Visual Pathway and Field Defects Made Easy: Five Rules
Last updated: October 2026
Field defects look like a picture quiz to memorise, but the whole pathway follows five rules. Learn them and you can predict the field from the lesion, or the lesion from the field, without a chart.
The five rules
1. In front of the chiasm: one eyeA lesion of the retina or optic nerve affects only that eye. Expect a RAPD, reduced acuity and colour, and a defect that often respects the horizontal meridian.
2. At the chiasm: both temporal fieldsOnly the crossing nasal fibres are hit, so both temporal fields go: bitemporal hemianopia, respecting the vertical meridian.
3. Behind the chiasm: same side of both fieldsEach tract, radiation and cortex carries the opposite half of the world, so the result is a homonymous defect on the side opposite the lesion.
4. The further back, the more alikeFibres from matching points of the two eyes lie further apart in the tract and closer together in the cortex, so tract lesions are incongruous and occipital lesions congruous.
5. Lower fibres go low, and see the skyInferior retinal fibres (upper field) loop through the temporal lobe; superior fibres (lower field) run through the parietal lobe. Temporal = top, parietal = pavement.
Lesion to field: the eight classic sites
| # | Field (as the patient sees it) | Lesion | Defect | Typical cause |
|---|---|---|---|---|
| 1 | Right optic nerve | Blind right eye (or central scotoma); right RAPD | Optic neuritis, NAION, compression, trauma | |
| 2 | Junction of right optic nerve and chiasm | Junctional scotoma: right central loss + left superotemporal defect | Meningioma, pituitary tumour extending forward | |
| 3 | Body of the chiasm | Bitemporal hemianopia | Pituitary adenoma (superior first), craniopharyngioma (inferior first) | |
| 4 | Right optic tract | Left homonymous hemianopia, incongruous; RAPD in the left eye; band atrophy of the left disc | Tumour, aneurysm, demyelination | |
| 5 | Right temporal lobe (Meyer's loop) | Left superior homonymous quadrantanopia, "pie in the sky" | Temporal lobe tumour, temporal lobectomy | |
| 6 | Right parietal radiation | Left inferior homonymous quadrantanopia, "pie on the floor" | MCA stroke, tumour | |
| 7 | Right occipital cortex | Left homonymous hemianopia, congruous, macular sparing | PCA stroke | |
| 8 | Right occipital pole (tip) | Left homonymous central scotomas, congruous | Watershed infarct, trauma to the occiput |
Fields are drawn as the patient sees them: left eye on the left, the temporal field of each eye on the outside. Black = lost.
The meridian rule: the quickest localising clue
| Defect respects the... | Think of | Examples |
|---|---|---|
| Horizontal meridian | Retina or optic nerve (vessels and nerve fibres are arranged above and below the raphe) | Altitudinal defect in NAION, BRAO, glaucomatous nasal step and arcuate scotoma |
| Vertical meridian | Chiasm or anything behind it (crossed and uncrossed fibres separate at the vertical line through fixation) | Bitemporal and homonymous hemianopias, quadrantanopias |
Memory hook: Horizontal = Head of the nerve (or retina); Vertical = Very far back (chiasm and beyond).
Optic nerve patterns at a glance
| Pattern | Usual cause |
|---|---|
| Central or centrocaecal scotoma | Optic neuritis, toxic or nutritional neuropathy, LHON |
| Altitudinal (usually inferior) | NAION, giant cell arteritis |
| Arcuate, nasal step, paracentral | Glaucoma, optic disc drusen |
| Enlarged blind spot | Papilloedema, peripapillary atrophy, AZOOR/MEWDS |
| Generalised constriction | Advanced glaucoma, retinitis pigmentosa, functional loss (tubular, non-expanding) |
Chiasm details examiners love
- Direction of growth decides which quadrant goes first. A pituitary adenoma pushes up from below, so the superior temporal quadrants go first; a craniopharyngioma presses from above, so the inferior temporal quadrants go first.
- Junctional scotoma: a lesion where one optic nerve joins the chiasm gives central loss in that eye plus a superotemporal defect in the other. The classic explanation is Wilbrand's knee (crossing inferonasal fibres looping into the opposite nerve). Its anatomy is debated, but the clinical sign is real: always test the "good" eye's temporal field in a unilateral optic neuropathy.
- Prefixed and postfixed chiasm: a prefixed chiasm (over the tuberculum) means a pituitary tumour reaches the tracts first; a postfixed chiasm (over the dorsum) means it hits the optic nerves first.
- Bitemporal defects that cross the vertical meridian are not chiasmal: think tilted discs, which give a pseudo-bitemporal defect.
Behind the chiasm: the extra clues
- Optic tract: incongruous hemianopia + RAPD in the eye with the temporal field loss (the opposite eye, because the temporal field is bigger) + band or bow-tie atrophy of that disc.
- LGN (dual blood supply): lateral choroidal artery infarct gives a horizontal wedge-shaped sectoranopia; anterior choroidal artery infarct gives the reverse (upper and lower sectors lost, horizontal wedge spared).
- Macular sparing in occipital stroke: the occipital pole has a dual supply (posterior and middle cerebral arteries) and a large macular representation.
- Temporal crescent: the most anterior part of the occipital cortex serves the outermost temporal field (beyond about 60°) of the opposite eye, which only that eye can see. A lesion here gives the only monocular retrochiasmal defect; sparing it is equally localising.
- Both occipital lobes: cortical blindness with normal pupils. Denying the blindness is Anton syndrome; seeing only moving objects is the Riddoch phenomenon.
- Retrochiasmal lesions do not reduce acuity unless both sides are involved, and there is no RAPD (except in tract lesions), because the pupil fibres leave before the LGN.
Where residents slip
- Calling an incongruous homonymous defect occipital: incongruity points forwards, to the tract.
- Forgetting to look for a RAPD in hemianopia: present only with a tract lesion, in the eye with the temporal loss.
- Missing the junctional scotoma: in every unilateral optic neuropathy, check the fellow eye's upper temporal field.
- Reading a pseudo-bitemporal defect from tilted discs as chiasmal: true chiasmal loss stops sharply at the vertical meridian.
- Ordering a scan only of the orbits for a homonymous defect: the lesion is in the brain, so image the brain.
Test yourself: flip cards
Quick quiz
Related: Visual field interpretation reference, Neuro-ophthalmology hub, Pupils made easy, Optic neuritis vs NAION vs AAION.
Key sources
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 5: Neuro-Ophthalmology (current edition)
- Miller NR, Newman NJ, et al. Walsh and Hoyt's Clinical Neuro-Ophthalmology: The Essentials, 3rd ed. Wolters Kluwer; 2016
- Horton JC. Wilbrand's knee of the primate optic chiasm is an artefact of monocular enucleation. Trans Am Ophthalmol Soc. 1997;95:579-609
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