The visual pathway begins in the retina and passes through the optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, optic radiation, and visual cortex before terminating in the occipital lobe. Lesions in different parts of the visual pathway can cause different visual field defects, such as hemianopia or quadrantanopia. The more posterior the lesion, the more likely the visual field defects will be congruous between the two eyes.

1. By
Mutahir Shah
Resident M Phil VS
2nd Semester
Pakistan Institute of Community
Ophthalmology
Visual Pathway and its Defects

2. Brief Anatomy of Visual Pathway
 Afferent Visual Pathways:-
 It is important to recognize that any disturbance in
afferent function may result in the same symptoms of
vision loss as observed with pathology affecting the
retina, optic nerve,and visual pathway.
 Retina
 The afferent visual pathway begins within the retina.
 Optic Nerve
 The optic nerve begins anatomically at the optic
disc but physiologically and functionally within the
ganglion cell layer that covers the entire retina.
 The first portion of the optic nerve, representing
the confluence of approximately 1.0- 1.2 million
ganglion cell axons.

3.  The combination of small channels and a unique blood
supply (largely from branches of the posterior ciliary
arteries) probably plays a role in several optic
neuropathies.
 Retinal fibers enter
optic discs in a
specific manner.
 Nerve fiber
bundle (NFB)
defects are of the
following:
1. Papillomacular
bundle.
2. Sup. & Inf. Arcuate
bundle.
3. Nasal bundle.

4. PAPILLOMACULAR BUNDLE-
DEFECTS:

5. Optic Chiasm
 The optic chiasm measures approximately 12 mm wide,
8 mm long in the anteroposterior direction, and 4 mm
thick.
 Within the chiasm, the fibers coming from the nasal
retina (approximately 53% of total fibers) cross to the
opposite side to join the corresponding contralateral
fibers.
 Extramacular superonasal fibers cross directly to
the opposite tract.
 Extramacular temporal fibers remain uncrossed
in the chiasm and optic tract.
 The macular projections are located centrally in
the optic nerve and constitute 80%-90% of the
total volume of the optic nerve and the chiasma!

6. The extramacular fibers from the inferonasal
retina cross anteriorly in the chiasm at the
"Wilbrand knee" before passing into the optic
tract.

7.  Optic Tract
 Starting from the posterior part of the chiasma upto
the LGB.
 Fibers (both crossed and uncrossed) from the upper
retinal projections travel medially in the optic tract;
lower projections move laterally.
 The macular fibers adopt a dorsolateral orientation as
they course toward the lateral geniculate body
 All retrochiasmatic lesions result in a contralateral
homonymous hemianopia.
 Optic tract lesions tend to produce markedly
incongruous field defect.

8. Lateral Ganiculate Body or
Nucleus
 The LGN is a peaked, mushroom-shaped
structure that is divided into 6 levels.
 The 4 superior levels are the termini of P-cell
axons, which are the ganglion cells with smaller
receptive fields and are responsible for mediating
maximal spatial resolution and color perception.
 The 2 inferior layers receive input from the M-cell
fibers, which are the ganglion cells with larger
receptive fields and are more sensitive to
detecting motion.
 Axons originating in the contralateral eye
terminate in layers 1, 4, and 6; the ipsilateral
fibers innervate 2, 3, and 5.

9.  As the fibers approach the LGN, the superior fibers
move superomedially and the inferior fibers swing
inferolaterally.
 Overall, the retinal representation rotates almost 90°,
with the superior fibers moving medially and the inferior
fibers laterally.
 The macular fibers tend to move super laterally
 Macular vision is subserved by the hilum and peripheral field
by the medial and lateral horns.

10. LGN field defects:
1. Incongruous homonymous hemianopia.
2. Unique sector & sector-sparing defects due to dual
blood supply of LGN from anterior & posterior
choroidal arteries.

11. Optic Radiation
 Following a synapse in the LGN, the axons travel
posteriorly as the optic radiations toterminate in
the primary visual (calcarine) cortex in the
occipital lobe.
 The most inferior of the fibers first travel
anteriorly, then laterally and posteriorly to loop
around the temporal horn of the lateral ventricles
(Meyer loop) .
 More superiorly, the fibers travel posteriorly
through the deep white matter of the parietal lobe.
 The macular (central) fibers course laterally, with
the peripheral fibers concentrated more at the
superior and inferior aspects of the radiations.

12. Right superior quadrantanopia >> temporal lobe lesion
Left inferior quadrantanopia >> parietal lobe lesion

13. Visual Cortex
 The visual cortex, the thinnest area of the human
cerebral cortex, has 6 cellular layers
 It occupies the superior and inferior lips of the
calcarine fissure on the posterior and medial surfaces
of the occipital lobes.
 Macular function is extremely well represented in the
visual cortex and occupies the most posterior position
at the tip of the occipital lobe.
 The most anterior portion of the calcarine fissure is
occupied by contralateral nasal retinal fibers only.
 The visual cortex usually have dual blood supply.
 The ‘macular’ visual cortex is supplied by terminal
branches of posterior & middle cerebral arteries.

14. A lesion affecting the tip of the occipital lobe tends to produce a
central homonymous hemianopia
Left homonymous hemianopia with macular sparing

16. Related terms used in Visual Field defects
 Scotoma –
 It is a defect in Visual Field surrounded by normal
visual field.
 Relative scotoma - an area where objects of low
luminance cannot be seen but larger or brighter ones
can.
 Absolute scotoma - nothing can be seen at all within
that area.
 Hemianopia - binocular visual defect in each eye's
hemifield.
 Bitemporal hemianopia –
 the two halves lost are on the outside of each eye's
peripheral vision, effectively creating a central visual
tunnel.

17.  Altitudinal hemianopia - refers to the dividing line
between loss and sight being horizontal rather than
vertical, with visual loss either above or below the line.
 Quadrantanopia - is an incomplete hemianopia
referring to a quarter of the schematic 'pie' of visual
field loss.
 Sectoral defect - is also an incomplete hemianopia
 Congruous: when the defect is not complete (does
not occupy the entire half of the field) & the defect
extends to the same angular meridian in both eyes

18.  Congruity describes incomplete homonymous
hemianopic defects that are identical in all attributes:
location, shape, size, depth, slope of margins.
 Remember: the more posterior ‘toward the occipital
cortex’ the lesion in the postchiasmal visual
pathways, the more likely the defects will be
congruous.

21. b. Anterior parietal
radiation les ion
a.Temporal radiation
lesion
c. Main radiation lesion
d. Anterior visual cortex les
e. Macular cortex
lesion
Temporal horn of
lateral ventricle
Lateral geniculate body
Calcarine fissure
a
b
c
d
e