Human Vision
Retina
Retinal Mosaic
Image Processing within the Retina
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Human Retina
• Lines posterior wall
• Consists of 6-10
layers of cells
• Macular and fovea
• Optic Nerve Head
• Highly vascular
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Retinal Vasculature
• Blood flows toward fovea
• Does not cover fovea
• 4 major arteries and many
capillary beds
• High density of blood flow
• Accounts for about 2030% of blood to eye
• Choriod supplies 65-85%
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Optic Head
• 1-2 mm wide
• Consists of Optic
nerve and artery
• Nerves are ganglion
cell axons
• Center most are from
fovea
• Note edges
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Retina (2)
• Retina is about 0.5
mm thick
• Consists of 6-10
layers
• Light sensors are
farthest from lense
beneath surface of
retina
• Many layers and
complex connections
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Simplified Retinal Mosaic
• Light enters from bottom
• Photoreceptors near top
• Top layers are pigment
epithelium lining the
choriod
• Pigment absorbs light
and participates in photon
capture cascade
• Also provides nutrients
and waste disposal
• Many nerual layers
provide primitive image
processing
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Detailed Structure
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Consists of 3 layers of nerve cell
bodies
separated by 2 layers of synaptic
connections called plexiform layers
Rods/Cones in top cell body layer
Bipolar, horizontal and amacrine in
middle layer
Ganglion and a few amacrine cell
bodies in bottom layer
Rods/Cones connected to many
other nerve cells.
Both excitatory and inhibitory type
connections
NOT ONE TO ONE !
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Artist Depiction of Types of Image
Processing at each layer
• Still trying to
understand structure
and function
• We’ll cover this next
week
• Don’t consider this as
fact but rather as a
poetic depiction
• It is VERY complex
and varies regionally
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Morphology of Central versus
Peripheral Retina
• Central on left, is
MUCH thicker and
more complex
• Has cones and rods
• Periphery has only
rods and different
type of vision
processing
(movement versus
image)
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Rod versus Cone Distribution
• Cones predominate at
fovea: region of
highest visual acuity
• Rods predominate at
periphery, responsible
for detecting
movement, optical
flow, etc.
• Image processing
varies by region
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Macula and Fovea
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Foveal pit (200 microns)
Area of highest acuity
No Rods, all cones
Midget cells (2 wavelengths of
light wide)
• Absence of other cell layers:
all pushed to side to enhance
light and detail
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Foveal Pit
• Hexagonal mosaic
• Regularity broken by
rare blue cone
• 4 in this image are
marked by blue dots
• Are extremely rare in
this region
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Macula Lutea
• Macula = fovea pit, slope, parafovea and
perifovea
• Covered by a protective layer of yellow
pigmentation (macula lutea)
• Acts as a short wavelength filter (UV)
• Protects against bright light and UV damage
• If cones were damaged we’d become
functionally blind
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Foveal Receptor
With and without Lutea
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Top is with Lutea
S-cones are yellow
M-cones are pink
L-cones are blue
Sometimes described
as “wearing
sunglasses”
• Lutea consists of
xanthophyll pigment
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Photoreceptors
• Mammalian retinas have
2-5 types of cones and
one type of rod
• Cones cell bodies
located below OLM w/
inner and outer
segments protruding into
sub-retinal space to
pigment epithelium
• Rod sit within cones,
their outer segments
stretch to pigment
epithelium
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Inner and Outer Segments
• Outer segments contain
discs that are contain
light sensitive elements.
• Inner segment is where
discs materials are
synthesized and
prepared for delivery to
outer segment
• Outer segment is the
actual photosensor
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Photoreceptors Rods/Cones
• Consist of:
1. Outer Segment
(stacked
rhodopsin/opsin filled
membranes)
2. Inner Segment
(mitochondria,
ribosomes and
membranes
3. Cell body (nucleus)
4. Synaptic terminal
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Size of Rods/Cones
• Rods inner segment =
2 microns diameter
• Cones = 6 microns
• Midget cones (in
fovea) = 1.5 microns
• Cilium form interface
between inner and
outer segments
• Control disc formation
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Rod and Cone Outer Segments
• Rod discs are
separate and free
floating
• Cone discs remain
attached to outer
segment membrane
• Process of shedding
is different
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Rod Discs
• Discs are folded
double membranes
embedded with
Rhodopsin
• The other part of the
visual pigment,
retinal, is provided to
disc by pigment
epithelium across the
subretinal space
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Transport of Retinal
• Retinal is a biproduct of
Vitamin A
• Produced in
pigment
epithelium
• Transported
across
subretinal space
via carrier
molecules
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Visual Transduction
• Visual pigment
consists of
opsin and
retinal bound
together in outer
segment discs
• Light absorption
causes 11-cis
retinal to be
transformed into
the all trans
structure
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Visual Pigment structure in Disc
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Phagocytosis of
Outer Segment
• Disc created at cilium
junction at base of OS
• Old discs are pinched off
top of segment by
pigment epithelium.
• Rods shedding at first
morning light
• Cone shedding at sunset
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Trichromate Cone Types
• Cones vary by spectral
sensitivity, which is broad.
• L = 564 nm
• M = 533 nm
• S = 437 nm
• Rod = 498 nm
• M and L cones broadly
overlap
• Land’s Retinex Theory of
Color.
• S cones found only in
trichromates
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Distribution of Cone Types
• L and M cones are “roughly”
equally distributed where
cones are found (e.g.
macula)
• There are 1/16th fewer S
cones than L or M cones
(under sampled)
• Virtually no S-cones in foveal
pit, do not begin to appear
until foveal slope and rim
• Bi-product of achromatic
optical properties of eye
• S cones appear to be
distributed in a gaussian
distribution corresponding to
defocused light
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Specialized Color Vision
• The spacing of red and green cones in the retina
appears to be optimized for discerning yellow,
orange, or red fruit against a green background
(ref: Biophotonics International, June 2002).
• This is typical of mammals that eat fruit. The
selection of fruit does not require high resolution
of color so we don’t need a full complement of
color sensors.
• Many species of Monkeys are dichromates,
lacking S-cones, but are perfectly adapted for
rummaging within the jungle for food
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Photoreceptor distribution
Rods and Cones
• Data is from Monkeys
• L cones 33%
• M cones peak in fovea (64%)
and vary between 52-55%
elsewhere
• S cones not reported
• L to M cone ratio implies
green is oversampled by 2:1
• But data varies and reverse
ratios have been reported.
• Rod densities peak in ring
around fovea about 18
degrees (4.5 mm) from
foveal pit.
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Visual Pathway
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1.
2.
3.
4.
Consists of 3-4
Elements:
Retina
Optic Chiasm
Lateral Geniculate
Body
Striated Visual
Cortex
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Retinal Mosaic
• Three layers of cell
bodies: ONL, INL and
GCL
• Two layers of synapses:
OPL and IPL
• Bipolar connects
Rod/Cones to Ganglion
cell(s)
• Amacrine’s have no axon!
• Some have “electrical
synapses”
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Retinal
Convergence/Divergence
• A single ganglion cell receives input from 121,000 bipolar cells!
• Photoreceptor cells sends input into 15-20
bipolar cells
• This implies that each cone participates in 15-20
types of low level vision processing
• A bipolar cell may receive input from dozens or
hundreds of photoreceptor cells
• We will examine dendritic arbor spanning soon,
BUT the longest connections within the retina
are no longer than 2 mm
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Visual Streams
• The retinal mosaic performs rudimentary
functions
• Enhancing some aspects of information
• Ganglion cells can be grouped by the visual
processing they perform
• Some respond to moving objects, others to
patterns: might be up to 20 such types
• We group ganglion cells that perform the same
image processing and call these Visual Steams
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Rod Pathway
An Example
• Rod cluster (~20) connects
to rod bipolar (center)
• Rod bipolar sends output
to AII amacrine
• Amacrine communicates to
2 types of ganglions: ON G
and OFF G
• Example of rod pathway
merging with cone pathway
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Image Contrast
Adaptation
• Visual Streams must be sensitive to image
patterns over a 6 order mag range
• Neurons have a dynamic range of 2 orders
• Neurons in peripheral field solve this by
signaling local contrast rather than absolute
stimulus level
• Contrast information reduces dynamic range
and is sufficient for detecting properties of
surfaces, which are visual entities we want to
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identify and recognize.
Ganglion Response to Light
• Microelectrodes used
to measure neuron
activity
• Typically measure
receptive field
• With small flashes of
light or simple moving
bars, to determine
ganglion’s response.
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Center Surround
Ganglion Cells
• Discovered by Kuffler
(1953)
• Two main types: oncenter, off-surround
and off-center, onsurround
• Dendritic fields in IPL,
but on-center at the
lower half, and offcenter at upper half
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Several Types
Center Surround
• Luminance based
• Color based
• Color is either
Red/Green or
Blue/Yellow
• Yellow is actually the
combined output from
both L and M cones
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Proposed Structure For
Center Surround
• Hypothetical circuit
• Horizontal cell contacts bipolars
and also projects back onto
receptors
• If off center, synapses onto
bipolar from center patch is
excitatory – the receptors are
turned OFF by the light
• Note 2 input paths to ganglion
cells: one direct the other
through an amacrine
• Center is supplied by direct
input. Surround by an indirect
path
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Center Surround Ganglion
Are fundamental
• Overlapping receptive fields enhance acuity
• Receptive field vary in size: smallest in fovea,
progressively larger toward periphery
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Parasol and Midget
Ganglion Cells
• Classify ganglion by
size of dendritic arbor
leads to two types:
Midget and Parasol
• Midget size is always
smaller at same
location in retina
• Both types are found
throughout retina,
providing complete
visual field
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Midget/Parasol
• Size of dendritic field varies
from small to large as
function of position from
retina
• Smaller field of Midget
ganglion cells means there
are more of them to
completely sample visual
field (7-9 times more)
• Thus we have two complete
mappings of the visual field:
one at high resolution, the
other at low!
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Mach Bands
• This phenomenon was
discovered by the
famous physicist, Ernst
Mach and it is in his
honor that these dark
and bright bars are
called Mach Bands.
These Mach Bands can
be explained by centersurround receptive field
interactions.
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Mach Bands
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The solid black curve
represents the amount of light
being reflected from the figure
at the top. The red curve
represents the brightness's of
this figure as it is usually
perceived.
To the left of the point where
the figure just starts to get
lighter people usually see a
dark bar that is slightly darker
that the area to the left of it.
At the point where the
brightness just stops
increasing, people usually
perceive a bright bar.
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Mach Bands: Lateral Inhibition
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The receptive fields are represented as a
disk (+) and annulus (-). The center disk
is an excitatory area and the annulus an
inhibitory area. The receptive fields in the
uniformly white and uniformly black
areas receive about the same stimulation
in their excitatory centers and inhibitory
surrounds. Therefore the center
excitations are in balance with the
surround inhibitions.
The receptive field over the bright Mach
Band gives a stronger response in the
center because part of the surround is in
the darker area. Therefore it receives
less inhibition from the surround than did
the center at the extreme left and right
ends. The receptive field over the dark
band receives more surround inhibition
because part of the surround is in the
brighter area. Therefore, the excitatory
response is less and this results in our
seeing that the area as darker.
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Hermann Grid
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Look closely at this matrix of
black squares. What do you
see? While scanning over the
matrix do you see something
peculiar in the intersections of
the white crosses formed by
the black squares? If you see
dark blobs, don't be surprised,
that is what most people see.
This figure is called the
Hermann grid after L. Herman
(1870). The dark blobs can be
explained by reference to
receptive fields and lateral
inhibition.
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Hermann
Grid
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To understand the receptive field explanation
for the Hermann grid illusion requires a basic
understanding of receptive fields. Once
again, it is a matter of lateral inhibition
between the center and surround of the
receptive field. Note the lower right part of the
diagram. The receptive field that lies at the
intersection of the white cross has more light
falling on its inhibitory surround than does the
receptive field that lies between the two black
squares. Consequently, the excitatory center
of this receptive field between the squares
yields a stronger response than that which
lies at the intersection of the white cross. This
explanation is appropriate for those
circumstances where the receptive fields are
larger than the spaces between the squares.
Receptive fields in the central fovea are much smaller than in the rest of the retina.
This is represented in the upper right of the diagram. In the Hermann grid you
probably did not see a dark area when you looked directly at the intersection of the
white cross, but did see dark areas in your peripheral vision. To prove to yourself that
this is not some trick, just get up and walk away from your computer screen. When
you are at some distance from it, you will undoubtedly notice that the dark blob is
visible even at your fixation point. I leave it to you to figure out why this happens.
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Fixation and Stabilization
• Close one Eye. Stare at the right target, at the
spot in the center. Keep staring for 30 seconds.
• Now try doing the same to the left target. Do
you notice a difference?
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Simultaneous Contrast
• All of the round dots are exactly the same
shade of gray
• They look different because of the lateral
interactions with the surround.
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Radiating Lines
• The Luminance plot
represent the amount of
light reflected from the
identified areas. The
amount of light is
constant around each
perimetric strip.
• The Lightness curves
represent the perceived
brightness. Note that near
the diagonal the apparent
amount of light increases.
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Simultaneous Contrast
• The circular area in each quadrant in the left figure is
exactly the same color. The different appearance is due
to the surround and the lateral interactions that occur.
On the right we changed the background colors so that
they are all identical. You can now see that the circular
areas are, indeed, identical.
• This phenomenon is called simultaneous color contrast.50
Next Week
• And will trace neural pathways from eyes to
occipital lobe (visual cortex)
• We’ll attempt to provide some understanding of
some of the mechanism that have been
discovered for perception
• This is an area of active research and is not well
resolved
• Let us give tribute to the Cat who has been the
subject of so many seminal studies of vision.
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