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USMLE Step 1Neuroanatomy and lesion localisationEP 10 of 16

Visual pathway lesions and field defects

Predict the visual field defect from the site of the lesion. Before the chiasm, one eye; at the chiasm, both outer fields; after the chiasm, the same side in both eyes, opposite the lesion.

  • 4:42 video
  • 3 min read
  • 12 sources
  • Published 15 Nov 2026
Video lesson · 4:42 · Watch on YouTubeSeries: Neuroanatomy & Lesion Localisation · @medservatory
Chapters (12)
By the end of this lesson

Predict the visual field defect produced by lesions of the optic nerve, optic chiasm, optic tract, Meyer loop, parietal radiations and visual cortex.

The short version

  1. The eye's optics (cornea and lens) invert the image, so the nasal half of each retina sees the temporal half of the visual field.
  2. At the chiasm the nasal-retinal fibres cross; beyond it each optic tract carries the opposite visual field from both eyes.
  3. Before the chiasm, one eye; at the chiasm, bitemporal hemianopia; after the chiasm, homonymous and opposite the lesion.
  4. Meyer loop (temporal) carries the upper field, so a temporal lesion gives a contralateral superior quadrantanopia (pie in the sky); parietal fibres carry the lower field, so a parietal lesion gives a contralateral inferior quadrantanopia (pie in the floor).
  5. A visual cortex lesion gives a homonymous hemianopia, often with macular sparing, and the further back the lesion, the more congruous the defect.

A normal eye chart, but both outer fields lost

A 41-year-old woman keeps scraping both wing mirrors. Each eye reads the chart perfectly, yet she has lost the outer half of her vision in both eyes.

This lesson is about visual pathway lesions: how to predict the visual field defect from the site of the lesion, by walking the pathway and asking at each step what is lost.

The optics and the crossing at the optic chiasm

The eye's optics (cornea and lens) invert the image, so the nasal half of each retina sees the temporal half of the visual field.[1][5]

At the optic chiasm, fibres from the nasal retina cross to the other side, and fibres from the temporal retina stay on the same side.[1][5]

Beyond the chiasm: tract, lateral geniculate body and radiations

The tract fibres synapse in the lateral geniculate body, then fan out as the optic radiations to the visual cortex in the occipital lobe, along the calcarine sulcus.[4]

  1. Retina to chiasmThe optic nerve carries fibres from one eye only.
  2. ChiasmNasal-retinal fibres cross; temporal-retinal fibres stay put.
  3. Optic tractCarries the opposite visual field from both eyes.
  4. Lateral geniculate bodyThe fibres synapse here.
  5. Optic radiationsFan out to the visual cortex in the occipital lobe, along the calcarine sulcus.

The lesion walk: from optic nerve to optic tract

Walk down the pathway and ask at each step what is lost.

Site of lesion and visual field defect
Site of lesionDefect
Optic nerve (before the chiasm)Loss of vision in that one eye[2][7]
Chiasm (midline)Bitemporal hemianopia: the crossing nasal fibres go, so both temporal fields go[2][8]
Optic tract, or the whole radiationHomonymous hemianopia: the same half of the field lost in both eyes, on the side opposite the lesion[2][3][9]

A pituitary tumour is a classic cause of a chiasmal lesion, because it presses on the chiasm in the midline.[8]

Meyer loop and the parietal fibres: pie in the sky, pie in the floor

The radiation then splits. The lower fibres loop through the temporal lobe (Meyer loop) and carry the upper visual field. A temporal lesion cuts them and takes the upper quadrant on the opposite side in both eyes: a contralateral homonymous superior quadrantanopia, pie in the sky.[4][10][11]

The upper fibres run straight back through the parietal lobe and carry the lower field. A parietal lesion takes the lower quadrant on the opposite side in both eyes: a contralateral homonymous inferior quadrantanopia, pie in the floor.[4][6][11]

Where the fibres run and what they carry
FibresRouteField carriedLesion gives
Lower fibres (Meyer loop)Through the temporal lobeUpper fieldSuperior quadrantanopia (pie in the sky)
Upper fibresStraight back through the parietal lobeLower fieldInferior quadrantanopia (pie in the floor)

The visual cortex: macular sparing and congruity

At the visual cortex, a posterior cerebral artery stroke causes a homonymous hemianopia, and the central vision is often spared. This is called macular sparing. The usual explanation is that the occipital pole also receives blood from the middle cerebral artery.[3][9][11][12]

The further back the lesion, the more alike the two eyes' defects: the defect is more congruous.[3][6][11]

Three shortcuts, and back to the patient

  1. Before the chiasm: one eye.
  2. At the chiasm: both outer fields.
  3. After the chiasm: the same side in both eyes, opposite the lesion.

Back to the patient: normal acuity and both temporal fields gone. That is a chiasmal lesion, where the crossing fibres run, and a pituitary tumour pressing on the chiasm fits.[2][8]

Practice question

Original · USMLE style
Your turnQB-MS-S1-NEURO-005-01

A 46-year-old man has had two generalized seizures. MRI shows a tumour confined to the right temporal lobe. His visual acuity is normal.

Which of the following visual field defects is most likely on formal testing?
Pick one option. Your answers stay on this device.

Common questions

What visual field defect does a lesion of the optic chiasm cause?

A midline chiasmal lesion cuts the crossing nasal fibres, so both temporal fields are lost: a bitemporal hemianopia. A pituitary tumour is a classic cause.

What is a homonymous hemianopia?

Loss of the same half of the visual field in both eyes, on the side opposite the lesion. It follows a lesion of the optic tract, of the whole optic radiation or of the visual cortex.

What is pie in the sky?

A contralateral homonymous superior quadrantanopia. The lower fibres of the radiation loop through the temporal lobe as Meyer loop and carry the upper field, so a temporal lesion takes the upper quadrant.

What is pie in the floor?

A contralateral homonymous inferior quadrantanopia. The upper fibres run straight back through the parietal lobe and carry the lower field, so a parietal lesion takes the lower quadrant.

What is macular sparing?

Central vision is often spared in a homonymous hemianopia from a posterior cerebral artery stroke. The usual explanation is that the occipital pole also receives blood from the middle cerebral artery.

Transcript

Read the full narration, by chapter634 words

0:00The case: a normal eye chart, but both outer fields lost

A 41-year-old woman keeps scraping her wing mirrors on both sides of the car. Each eye reads the chart perfectly, but she has lost the outer half of her vision in both eyes. One small lesion, two blind outer fields. Where is it? Today: predict the visual field defect from the site of the lesion.

0:20The optics: the lens inverts the image, and the chiasm crossing

Start with the optics. The lens inverts the image, so the nasal half of each retina sees the temporal half of the visual field. At the optic chiasm, fibres from the nasal retina cross to the other side, and fibres from the temporal retina stay put.

0:37Beyond the chiasm: optic tract, lateral geniculate body, radiations

So beyond the chiasm, each optic tract carries the opposite visual field from both eyes: the right optic tract carries the left field. The fibres synapse in the lateral geniculate body, then fan out as the optic radiations to the visual cortex in the occipital lobe, along the calcarine sulcus.

0:57The lesion walk: optic nerve, then the chiasm (bitemporal hemianopia)

Now walk down the pathway, and ask at each step what is lost. Optic nerve, before the chiasm: loss of vision in that one eye. Chiasm, in the midline: the crossing nasal fibres go, so both temporal fields go. That is a bitemporal hemianopia, and a pituitary tumour is a classic cause.

1:20Optic tract: homonymous hemianopia

Optic tract, or the whole radiation: loss of the same half of the field in both eyes, on the side opposite the lesion. That is a homonymous hemianopia.

1:30Meyer loop: superior quadrantanopia, pie in the sky

The radiation then splits. The lower fibres loop through the temporal lobe, Meyer loop, and carry the upper visual field. A temporal lesion cuts them and takes the upper quadrant: a superior quadrantanopia, pie in the sky.

1:47Parietal fibres: inferior quadrantanopia, pie in the floor

The upper fibres run straight back through the parietal lobe and carry the lower field. A parietal lesion takes the lower quadrant: an inferior quadrantanopia, pie in the floor.

1:59Visual cortex: macular sparing and congruity

At the visual cortex, a posterior cerebral artery stroke causes a homonymous hemianopia, and the central vision is often spared, called macular sparing. The usual explanation is that the occipital pole also receives blood from the middle cerebral artery. And the further back the lesion, the more alike the two eyes' defects: more congruous.

2:22Three shortcuts, and back to our patient

Three shortcuts. Before the chiasm, one eye. At the chiasm, both outer fields. After the chiasm, the same side in both eyes, opposite the lesion. Back to our patient. Normal acuity, and both temporal fields gone. That is a chiasmal lesion, where the crossing fibres run, and a pituitary tumour pressing on the chiasm fits.

2:47Practice question

Your turn. A 46-year-old man has had two seizures. MRI shows a tumour in his right temporal lobe. His visual acuity is normal. Which visual field defect is most likely on formal testing? A, left homonymous superior quadrantanopia. B, left homonymous inferior quadrantanopia. C, right homonymous superior quadrantanopia. D, bitemporal hemianopia. E, left homonymous hemianopia with macular sparing. Pause here and decide.

3:27The options explained, and the answer

B is out: the lower field is carried by the parietal fibres, so an inferior quadrantanopia points to the parietal lobe, not the temporal lobe. C is out: wrong side. A right-sided lesion affects the left visual field in both eyes. D is out: a bitemporal defect comes from a midline chiasmal lesion, not a lesion in one temporal lobe. E is out: a hemianopia with macular sparing suggests the occipital cortex, and a temporal tumour affects only the lower loop of the radiation. So the answer is A. Meyer loop carries the upper field, so a right temporal lesion takes the upper left quadrant of both eyes: pie in the sky.

4:13Recap and next lesson

Three things to keep. One: before the chiasm, one eye; at the chiasm, bitemporal; after it, homonymous and opposite the lesion. Two: temporal loop, pie in the sky; parietal, pie in the floor. Three: the visual cortex gives a hemianopia, often with macular sparing. Next, the basal ganglia and their direct and indirect pathways.

Sources

Numbers in the text, e.g. [1], point here
  1. Optic chiasm. Wikipedia. en.wikipedia.org/wiki/Optic_chiasm
  2. Visual field. Wikipedia. en.wikipedia.org/wiki/Visual_field
  3. Homonymous hemianopsia. Wikipedia. en.wikipedia.org/wiki/Homonymous_hemianopsia
  4. Optic radiation. Wikipedia. en.wikipedia.org/wiki/Optic_radiation
  5. Connecting the retina to the brain. ASN Neuro 2014. europepmc.org/article/PMC/PMC4720220
  6. Homonymous hemianopia: challenges and solutions. Clin Ophthalmol 2014. europepmc.org/article/PMC/PMC4181645
  7. Monocular visual field defect and a highly incongruous homonymous defect. Case Rep Ophthalmol 2021. europepmc.org/article/PMC/PMC8255693
  8. Ophthalmic surveillance in non-functioning pituitary adenomas. BMJ Open 2026. europepmc.org/article/PMC/PMC13684206
  9. Visual field loss after stroke. europepmc.org/article/PMC/PMC3782154
  10. Epilepsy surgery, vision and driving. europepmc.org/article/PMC/PMC4030586
  11. Eyes and stroke: the visual aspects of cerebrovascular disease. Stroke Vasc Neurol 2017. europepmc.org/article/PMC/PMC5829892
  12. Posterior circulation ischaemic stroke diagnosis and management. Clin Med 2023. europepmc.org/article/PMC/PMC11046504

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), "Optical pathways", CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured to the lesson's colour key, outlined, labelled and highlighted by us). Plate: Gray's Anatomy (1918), plate 753 (projection fibres of the cerebral hemisphere), Henry Vandyke Carter, public domain, via Wikimedia Commons; labelled and overdrawn by us. Original diagrams: the field circles, the eye and lens sketch, the lesion marks, the Meyer loop and parietal fibre paths (schematic), the pie sketches, the defect table, the question card and the pocket card. No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-005.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 09Lacunar stroke syndromes: pure motor, pure sensory and more
  1. EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
  2. EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
  3. EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
  4. EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
  5. EP 05Brainstem crossed findings: the localisation rule4:50
  6. EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
  7. EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
  8. EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
  9. EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
  10. EP 10Visual pathway lesions and field defects4:42
  11. EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
  12. EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
  13. EP 13Brain herniation syndromes: what is squeezed, and where4:56
  14. EP 14CSF flow and hydrocephalus: where the flow fails5:12
  15. EP 15Corticospinal tract: from cortex to anterior horn4:59
  16. EP 16Hypothalamic nuclei and their functions5:24