The Pathway
Retina (photoreceptor → bipolar → ganglion cell) → Optic nerve → optic chiasma — nasal fibres decussate → Optic tract → Lateral geniculate body (layers 1–6) → Optic radiation → Visual cortex — area 17 (calcarine sulcus)
- Nasal (medial) fibres cross; temporal (lateral) fibres DO not — the single fact that explains every field defect
- Therefore each optic tract carries the opposite half of the visual field
- Optic radiation — the lower fibres loop forward through the temporal lobe as Meyer's loop and carry the upper quadrant of the field
- Visual association areas 18 and 19 interpret what area 17 receives
Lesions and Their Field Defects
| Site of lesion | Field defect | Typical cause |
|---|
| Optic nerve | Total blindness of that eye; loss of direct light reflex | Optic neuritis, trauma |
| Optic chiasma (central) | Bitemporal hemianopia — crossing nasal fibres damaged | Pituitary tumour, craniopharyngioma |
| Chiasma (lateral) | Ipsilateral nasal hemianopia | Internal carotid aneurysm |
| Optic tract | Contralateral homonymous hemianopia | Stroke, tumour |
| Temporal lobe (Meyer's loop) | Contralateral superior quadrantanopia — "pie in the sky" | Temporal lobe lesion |
| Parietal radiation | Contralateral inferior quadrantanopia | Parietal lesion |
| Occipital cortex | Homonymous hemianopia with macular sparing | Posterior cerebral artery occlusion |
- Macular sparing occurs because the macular area of the occipital pole has a dual blood supply from the middle and posterior cerebral arteries
Pupillary Light Reflex
Light in one eye → Retina → optic nerve → Pretectal nucleus (bypasses the LGB) → both Edinger–Westphal nuclei → Parasympathetic in CN III → ciliary ganglion → Sphincter pupillae contracts — both pupils
| Term | Meaning |
|---|
| Direct reflex | Constriction of the eye that is illuminated |
| Consensual reflex | Constriction of the other eye — because the pretectal nucleus projects bilaterally |
| Afferent | Optic nerve (II) |
| Efferent | Oculomotor nerve (III) |
Localising Value of the Pupillary Reflex
| Lesion | Direct (that eye) | Consensual (other eye) |
|---|
| Optic nerve (afferent) | Lost | Lost when that eye is lit, but preserved when the normal eye is lit |
| Oculomotor nerve (efferent) | Lost | Preserved in the other eye |
- Argyll Robertson pupil — small irregular pupil that accommodates but does not react to light; classical of neurosyphilis
- Marcus Gunn pupil (relative afferent defect) — on swinging the torch, the affected pupil paradoxically dilates
Visual Cortex and Processing
- Area 17 (striate cortex) — primary; retinotopically mapped, with the macula occupying a disproportionately large area at the occipital pole
- Organised in columns — ocular dominance columns and orientation columns (Hubel and Wiesel)
- Simple, complex and hypercomplex cells respond to bars and edges of particular orientation and movement
- Areas 18 and 19 — association; interpretation, depth, colour and movement
- Two streams — the dorsal ("where") stream to the parietal lobe, and the ventral ("what") stream to the temporal lobe
Other Visual Reflexes
| Reflex | Pathway | Function |
|---|
| Accommodation reflex | Cortex → Edinger–Westphal → III | Near vision triad |
| Corneal reflex | V afferent, VII efferent | Blink — protective |
| Dazzle reflex | Subcortical | Blink to sudden bright light |
| Optokinetic nystagmus | Cortex and brainstem | Follows a moving scene |
Binocular Vision and Depth Perception
- Binocular vision requires both foveae to receive corresponding images, which are fused centrally
- Cues to depth — retinal disparity (the chief binocular cue), convergence, and monocular cues such as relative size, overlap, motion parallax and perspective
- Diplopia results when the images fall on non-corresponding points, as in squint or a nerve palsy
- Amblyopia — if a squint is untreated in childhood, the brain suppresses one image and that eye never develops normal vision. Treatment must be within the critical period, before 7–8 years
Applied Aspects
- Pituitary adenoma classically presents with bitemporal hemianopia, because it presses on the crossing nasal fibres from below
- Papilloedema — swollen optic disc from raised intracranial pressure; vision is preserved until late, but the blind spot enlarges
- Third nerve palsy — ptosis, a "down and out" eye, and a fixed dilated pupil; a surgical third nerve palsy (aneurysm, uncal herniation) involves the pupil, a medical one (diabetes) usually spares it
- Cortical blindness — bilateral occipital damage; the patient is blind but pupillary reflexes are intact, since these do not involve the cortex
- Visual field charting is essential in any pituitary or intracranial lesion, and in glaucoma follow-up
- Optic atrophy — pale disc with reduced acuity and colour vision; may follow papilloedema, neuritis or compression
- Homonymous hemianopia means the lesion is behind the chiasma; heteronymous (bitemporal) means it is AT the chiasma
- Retrobulbar neuritis — "the patient sees nothing and the doctor sees nothing"; disc looks normal early