2. Visual Pathway
“Focus on the Eye”
• Concerned about
Cornea
Lens
Retina
There is more to vision than meets
the eye
3.
4. Retinal Fields vs. Visual Fields
What’s the difference
• Retinal Field – describes the area that includes
neural fibers of the retina that are receiving
light from some object
• Visual Field – describes the area in space
where the object lies
They are the reversal of one another
• The nasal retinal field receives light from the
temporal visual field
• The temporal retinal field receives light from
the nasal visual field
5. Temporal Temporal
Visual field Visual Field
Nasal Visual Nasal Visual
Field Field
Temporal Temporal
Retinal field Retinal Field
Nasal Retinal Nasal Retinal
Field Field
6. Optic Chiasm
Partial decussation (cross-over) of
Optic Nerve fibers occurs at the level
of the Optic Chiasm
• Only nasal retinal fibers (from the nasal
retinal field) cross over
• Temporal nasal fibers (from the
temporal retinal field) do not.
10. Optic Tract
Optic tract
It is important for the sense of sight.
By convention, the optic tract is defined as
that extent of the visual system pathway
from the optic chiasm to the lateral
geniculate nucleus of the thalamus.
Each optic tract contains axons from
ganglion cells in the retinas of both the left
and right eyes, but information from only
one half (i.e either left or right) of each
eye's visual field
13. Lateral Geniculate Body
After the optic tract, the next stop is
the Lateral Geniculate Body (or
Lateral Geniculate Nucleus)
14. LGN
• Optic nerve fibers from the optic
tracts terminate at two bodies in the
thalamus (a structure in the middle
of the brain) known as the Lateral
Geniculate Nuclei (or LGN for
short).
• One LGN lies in the left
hemisphere and the other lies in the
right hemisphere.
• Each has six layers
15. The optic tract wraps around the
cerebral peduncles of the
midbrain to get to the lateral
geniculate nucleus (LGN), which
is a part of the thalamic sensory
relay system.
There are two geniculate nuclei,
located on either side of the rear
end of the thalamus. They each
consist of six cellular layers,
forming a threefold
representation of the opposite
binocular visual hemifield in
exact anatomic registration.
16. This apparently
complicated arrangement
is engineered so that the
right LGN receives
information about the left
visual field, and the left
LGN receives information
about the right visual
field.
17. LGN
This layered structure is
exquisitely precise in two
ways.
• First, cells in different layers
that align (like the numbers in
the picture) have receptive
fields in the same area of
retina.
• Second, optic nerve fibers
from the two eyes are
segregated in different layers.
If you look carefully at the
projections to the LGN, you
will see that ipsilateral fibers
arrive in layers 2, 3, and 5,
while contralateral fibers
arrive in layers 1, 4, and 6
(no-one knows why).
18. LGN Cell Types
All cells in the LGN have concentric
receptive fields, just like the ganglion cells
whose fibers terminate in the LGN.
Layers 1 and 2 are made up of cells with
large bodies ("magnocellular") that have
monochromatic responses (ie. mediate
responses to light and dark)
Layers 3 to 6 are made up of cells with
small bodies ("parvocellular") that mediate
color vision.
19. Optic Radiations
Leaving the LGN are optic radiations
Optic radiations are a collection of axons
from relay neurons in the lateral
geniculate nucleus of the thalamus.
They carry visual information to the visual
cortex (also called striate cortex) along
the calcarine fissure.
There is one such tract on each side of the
brain.
20.
21. Meyer’s Loop
The optic radiations follow a very wide three
dimensional arc. Here is how the radiations are
conventionally drawn, and how they look from the
side
The longer loop actually dives into the temporal
lobe before it heads back to the occipital lobe.
This group of fibers is called Meyer's loop. Recall
that, since the lens inverts all images, the lower
half of the retina sees the upper half of the world.
This orientation is preserved through the pathway,
so that the lower optic radiations, or Meyer's loop,
are carrying information from the upper visual
world.
24. Retinotopic Mapping
In lower visual areas (e.g., V1 through V5) the
neurons are organized in an orderly fashion called
topographic or retinotopic mapping
• they form a 2D representation of the visual image
formed on the retina in such a way that neighboring
regions of the image are represented by neighboring
regions of the visual area
But this retinotopic representation in the cortical
areas is distorted.
The foveal area is represented by a relatively
larger area in V1 than the peripheral areas.
25. Visual Cortex
Much of the primate cortex is
devoted to visual processing.
• In the macaque monkey at least 50% of
the neocortex appears to be directly
involved in vision, with over twenty
distinct areas.
• Some of the areas concerned are quite
well understood, others are still a
complete mystery.
26. Visual Cortex
Nearly all visual information reaches the
cortex via V1, the largest and most
important visual cortical area.
Because of its stripey appearance this area
is also known as striate cortex, amongst
other things.
Other areas of visual cortex are known as
extrastriate visual cortex
• the more important areas are V2, V3, V4 and
MT (also known as .....V5!).
28. V1
In primates nearly all visual
information enters the cortex via
area V1. This area is located in the
occipital lobe at the back of the
brain. It is also known as:
- primary visual cortex
- area 17
- striate cortex.
29.
30. V1
This region represents about 5% of
the neocortex in man.
It is the most complex region of the
cortex with at least 6 identifiable
layers
• layer 1 is close to the cortical surface,
layer 6 adjoins the white matter below
33. Simple Cells
Simple cell receptive fields contain sub-regions
that exert an excitatory influence on the cell's
response (light grey in the picture), and sub-
regions that exert an inhibitory influence (dark
grey in the picture). The blue lines in the picture
are time traces that plot the onset and offset of
stimulation. The black vertical lines below them
indicate individual nerve impulses. The most
effective stimulus for this particular receptive
field (left) is one that puts a lot of light in the
excitatory region, and only a little in the
inhibitory region.
35. Simple Cells
It must have the right orientation, the
right position, and the right size. Stimuli
that are non-optimal in terms of position
(middle left), or orientation (middle right),
or size (right) are less effective. Simple
cell receptive fields could be 'built' in the
cortex by collecting responses from LGN
cells whose receptive fields fall along a line
across the retina, but the exact wiring is
still the subject of debate.
36. Complex Cells
Complex cells are the most numerous in
V1 (perhaps making up three-quarters of
the population). Like Simple cells, they
respond only to appropriately oriented
stimuli, but unlike Simple cells, they are
not fussy about the position of the
stimulus, as along as it falls somewhere
inside the receptive field (left and middle-
left examples above). Many complex cells
are also direction-selective, in the sense
that they respond only when the stimulus
moves in one direction and not when it
moves in the opposite direction
38. Orientation Cells
Hubel and Wiesel were the first to discover that cells in V1
are arranged in a beautifully precise and orderly fashion.
Hubel and Wiesel found that as one advances deeper into
the cortex through successive layers perpendicular to the
surface, all cells that have orientation tuning prefer the
same orientation.
On the other hand, moving across the surface of the
cortex, orientation tuning mostly changes in an orderly
fashion (as shown by the small lines in the picture).
Hubel and Wiesel used the term "orientation columns" to
describe this arrangement, but they are really slabs rather
than columns.
