The visual pathway, pupillary reflexes and eye movements !
- Concept
- Exam priority: medium
Signals go from the eye to the back of the brain; where the path is damaged decides which part of the view is lost.
Updated 2026-11-01
Also called: Visual field defects, Light reflex
See it in 3D: Optic nerve · Optic chiasm · Geniculate bodies · Visual cortex · Oculomotor nerve (III) · Abducens nerve (VI) · Trochlear nerve (IV)
What it is
Ganglion cell axons form the optic nerve; at the chiasm the fibres from the nasal retina cross, so each optic tract carries the opposite visual field. The tract reaches the lateral geniculate body, and the optic radiations run to the visual cortex in the occipital lobe. A side branch to the pretectal area drives the pupillary light reflex, and cranial nerves III, IV and VI move the eyes.
Key facts
- Oculomotor nerve (III): medial, superior and inferior rectus, inferior oblique, eyelid elevator and pupil constriction. Trochlear (IV): superior oblique. Abducens (VI): lateral rectus.
- Compressive CN III palsy (posterior communicating aneurysm) dilates the pupil early because parasympathetic fibres run on the surface of the nerve.
- Horner syndrome (sympathetic lesion): ptosis, miosis and loss of sweating on the same side.
- Medial longitudinal fasciculus lesion (internuclear ophthalmoplegia, classic in multiple sclerosis): the adducting eye fails on lateral gaze.
Where the lesion is, what is lost
| Lesion | Field defect | Typical cause |
|---|---|---|
| Right optic nerve | Right eye blind | Optic neuritis, ischemia, compression |
| Optic chiasm (centre) | Bitemporal hemianopia | Pituitary adenoma |
| Right optic tract | Left homonymous hemianopia | Stroke, tumour |
| Right Meyer loop (temporal) | Left upper quadrantanopia | Temporal lobe lesion |
| Right parietal radiations | Left lower quadrantanopia | Parietal lesion |
| Right occipital cortex | Left homonymous hemianopia with macular sparing | Posterior cerebral artery stroke |
Clinical notes
- Bitemporal hemianopia makes people bump into things on both sides; it is a classic sign of a pituitary tumour.
- A blown pupil in a patient with head trauma suggests uncal herniation compressing CN III.
- Amblyopia: if one eye sends a blurred or misaligned image in early childhood, the cortex suppresses it; early detection matters.
Test yourself
Think of the answer, then tap the question to see it.
Why does a pituitary adenoma cause loss of both temporal fields?
It presses the chiasm from below, where crossing fibres from both nasal retinas run; nasal retina sees the temporal field.
Light shone in the left eye constricts both pupils, but light in the right eye constricts neither. Where is the lesion?
In the right optic nerve (afferent limb). The efferent pathways are intact, since both pupils respond to light in the left eye.
Why does an occipital infarct often spare central vision?
The macular cortex at the occipital pole often receives collateral blood from the middle cerebral artery, and macular representation is large.
Why does a compressive oculomotor palsy dilate the pupil, while a diabetic (ischemic) one often spares it?
Pupillomotor parasympathetic fibres lie on the outside of the nerve, so compression hits them first; ischemia damages the core of the nerve.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Optic nerve
- Optic chiasm
- Geniculate bodies
- Visual cortex
- Oculomotor nerve (III)
- Abducens nerve (VI)
- Trochlear nerve (IV)
Related topics
- The retina and phototransduction
- Optics of the eye, accommodation and refractive errors
- Sympathetic and parasympathetic effects on each organ
- The vestibular system, tone and posture
Related conditions
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: 16.3 The Cranial Nerve Exam (OpenStax (Rice University), 2022); Neuroanatomy, Optic Chiasm (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Nucleus Lateral Geniculate (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Cranial Nerve 3 (Oculomotor) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 52). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.