Visual processing in the thalamus and cortex: LGN, V1 and the two streams

Concept
Exam priority: medium

The eye's signal goes through a relay in the thalamus to the back of the brain, which splits it into 'where is it moving' and 'what is it'.

Updated Nov 13, 2026

Also called: Lateral geniculate nucleus, Primary visual cortex (V1), Dorsal and ventral streams

See it in 3D: Geniculate bodies · Visual cortex · Optic chiasm · Posterior cerebral artery · Thalamus · Midbrain

Key points

Side routes: retinohypothalamic tract → suprachiasmatic nucleus (circadian rhythm); pretectum → pupillary light reflex; superior colliculus → reflex orienting of eyes and head; pulvinar → attention.
LGN has 6 layers: 1–2 magnocellular (motion, contrast), 3–6 parvocellular (detail, red–green); the contralateral eye feeds layers 1, 4, 6.
Eyes stay segregated in the LGN and in V1 layer 4 (ocular dominance columns); they combine only above layer 4.
The macula takes a disproportionately large share of V1 (posterior pole), which is why macular sparing occurs in posterior cerebral artery strokes with collateral flow from the middle cerebral artery.
Dorsal stream (V5/MT, parietal) lesion: loss of motion perception (akinetopsia); ventral stream (V4, fusiform) lesion: cerebral achromatopsia, prosopagnosia.

What it is

Most optic tract fibres relay in the lateral geniculate nucleus (LGN) of the thalamus and project to the primary visual cortex (V1) around the calcarine sulcus. V1 breaks the image into edges, orientations, colour and depth, and hands this to higher areas along two streams: a dorsal stream to the parietal lobe for motion and position, and a ventral stream to the temporal lobe for detail, colour and object identity. Smaller side routes serve the body clock, the pupils and reflex eye movements.

Key facts

The LGN is not a passive relay: cortical feedback and brainstem arousal inputs gate how much passes through.
V1 (Brodmann 17, striate cortex) has six layers; LGN axons end in layer 4C (magno in 4Cα, parvo in 4Cβ).
Columns in V1 group neurons with the same preferred orientation; colour-sensitive 'blobs' sit in layers 2–3.
Simple cells respond to a bar or edge of one orientation at one position; complex cells respond to that orientation anywhere in a larger field, and many prefer a direction of motion.
Secondary areas (V2, V3, V4, V5/MT, inferotemporal cortex) build up shape, colour constancy, motion and faces.
Destroying V1 on both sides causes cortical blindness with preserved pupillary reflexes; some patients still point to targets they deny seeing (blindsight).

Two streams of visual processing

Feature	Dorsal ('where/how')	Ventral ('what')
Main input	Magnocellular	Parvocellular and koniocellular
Path	V1 → V2 → V5/MT → posterior parietal cortex	V1 → V2 → V4 → inferotemporal cortex (fusiform)
Codes	Motion, depth, position, guiding reach and eye movements	Fine detail, colour, form, faces and objects
Lesion	Akinetopsia, optic ataxia, simultanagnosia	Achromatopsia, visual agnosia, prosopagnosia

Clinical notes

Posterior cerebral artery infarct: contralateral homonymous hemianopia, often with macular sparing.
Bilateral occipital infarcts may cause cortical blindness; denying the blindness is Anton syndrome.
Amblyopia from untreated strabismus is a cortical problem: the deprived eye loses its share of V1 during the critical period.

Test yourself

Think of the answer, then tap the question to see it.

A patient with a left posterior cerebral artery stroke has a right homonymous hemianopia but can read with central vision. Why?

Macular sparing: the macular part of V1 at the occipital pole often gets collateral flow from the middle cerebral artery and survives.

Which LGN layers receive input from the eye on the same side?

Layers 2, 3 and 5; layers 1, 4 and 6 receive the contralateral eye.

How do simple and complex cells in V1 differ?

Simple cells fire for an edge of one orientation at a fixed position; complex cells keep responding to that orientation as it moves within a wider field.

Which visual pathway keeps sleep–wake timing in step with daylight?

The retinohypothalamic tract from melanopsin ganglion cells to the suprachiasmatic nucleus, which sets melatonin release from the pineal gland.

A patient after bilateral occipital strokes has normal pupillary light reflexes but no vision. How can the reflexes be intact?

The pupillary reflex uses the pretectal route in the midbrain, which leaves the optic tract before the LGN and V1.

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The structures of Durgical's 3D anatomy where it happens.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: Neuroanatomy, Visual Cortex (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Nucleus Lateral Geniculate (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Posterior Cerebral Arteries (StatPearls (NCBI Bookshelf), 2024).

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