6; ipsilateral eye → 2, 3, 5. The remaining ≈10% of tract fibres bypass the LGN to the pretectum (pupillary reflexes), superior colliculus (orienting, saccades) and suprachiasmatic nucleus (circadian rhythm, via the retinohypothalamic tract).

Optic radiation and V1. From the LGN the geniculocalcarine fibres fan out through the retrolenticular internal capsule. Inferior fibres (from the inferior retina = superior visual field) loop forward into the temporal lobe around the temporal horn (Meyer loop); superior fibres (inferior visual field) run through the parietal lobe. They end in the primary visual cortex (V1, striate cortex, Brodmann area 17) on the banks of the calcarine sulcus — cuneus above (inferior field), lingual gyrus below (superior field), with the macula represented at the occipital pole. V1 sends information to extrastriate areas: a ventral "what" stream (temporal lobe — form, colour, faces) and a dorsal "where/how" stream (parietal lobe — motion, spatial location).

Location: From the retina through the orbit (optic canal), middle cranial fossa (chiasm above the sella turcica), around the cerebral peduncle (optic tract) to the thalamus (LGN), then through the temporal and parietal white matter to the occipital lobe.

Parts & subdivisions

Optic nerve (CN II)
Lesion → ipsilateral monocular blindness (or central scotoma in optic neuritis) with RAPD.
Optic chiasm
Midline compression from below (pituitary macroadenoma) → bitemporal hemianopia, often starting in the superior quadrants; from above (craniopharyngioma, often in children) → starting inferiorly.
Optic tract
Lesion → contralateral homonymous hemianopia (incongruous), may have contralateral RAPD.
Lateral geniculate nucleus
Thalamic relay with 6 layers (1–2 magno, 3–6 parvo); contralateral eye to 1, 4, 6. Lesions → homonymous field defects (sectoranopias).
Meyer loop (temporal radiation)
Lesion (temporal lobe tumour, stroke, temporal lobectomy) → contralateral superior homonymous quadrantanopia — "pie in the sky".
Parietal radiation
Lesion → contralateral inferior homonymous quadrantanopia — "pie on the floor".
Primary visual cortex (V1, area 17)
Lesion (posterior cerebral artery infarct) → contralateral homonymous hemianopia with macular sparing (dual PCA/MCA supply of the occipital pole). Bilateral lesions → cortical blindness (Anton syndrome if denied).

Anatomy

Arterial supply: Optic nerve — ophthalmic artery branches and pial plexus; chiasm — superior hypophyseal arteries and circle of Willis branches; optic tract — anterior choroidal artery; LGN — anterior and posterior choroidal arteries; optic radiation — MCA and PCA branches; V1 — calcarine branch of the PCA (with MCA collaterals at the occipital pole).
Relations: Chiasm: above the pituitary gland and diaphragma sellae, below the hypothalamus and third ventricle floor, internal carotid arteries lateral, anterior cerebral and anterior communicating arteries above/anterior.

Function

Convey retinal image information to the cortex for conscious vision
Segregate parallel channels (magno-, parvo-, koniocellular) for motion, form and colour
Combine input from both eyes for binocular vision and stereopsis (ocular dominance columns in V1)
Supply non-image-forming targets: pretectum (pupils), superior colliculus (orienting), SCN (circadian)

Clinical significance

Localising visual field defects

Monocular defect → lesion anterior to the chiasm. Bitemporal → chiasm. Homonymous (same side in both eyes) → retrochiasmal (tract, LGN, radiation, cortex): the more posterior the lesion, the more congruous the defect. Quadrantanopia localises to temporal (superior) or parietal (inferior) radiation.

Pituitary adenoma

Gradual bitemporal hemianopia (patients bump into things at the side or miss road signs), sometimes with endocrine features (acromegaly, prolactinoma, Cushing disease). Pituitary apoplexy causes sudden headache, visual loss and ophthalmoplegia.

Optic neuritis and multiple sclerosis

Inflammatory demyelination of the optic nerve is the presenting feature of MS in ≈20%. Visual evoked potentials show delayed P100 latency.

Mnemonics

LGN layers — "See I? I see. See I?"

Layers 1–6 receive Contralateral, Ipsilateral, Ipsilateral, Contralateral, Ipsilateral, Contralateral eye input (C-I-I-C-I-C).

Meyer loop

"PITS" — Parietal = Inferior quadrant, Temporal = Superior quadrant (contralateral, homonymous).

Test yourself

A pituitary macroadenoma compressing the optic chiasm from below classically produces which visual field defect?

Left homonymous hemianopia with macular sparing
Bitemporal hemianopia, often beginning in the superior quadrants
Right superior homonymous quadrantanopia
Monocular blindness of the right eye

Answer: Bitemporal hemianopia, often beginning in the superior quadrants

The crossing nasal-retina fibres (which see the temporal fields) are compressed at the chiasm. Inferior nasal fibres lie lowest in the chiasm, so compression from below affects the superior temporal fields first.

A temporal lobe lesion damaging Meyer loop on the left side produces which defect?

Right superior homonymous quadrantanopia
Right inferior homonymous quadrantanopia
Left superior homonymous quadrantanopia
Bitemporal hemianopia

Answer: Right superior homonymous quadrantanopia

Meyer loop carries fibres from the inferior retina (superior visual field) through the temporal lobe. A left-sided lesion therefore causes a right (contralateral) superior quadrantanopia — "pie in the sky". Parietal radiation lesions give inferior quadrantanopias.