The visual pathway conveys visual signals from the retina to the visual cortex. Light is detected by photoreceptors (rods and cones) and converted to electrical signals. These signals pass through bipolar and ganglion cells in the retina and then through the optic nerve, optic chiasm, optic tracts and lateral geniculate body to the visual cortex via the optic radiations. Lesions along this pathway can cause different visual field defects including hemianopia and quadrantic defects depending on the location of the lesion. Common causes of lesions include tumors, vascular abnormalities and infections.

1. Visual Pathway

2. Anatomy Of Visual Pathway
 Visual pathway conveys visual (light) impulses
from the retina to the visual cortex in the
occipital lobe.
 Visual impulses are generated in rods & cones,
acts as transducers converting light signals to
electrical signals .
 Rods and cones of retina acts as
photoreceptors

3.  Rods concerned with scotopic vision (dim light)
numerous at ora serrata (120 million)
 Cones concerned with photopic vision (colour
vision) numerous at fovea centralis (7 million)
 Both rods and cones absent at optic disc

4.  First order neuron –
bipolar cells of retina
 Second order neuron -
ganglion cells of retina
 3rd order neuron – neurons
in the 6 layers of L G B.

5. VISUAL PATH
Optic nerve
↓
optic chiasma
↓
optic tract
↓
Lateral geniculate body
↓
optic radiation
↓
visual cortex

8. Optic Nerve
 4 parts –
1. Intra ocular portion ( 1mm )
i. Surface nerve fiber layer
ii. Prelaminar region
iii. Lamina cribrosa
iv. Retrolaminar region
2. Intra orbital portion ( 25-30mm )
◦ Closely surrounded by origin of rectus muscles.
3. Intra canalicular part ( 5-9mm )
◦ Sphenoid & ethmoid sinuses medial to it.
4. Intra cranial part ( 10-16mm )

9.  Consists of approx. 1.2 million axons.
 80 % of fibres originate from macular region.
 No neurilemma – no regeneration.
 Acquire myelin sheaths proximal to lamina
cribrosa.
 Covered by meningeal sheaths.

10.  Fibres from the peripheral parts of retina →
periphery of optic nerve.
 From parts of retina near optic disc → central
area of nerve.
 Fibres from macula ( Papillomacular bundle ) -
◦ Enter nerve on outer aspect , apex towards centre
of nerve.
◦ Become centrally – posterior part.
 Fibers from the 4 quadrants of retina run in
the same relative positions of optic nerve

12. Optic Chiasma
 Flattened structure , 12mm horizontally & 8mm
anteroposteriorly.
 Lies over tuberculum & diaphragma sella.
 Nasal fibres decussate.
 Von Willebrand knee – fibres from inferonasal
retina of other side which loop forward slightly
into opposite optic nerve after crossing over.

13. Optic tracts
 Starts at the posterolateral end of optic chiasma
 Optic tract winds round the upper part of cerebral
peduncle to reach the LGB.
 Pupillary reflex fibres pass onto pretectal nucleus.
 Some fibers terminate in superior colliculus.
 Mode of termination of fibers of optic tract
supr quadr of retina – lateral part of L G B
infr quad of retina – medial part of L G B
Macular fibres – central & posterior part of
LGB

14. Lateral Geniculate Body
 Seen as small ovoid projection at posterolateral part of
thalamus.
 It is the thalamic relay station for visual impulses.
 Grey matter split to form 6 lamina.
 1& 2 lamina larger cells – magnocellular ( motion)
 3 to 6 -smaller neurons –parvocellular ( colour, fine
details,shape )
 Crossed fibers of optic tract make synapse with cells in
1,4,and 6 layers.
 Uncrossed/ fibres of same side with 2,3,and 5.
 Fibers from fovea centralis has precise point to point

15. Optic radiations
 Extend from LGB to visual cortex ( Geniculo-
calcarine tract ).
 Pass thru the retrolentiform part of internal capsule.
 Ventral fibres ( lower quadrant of retina) run
forwards into temporal lobe – Meyers Loop.
 Dorsal fibres ( upper retinal quadrant ) run directly
thru parietal lobe into visual cortex.
 Pass close to posterior cornu of lateral ventricle.

17. Visual cortex
 Medial aspect of occipital lobe, in & about the
calcarine fissure.
 Line of Gennari – interpolated in grey matter.
 Visuosensory area - striate area 17,
Visuopsychic area – peristriate area 18 &
parastriate area 19.
 Rt ½ of field of vision is represented in the
visual cortex of Lt hemisphere & vice versa

18.  Spatial arrangement maintained-
◦ Parts above & below calcarine fissure
represent upper & lower quadrants of
both retinae.
◦ Peripheral retinal fibers end in antr part of
visual area
◦ Macular fibers postrly

19. Blood supply of visual
pathway
 Mainly supplied by pial network of vessels.
 Orbital part of optic nerve also supplied by an
axial system – central artery of retina.
 Blood supply of optic nerve head –
i. Surface layer – capillaries from retinal arterioles.
ii. Prelaminar region – centripetal branches of
peripapillary choroid + vessels of lamina cribrosa.
iii. Lamina cribrosa – posterior ciliary A & arterial circle
of Zinn.
iv. Retrolaminar part – centrifugal branches from CRA
& centripetal br from pial plexus.

20. Lesions Of Visual
Pathway

21. Lesions of optic nerve
 One optic nerve damaged → loss of vision on
affected side, loss of ipsilateral direct &
contralateral consensual pupillary reflexes.
 Lesion in proximal part of optic nerve near chiasma
→ ipsilateral blindness & contralateral
superotemporal field defect – Traquair Junctional
scotoma.
 Due to involvement of crossed fibres of opp side as
they loop forward into the nerve.

22.  Causes of optic nerve lesions –
◦ Optic atrophy
◦ Traumatic avulsion of optic nerve
◦ Indirect optic neuropathy
◦ Acute optic neuritis
◦ ischemia

23. Lesions of optic chiasma
Sagittal lesions of chiasma :-
 Bitemporal heteronymous hemianopia & bitemporal
hemianopic paralysis of pupillary reflexes.
 Lead to partial descending optic atrophy.
◦ pallor of optic disc
 Common causes –
◦ Tumours of pituitary gland
◦ Suprasellar tumours – craniopharyngiomata , suprasellar
meningiomata
◦ Gliomas of third ventricle, 3rd ventricular dilatation
◦ Chronic chiasmal arachnoiditis.
◦ Ectopic pinealomas, dermoid tumours.

24.  Pattern of visual defect varies.
 As one side is compressed before the other,
earliest defect – unilateral central scotoma.
◦ Involvement of von Willebrand knee.
 Altitudinal hemianopia
◦ Usually loss of upper halves of field – intra/ extra
sellar tumours.
◦ Rarely loss of lower halves of field – suprasellar
tumour.

25. Lateral lesions of chiasma :-
 Binasal hemianopia, with binasal paralysis of
pupillary reflexes.
 Partial descending optic atrophy.
 Common causes –
◦ Distension of 3rd ventricle
◦ Atheroma of carotids or posterior communicating
arteries

26. Lesions of Optic tract
 Incongruous Homonymous hemianopia
◦ As arrangement of nerve fibres in the tract is not regular.
 Contralateral hemianopic pupillary reaction (
Wernicke’s reaction).
 Partial descending optic atrophy.
◦ Atrophy of crossed retinal fibres – bow tie optic atrophy.
 Association with 3rd nerve palsy & hemiplegia.

27.  Causes –
◦ Syphilitic meningitis or gumma & tuberculous.
◦ Tumours of optic thalamus
◦ Aneurysms of superior cerebellar or posterior
cerebral arteries.
◦ Tentorial meningiomas

28. Lesions of Optic radiations
 Involvement of total optic radiation → congruous
homonymous hemianopia.
 Lesions of Meyers loop ( inferior fibres) → superior
homonymous quadrantic defects – Pie in the sky on the
opp side.
 Lesions of temporal lobe → complete superior
homonymous quadrantanopia.
 Lesions of parietal lobe (superior fibres) → inferior
quadrantic hemianopia – Pie on the floor.
 No optic atrophy.

29. Left homonymous hemianopia

30. Pie in the floor Pie in the sky

31. Lesions of Visual Cortex
 Congruous Homonymous hemianopia , usually
sparing macular area.
 Pupillary reflexes normal, no optic atrophy.
 Causes –
◦ Injury by fall on the back of head
◦ Gunshot wounds
◦ Cerebral softening
 When angular gyrus involved → word
blindness.

32.  Riddoch Phenomenon –
◦ Appreciation of a dim kinetic target is retained
within the defective visual field, with loss of
appreciation of a static bright target.
◦ Typical of an occipital lesion.
 If lesion is more anteriorly in the occipital
cortex,
Contralateral temporal crescentic field
defect