The visual system
The retina
• Light passes through
the lens, through the
inner layer of ganglion
cells and bipolar cells
to reach the rods and
cones.
The retina
• 0.5 mm thick
The retina
• 0.5 mm thick
• The photosensors (the rods
and cones) lie outermost in
the retina.
The retina
• 0.5 mm thick
• The photosensors (the rods
and cones) lie outermost in
the retina.
• Interneurons
The retina
• 0.5 mm thick
• The photosensors (the rods
and cones) lie outermost in
the retina.
• Interneurons
• Ganglion cells (the output
neurons of the retina) lie
innermost in the retina
closest to the lens and front
of the eye.
The retina
• Receptive field:
– The location where a visual
stimulus causes a change in the
activity of the visual neuron
The retina
• Receptive field:
– The location where a visual
stimulus causes a change in the
activity of the visual neuron
– For a rod or a cone, you could
think of it as a pixel.
The retina
• Receptive field:
– The location where a visual
stimulus causes a change in the
activity of the visual neuron
– For a rod or a cone, you could
think of it as a pixel.
– For the rest of the visual
system, receptive fields are
more complicated and more
interesting!
Retinal interneurons
• Photoreceptors synapse onto many interneurons.
Retinal interneurons
• Photoreceptors synapse onto many interneurons.
• The interneurons synapse onto one another and
onto ganglion cells.
Ganglion cells
• There are about a million ganglion cells.
Ganglion cells
• There are about a million ganglion cells.
• There are at least 18 different morphological types
of ganglion cell in the human retina.
Ganglion cells
• Most ganglion cells have center-surround
receptive fields.
Ganglion cells
• Most ganglion cells have center-surround
receptive fields.
• About 50% of those are OFF-center ON-surround.
Ganglion cells
• Most ganglion cells have center-surround
receptive fields.
• About 50% of those are OFF-center ON-surround.
• These are like the “bug detectors” in the Lettvin
paper.
OFF-center ON-surround ganglion cells
• ON-center OFF-surround ganglion cells
• Many cells respond best to a small spot of light on
a dark background.
Sustained vs. transient responses
• Some respond
transiently
• Some give sustained
responses.
transient
cell
sustained
cell
Other types of ganglion cells
• Some ganglion cells don't have center-surround
receptive fields and are involved in detecting
novel stimuli but not in form detection.
• Some ganglion cells have huge receptive fields
and are involved in setting circadian rhythms
• Some respond best to particular color
combinations (red/green or yellow/blue).
Pathway from the eye to the cortex
Striate cortex
Also called V1, primary visual cortex, or area 17
Cortical circuitry
• The basic arrangement
of circuitry is
perpendicular to the
surface of the brain:
columnar.
• Axons from lateral
geniculate terminate in
layer 4.
pia
1
2
3
4
5
6
white matter
Receptive fields in striate cortex
• Most cells in layer 4 have circular centersurround receptive fields.
• Most cells in the other layers respond only
weakly to spots of light or dark. Instead,
they respond best to lines or edges.
An elongated bar in the correct orientation
and the correct position is the best stimulus
for a simple cell.
Examples of other types of orientationselective receptive field preferences
Model of how center-surround cells can
be building blocks for simple cells.
Complex cells also prefer oriented lines but they
don’t require a particular location.
Many complex cells are directionally-selective.
Model of how simple cells could be buildingblocks for complex cells
Orientation columns
• Cortex -- as a whole -- is organized into columns.
• In striate cortex, the orientation column is the
basic unit.
– All of the cells outside of layer 4 will respond
best to stimuli of the same orientation.
– Some cells will be direction selective, others
not.
1 mm
Output of striate cortex: more than 40
other regions!
"where"
(parietal)
"What"
(temporal)
The “where” stream
• Cells in some of these regions respond almost
exclusively to moving objects.
– Some respond to circular or spiraling
movement.
– Some respond to “visual flow.”
– Some respond best to approaching or receding
objects.
– ….
Area MT is a well-studied “motion area.” Most of the cells in
MT respond best to moving stimuli, and most of those cells
have well-defined direction preferences.
Area MT, like many other cortical areas, shows
columnar organization.
Cells in area MST often respond best to more
complex types of motion.
Lesions of the “where” stream
•
•
•
•
loss of speed motion perception
visual neglect in peripersonal space
loss of ability to follow moving objects
loss of ability to use visual information to grasp
objects
The “what” stream
• Some areas have many cells that are colorselective.
• Some areas have cells that respond to
complex shapes.
• Some areas are particularly important for
face perception.
Lesions of the ventral stream
• Achromatopsia: loss of color vision
• Prosopagnosia: loss of face recognition
– Some regions have cells that respond best to
objects such as particular faces.
Example of a ventral occipito-temporal cell that responds best
to faces
Recommended reading
• Lettvin, J. Y., H. R. Maturana, et al. (1959). "What the
frog's eye tells the frog's brain." Proc. Inst. Radio Engr.
N.Y. 47: 1940-1951.
• Hubel, D. H. (1982). "Exploration of the primary visual
cortex, 1955-78." Nature 299(5883): 515-524.
Supplementary material starts
here.
Organization of orientation columns
• Adjacent columns usually have cells with slightly
different orientation preferences.
• This orderly pattern is interrupted by occasional
“color blobs” -- columns of color-selective cells
with center-surround receptive fields.
1 mm
In vivo imaging
• Present
horizontal bars.
• Collect image
intensity data.
• Repeat with
vertical bars.
• Subtract the
difference on a
point-by-point
basis.
Orientation columns form a “pinwheels” with 360°
of orientations surrounding a zone of non-oriented
cells.
Center: non-oriented, color ‘blobs’.
Ocular dominance columns
• Left-eye and right-eye layers of the lateral
geniculate project to adjacent zones in layer 4.
• Each zone is called an ocular dominance column
and is about 1 mm wide.
a
R
b
L
R
L
R
L
Hubel and Wiesel noticed that they tended to find
groups of cells dominated by one eye, then a group
dominated by the other eye, etc.
Ocular dominance columns as seen by in vivo
imaging.
In humans, each stripe is about 1-2 mm wide.
Ocular dominance
• Each ocular dominance column contains:
– about 20 orientation-selective columns
– about 1 color blob
Development of ocular
dominance columns
• At birth, the geniculo-cortical projection to
layer 4 is much less well segregated than in
the adult.
R
L
R
L
immature
R
L
R
L
R
L
mature
R
L
Development of ocular
dominance columns
• The connections from the geniculate sort
themselves out by remodeling of axons.
Development of ocular
dominance columns
• The connections from the geniculate sort
themselves out by remodeling of axons.
– New branches are made.
Development of ocular
dominance columns
• The connections from the geniculate sort
themselves out by remodeling of axons.
– New branches are made.
– Some old branches are retracted.
Development of ocular
dominance columns
• The connections from the geniculate sort
themselves out by remodeling of axons.
– New branches are made.
– Some old branches are retracted.
• The basic segregation of geniculocortical
projections is complete at 4-6 months
postnatal.
Development of ocular
dominance columns
• Each eye will normally wind up controlling
about 50% of layer 4.
Development of ocular
dominance columns
• Each eye will normally wind up controlling
about 50% of layer 4.
• However, if one eye isn't functioning
properly during development, it won't get its
share of cortical territory.
Result of uncorrected left eye
cataract
• Instead of a 50:50 division of left-eye & right-eye inputs
from the lateral geniculate to layer 4, the deprived eye
occupies only ~20% of the territory.
R
L
R
L
immature
R
L
R
L
R
L
mature
R
L
Result of uncorrected left eye
cataract
• Instead of a 50:50 division of left-eye & right-eye inputs
from the lateral geniculate to layer 4, the deprived eye
occupies only ~20% of the territory.
• The open eye occupies ~80%.
R
L
R
L
immature
R
L
R
L
R
L
mature
R
L
Result of uncorrected left eye cataract
• The good eye takes over 100% of the cells in the other
layers.
R
L
R
L
immature
R
L
R
L
R
L
mature
R
L
Take-over by non-deprived eye
• Why do all of the cells in the layers above
and below layer 4 respond only to the good
eye?
Take-over by non-deprived eye
• Why do all of the cells in the layers above
and below layer 4 respond only to the good
eye?
– Anatomy: Most of the projections to the other
layers come from the layer 4 cells that are
controlled by the good eye.
Take-over by non-deprived eye
• Why do all of the cells in the layers above
and below layer 4 respond only to the good
eye?
– Anatomy: Most of the projections to the other
layers come from the layer 4 cells that are
controlled by the good eye.
– Physiology: Intrinsic inhibitory circuits
exaggerate the imbalance between the two eyes.
The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate
most cells in striate cortex.
The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate
most cells in striate cortex.
• Since most of the visual projections to other parts of cortex
come from these cells, those regions also become
monocular.
The deprived eye becomes functionally blind.
• The deprived eye completely loses its ability to activate
most cells in striate cortex.
• Since most of the visual projections to other parts of cortex
come from these cells, those regions also become
monocular.
• The child has no binocular (stereoscopic) depth perception.
Critical periods
• This dramatic effect happens only if there is a deficit
during the critical period of development, the first few
months of postnatal life.
Critical periods
• This dramatic effect happens only if there is a deficit
during the critical period of development, the first few
months of postnatal life.
• A cataract that develops in an adult will produce no
permanent deficits.
Critical periods
• This dramatic effect happens only if there is a deficit
during the critical period of development, the first few
months of postnatal life.
• A cataract that develops in an adult will produce no
permanent deficits.
• Even if the cataract is present for years, vision will be
restored when the cataract is removed.
Critical periods
• If a cataract in a baby is removed soon enough (within 1 or
2 months), there will be a partial or even complete
recovery from effects.
Critical periods
• If a cataract in a baby is removed soon enough (within 1 or
2 months), there will be a partial or even complete
recovery from effects.
• The worse the cataract and the longer the delay to removal,
the less recovery will occur.
Axons that fire together, wire
together
• What mechanisms allow visual input to
influence formation of ocular dominance
columns?
Axons that fire together, wire
together
• What mechanisms allow visual input to
influence formation of ocular dominance
columns?
– Left-eye and right-eye inputs compete for
synaptic territory on cells in layer 4.
Axons that fire together, wire
together
• What mechanisms allow visual input to
influence formation of ocular dominance
columns?
– Left-eye and right-eye inputs compete for
synaptic territory on cells in layer 4.
– Normally, vigorous activity from each eye
helps axons cooperate in stabilizing territory.
Model of activity-mediated segregation
• Initially, inputs from both eyes via the lateral
geniculate connect to each cell in layer 4.
Model of activity-mediated segregation
• Initially, inputs from both eyes via the lateral
geniculate connect to each cell in layer 4.
• By the time of birth, about half the cells have
more left-eye input, while others have more righteye input.
right eye
left eye
Model of activity-mediated segregation
• Consider a cell with mostly left-eye input.
right eye
left eye
Model of activity-mediated segregation
• Consider a cell with mostly left-eye input.
• If visual input is normal, the more numerous left-eye inputs
reinforce each other. The less numerous right eye inputs are
eliminated.
right eye
left eye
Effect of monocular deprivation
• Monocular deprivation tips the scales in favor of the good
eye.
right
eye
left
eye
Effect of monocular deprivation
• Monocular deprivation tips the scales in favor of the good
eye.
• The good eye takes over all the cells where it started out
with a majority of inputs and can also take over many other
cells since the other eye’s activity is very weak.
right
eye
left
eye
Other aspects of visual function have their
own critical periods.
• Uncorrected astigmatism can lead to permanent deficiencies in
acuity for lines of particular orientations.
Other aspects of visual function have their
own critical periods.
• Uncorrected astigmatism can lead to permanent deficiencies in
acuity for lines of particular orientations.
• This defect need not be corrected until early grade school age.
Strabismus
• Strabismus (cross-eyedness or walleyedness) can lead to loss of stereopsis.
Strabismus
• Strabismus (cross-eyedness or walleyedness) can lead to loss of stereopsis.
• Strabismus is often be treated by surgery
followed by patching the eye with the
stronger vision.
Strabismus
• Strabismus (cross-eyedness or walleyedness) can lead to loss of stereopsis.
• Strabismus is often be treated by surgery
followed by patching the eye with the
stronger vision.
• A better understanding of critical periods
has led to earlier attention to the problem in
recent decades.
ON-center OFF-surround ganglion cells
• Lighting up the center
of the field excites the
ganglion cell (turns it
"ON").
* * * *
photoreceptors
interneurons
+
ganglion cell
ON-center OFF-surround ganglion cells
• Lighting up the center
of the field excites the
ganglion cell (turns it
"ON").
* * * *
photoreceptors
interneurons
+
ganglion cell
ON-center OFF-surround ganglion cells
• Lighting up the
surrounding part of the
field inhibits the ganglion
cell (turns it "OFF").
* *
* *
photoreceptors
interneurons
-
ganglion cell
OFF-center ON-surround ganglion cells
* * * *
• The idea is the same,
but now light in the
center leads to
inhibition, and light in
the surround leads to
excitation.
photoreceptors
interneurons
+* *
ganglion cell
* *
photoreceptors
interneurons
-+
ganglion cell