Visual System
Overview of Pathways – Functional Anatomy
• Another sensory system, like touch and pain.
• Therefore, organized in similar way:
thalamus
sensory system (retina) ------- 1° visual cortex.
As in other sensory systems, projections to thalamus
and visual cortex is very topographically
organized.
Neural pathway
• The right visual
field maps on the
left visual cortex
and vice versa
Figure 10.36
Neural pathway – Regional Anatomy of the Visual
System
A. Retina
1. Retina regions
2. Retina layers and cell types.
B. Optic nerve and optic chiasm.
C. Superior Colliculus.
D. Lateral geniculate nucleus (of the
thalamus).
E. Optic radiations and projections to
the 1° visual cortex.
1. magnocellular/parvocellular
projections.
2. Column Organization of the 1°
visual cortex.
a. ocular dominance columns.
b. orientation columns.
c. color blobs.
F. Inputs to higher-order visual cortical
areas.
Photoreceptor cells
Inter-neurons
Ganglion cells
Optic nerve
Optic chiasm
Optic tract
LGN
Brachium of
superior colliculus
Optic
radiations
1° visual
Cortex
(calcarine fissure)
Superior
colliculus
2 Primary Visual Paths (See Fig. 7-1)
Pathway for Visual Perception: (Fig. 7-1A)
•Inputs to the 1° visual cortex
serve basic visual perception.
•After this initial input, pathways
may then ascend to 2° and higherorder cortical areas in
surrounding parts of the occipital
lobe.
•These other parts serve other aspects
of vision, such as colour vision
and visual motion.
•A decussating path (through corpus
callosum) terminates on contralateral 1°
cortex (unify images from 2 eyes).
•A descending path goes to
oculomotor centres in midbrain, in order
to focus visual image on the retina.
Pathway for Control of Eye Movement (Fig. 7-1B):
For control of eye movements:
Pathway to midbrain.
Superior colliculus: inputs in
voluntary control of saccades
(REMs to salient stimuli) and in
coordinating movements of head
in concert with this.
Pretectal nuclei participate in
pupillary reflexes
Regional Anatomy of the Visual System
A. Retina and optic nerve are derived from out-pocketings of
the diencephalon and are, therefore, part of the CNS, and
not the PNS (in case you were wondering).
B. As seen in Fig. 7-1B, temporal half of each eye projects
ipsilaterally, where as the nasal half projects
contralaterally.
C. Optic disk – where axon exits and blood vessels pass – a
“blind spot”.
D. Macula – area of highest visual acuity.
E. Fovea – centre of the macula.
F. Retinal layers and cell types (see preceding and following
slide)
Photoreceptor cells
Inter-neurons
Ganglion cells
Optic nerve
Optic chiasm
Optic tract
LGN
Brachium of
superior colliculus
Optic
radiations
1° visual
Cortex
(calcarine fissure)
Superior
colliculus
2 Primary Visual Paths (See Fig. 7-1)
Regional Anatomy of the Visual System: Retinal structure
• 3 cell layers,
2 synaptic layers:
Outer layer: photoreceptors- rods
(night vision) and cones (for
daylight and colour, densest just
around the fovea)
Middle layer: bipolar neurons:
horizontal (more superficial)
and amacrine (deeper). Both
perform lateral interactions,
which enhance visual contrast.
Inner layer: ganglion cells. Note
that light goes through to
outermost layer  hits
photoreceptor cells.
Retinal Layers:
Cone bipolar cells:
Input from small #
Of cones for high visual
Acuity.
Rod bipolar cells:
Input from several rods
for convergence and
increased sensitivity
at low illumination.
At outermost extreme:
Pigmented epithelium –
Phagocytic role for removing
old photoreceptors.
This function is absent in
retinosa pigmentosum,
which can  detached
retina
Neural processing
• The bipolar neurons and ganglion cells
process the signal
• In the fovea where the acuity is the highest:
1 cone  1 bipolar cell  1 ganglion cell
• At the periphery: many rods  1 bipolar
cell … acuity is much decreased
• Other cells in the retina participate in signal
processing
Fig. 7-3A
Concentration of rods and cones
Optic Nerve and Optic Chiasm
•
•
•
•
Ganglion cell axons (which are clear and
unmyelinated, while running along the inner
surface of the retina) gather together and exit at
the optic disk, where they become myelinated
and form the optic nerve (actually, the 2nd cranial
nerve, but since the CNS, officially a tract).
Optic nerves from both eyes converge at optic
chiasm: partial cross-over.
Images in the nasal hemiretina from both sides
cross over (temporal stay ipsilateral).
This allows for complete cross-over of each
visual field (see Fig. 7-3C).
Neural pathway
• The right visual
field maps on the
left visual cortex
and vice versa
Figure 10.36
Fig. 7-3C
Superior colliculus
• As noted above, some optic tract axons do not go
to the LGN of the thalamus, but instead, travel in
the brachium of the superior colliculus.
• Laminated - 1st layer receives direct input from
the retina (brachium). Other layers receive input
from 1st or from somatic sensory (AL system) and
auditory systems.
- sensory info from the different modalities is
lined up in the different layers  output to eye
and neck muscles so that one can properly orient
to the stimulus.
Superior Colliculus
Superior
colliculus
Lateral Geniculate Nucleus
A nucleus in the thalamus, which projects to the 1° visual
cortex and serves visual perception.
A recurrent theme: this nucleus is also laminated (6 layers)
with alternating input from ipslateral and contralateral
retina.
There is also a division between 2 important input systems:
Magnocellular – input from M ganglion cells with wide
dendritic arbours (integrates visual input info from wide
area for motion vision).
Parvocellular – input from P cells (ganglionic cells with
small dendritic arbours) – for discriminative aspects of
visual form and colour.
These systems project to distinct parts of the 1° visual
cortex.
Lateral geniculate nucleus
Optic Radiations and Projections to the
Primary Visual Cortex
• Meyer’s Loop: course anterior for a short
distance in order to move over the lateral
ventricles.
• 1° cortex, in columnar fissure, is also
Brodman’s Area 17.
• Densest projection to the 1° visual cortex
(like other sensory) is to Layer 4 (Stripe of
Gennari).
Optic radiations
Stripe of Gennari in area 17
(Fig. 7-10A)
Inputs to primary visual cortex
Parvocellular
Magnocellular
Note: both have distance
Projections and destinations.
These, in turn, project to higherorder areas for visual processing.
Columnar Organization
Neurons lined up with similar properties
across the different layers.
e.g., somatic sensory cortex: columns from
same location in body and same modality.
In visual system, 3 types of aggregates (2
transverse the whole thickness of cortex
and, therefore, are considered columns:
a. Ocular dominance columns.
b.Orientation columns.
c. Aggregates of colour-sensitive neurons
(“colour blobs”).
Ocular dominance columns: alternating input
from each eye
Orientation columns: inputs with similar
spatial orientation
“Colour blobs”: aggregates of coloursensitive neurons
(layers 2 and 3 only)
Inputs to Higher-Order Visual Cortical
Areas
• Higher-order areas surround area 17 (areas 18 and 19), and
can be distinguished as they lack a Stripe of Gennari.
1. Motion pathway – Magnocellular system  1° cortex 
higher-order areas for visual form in motion (including
medial temporal projection).
2. Colour pathway – parvocellular system  1° cortical
projections in 4Cβ  colour blobs  higher-order areas.
3. Form visual pathway – parvocellular system  4Cβ 
region between blobs  higher-order cortex and inferior
temporal lobe.
Two “streams” of projections outside the visual system:
- ventral: features “what”
- dorsal: spatial information: “where”
Higher order cortical areas
Visual Field Deficits after Lesions at Various
Levels of the Visual System
A. Optic n.: transection = monocular blindness.
B. Optic chiasm: (e.g., pituitary tumour). Transection 
bitemporal visual field deficit.
C. Optic tract or LGN: transection  contralateral visual
field deficits (homonymous hemianopsis).
D. Optic radiations: transection of Meyer’s Loop only
(temporal region)  contralateral upper quadrant
(‘quadranotopia’). Complete transection 
homonymous hemianopsia.
E. 1° Visual Cortex: most common = infarction 
homonymous hemianopsia with macular sparing (2°
greater cortical representation).
Visual field lesions and defects
Neural pathway – Regional Anatomy of the Visual
System
• What will happen if the left
optic nerve is severed?
• What will happen if a person
has a tumor in the pituitary
gland (just below the optic
chiasmata) and the inner fibers
are destroyed?
• What will happen if a person
suffers a brain tumor on the
right side of the brain around
the lateral geniculate body?